Loops in Java · JavaBook

Java Master Course — Chapter 7 of 50

Loops allow a Java program to repeat a block of code. They are one of the most important concepts in programming because many problems require the same operation to be performed multiple times.

1. What Is a Loop?#

Suppose you want to print:

Output

Java
Java
Java
Java
Java

Without a loop, you could write:

Java

System.out.println("Java");
System.out.println("Java");
System.out.println("Java");
System.out.println("Java");
System.out.println("Java");

This works, but it is repetitive.

With a loop:

Java

for (int i = 1; i <= 5; i++) {
    System.out.println("Java");
}

Much better.

The basic idea is:

Diagram

        Start
          ↓
      Check condition
          ↓
      ┌───┴───┐
    true    false
      │        │
      ↓        ↓
   Execute    Stop
    body
      │
      ↓
   Update
      │
      └──────► Check again

A loop repeatedly executes code while a condition remains true.

2. Why Do We Need Loops?#

Loops are useful whenever the same operation must happen repeatedly.

Examples:

Output

Print numbers 1 to 100
Read 10 values
Search through an array
Calculate a sum
Process every student
Repeat a menu
Validate input
Generate patterns
Run a game loop
Process records

Without loops, many programs would contain thousands of repeated statements.

3. Real-Life Example#

Imagine you need to water 100 plants.

You could think:

Output

Water plant 1
Water plant 2
Water plant 3
...
Water plant 100

A programmer thinks:

Output

Repeat watering until all plants are done.

That is the basic idea behind loops.

4. Main Loop Types in Java#

Java provides several looping mechanisms.

The most important are:

Output

while
do-while
for
enhanced for

There are also loop-control statements:

Output

break
continue

You can also create nested loops.

We will learn all of them.

5. `while` Loop#

The while loop is the simplest condition-controlled loop.

Syntax:

Java

while (condition) {
    // repeated code
}

Example:

Java

int i = 1;

while (i <= 5) {
    System.out.println(i);
    i++;
}

Output:

Output

6. How `while` Works#

Consider:

Java

int i = 1;

while (i <= 5) {
    System.out.println(i);
    i++;
}

Execution:

Output

i = 1

Check: 1 <= 5 → true
Print 1
i becomes 2

Check: 2 <= 5 → true
Print 2
i becomes 3

Check: 3 <= 5 → true
Print 3
i becomes 4

Check: 4 <= 5 → true
Print 4
i becomes 5

Check: 5 <= 5 → true
Print 5
i becomes 6

Check: 6 <= 5 → false
Stop

The condition is checked before every iteration.

7. Important Parts of a Loop#

Most counter-based loops have three important components:

Initialization
      ↓
Condition
      ↓
Update

Example:

Java

int i = 1;       // initialization

while (i <= 5) { // condition
    System.out.println(i);

    i++;         // update
}

Think:

Start
  ↓
Where do I begin?
  ↓
Should I continue?
  ↓
Do the work
  ↓
Change the counter
  ↓
Check again

8. Initialization#

Initialization gives the loop its starting state.

Example:

Java

int i = 1;

Other examples:

Java

int i = 0;
int count = 10;
int sum = 0;

Initialization normally happens before the loop begins.

9. Condition#

The condition determines whether another iteration should happen.

Example:

Java

i <= 10

As long as it is:

Output

true

the loop continues.

When it becomes:

Output

false

the loop stops.

10. Update#

The update changes the loop state.

Example:

Java

i++;

Without an update, a counter loop may never terminate.

Example:

Java

int i = 1;

while (i <= 5) {
    System.out.println(i);
}

This can run forever because i never changes.

11. Infinite Loop#

An infinite loop never reaches a false condition.

Example:

Java

while (true) {
    System.out.println("Hello");
}

This continues indefinitely unless something externally stops the program or the loop exits.

Another example:

Java

int i = 1;

while (i <= 5) {
    System.out.println(i);
}

The condition remains:

Output

1 <= 5

forever.

12. Intentional Infinite Loops#

Infinite loops are not always mistakes.

They can be intentional.

Example:

Java

while (true) {
    // repeatedly process application events
}

Usually there will be some internal exit condition:

Java

while (true) {
    System.out.print("Enter command: ");
    String command = sc.nextLine();

    if (command.equals("exit")) {
        break;
    }
}

This creates a loop that continues until the user enters exit.

13. Counter-Controlled Loop#

A common pattern is:

Java

int i = 1;

while (i <= 10) {
    System.out.println(i);
    i++;
}

Here:

Output

initial value = 1
final condition = <= 10
step = +1

This is called a counter-controlled loop.

14. Counting Backwards#

You can decrement instead.

Java

int i = 5;

while (i >= 1) {
    System.out.println(i);
    i--;
}

Output:

Output

15. Different Step Sizes#

You do not have to increase by exactly 1.

Example:

Java

int i = 0;

while (i <= 20) {
    System.out.println(i);
    i += 2;
}

Output:

Output

You can use:

Java

i += 5;

or:

Java

i -= 2;

depending on the problem.

16. `do-while` Loop#

The do-while loop is similar to while, but with one important difference:

The body executes at least once.

Syntax:

Java

do {
    // code
} while (condition);

Example:

Java

int i = 1;

do {
    System.out.println(i);
    i++;
} while (i <= 5);

Output:

Output

17. `while` vs `do-while`#

while:

Java

while (condition) {
    // body
}

The condition is checked first.

do-while:

Java

do {
    // body
} while (condition);

The body runs first, then the condition is checked.

18. Important Difference#

Consider:

Java

int i = 10;

while (i < 5) {
    System.out.println("Hello");
}

Output:

Output

The body never runs.

Now:

Java

int i = 10;

do {
    System.out.println("Hello");
} while (i < 5);

Output:

Output

Hello

Even though:

Output

10 < 5

is false.

Why?

Because do-while executes its body before checking the condition.

19. When Is `do-while` Useful?#

It is useful when an operation must happen at least once.

A classic example is a menu:

Show menu
   ↓
Read choice
   ↓
Process choice
   ↓
Ask whether to continue

The menu needs to appear at least once.

20. Menu Example with `do-while`#

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int choice;

        do {
            System.out.println();
            System.out.println("===== MENU =====");
            System.out.println("1. Say Hello");
            System.out.println("2. Say Bye");
            System.out.println("3. Exit");

            System.out.print("Enter choice: ");
            choice = sc.nextInt();

            switch (choice) {
                case 1 -> System.out.println("Hello!");
                case 2 -> System.out.println("Bye!");
                case 3 -> System.out.println("Exiting...");
                default -> System.out.println("Invalid choice");
            }

        } while (choice != 3);

        sc.close();
    }
}

The menu appears at least once.

21. `for` Loop#

The for loop is commonly used when you know or can clearly express the loop's initialization, condition, and update.

Syntax:

Java

for (initialization; condition; update) {
    // body
}

Example:

Java

for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}

Output:

Output

22. Anatomy of a `for` Loop#

Consider:

Java

for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}

Break it down:

Output

for (
     int i = 1;   → initialization

     i <= 5;      → condition

     i++          → update
)

The body:

Java

System.out.println(i);

runs on each iteration.

23. Exact Execution Order of `for`#

Example:

Java

for (int i = 1; i <= 3; i++) {
    System.out.println(i);
}

Execution:

Output

1. int i = 1
2. i <= 3 → true
3. print 1
4. i++
5. i <= 3 → true
6. print 2
7. i++
8. i <= 3 → true
9. print 3
10. i++
11. i <= 3 → false
12. stop

Important:

Initialization happens once.

Condition is checked before each iteration.

Update happens after the body.

24. `for` Loop vs `while` Loop#

These are equivalent:

Java

for (int i = 1; i <= 5; i++) {
    System.out.println(i);
}

and:

Java

int i = 1;

while (i <= 5) {
    System.out.println(i);
    i++;
}

The main difference is organization.

Use for when the initialization, condition, and update naturally belong together.

Use while when the stopping condition is more naturally expressed independently.

25. Counting from 0#

Many programming tasks start at zero.

Example:

Java

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

Output:

Output

Notice:

Output

i < 5

not:

Output

i <= 5

This produces exactly five iterations.

26. Number of Iterations#

For:

Java

for (int i = 0; i < 5; i++) {
}

values are:

Output

Total:

Output

5 iterations

For:

Java

for (int i = 1; i <= 5; i++) {
}

values are:

Output

Total:

Output

5 iterations

27. `<` vs `<=`#

This is one of the most common loop mistakes.

Java

for (int i = 0; i < 5; i++)

means:

Output

0 to 4

while:

Java

for (int i = 0; i <= 5; i++)

means:

Output

0 to 5

which is six iterations.

Always check whether the endpoint should be included.

28. Counting Down with `for`#

Java

for (int i = 5; i >= 1; i--) {
    System.out.println(i);
}

Output:

Output

29. Counting by 2#

Java

for (int i = 2; i <= 20; i += 2) {
    System.out.println(i);
}

Output:

Output

30. Reverse Counting by 2#

Java

for (int i = 20; i >= 2; i -= 2) {
    System.out.println(i);
}

Output:

Output

31. Multiple Variables in a `for` Loop#

A for loop can initialize multiple variables.

Example:

Java

for (int i = 1, j = 10; i <= 5; i++, j--) {
    System.out.println(i + " " + j);
}

Output:

Output

Both counters are updated each iteration.

32. Omitting Parts of `for`#

Java allows parts of the for header to be omitted.

For example:

Java

int i = 1;

for (; i <= 5; i++) {
    System.out.println(i);
}

Initialization was moved outside.

You can also write:

Java

for (int i = 1; ; i++) {
    if (i > 5) {
        break;
    }

    System.out.println(i);
}

The condition is omitted, creating an infinite loop unless break exits it.

This is legal, but use it only when it improves clarity.

33. Infinite `for` Loop#

This:

Java

for (;;) {
    System.out.println("Hello");
}

is an infinite loop.

It is equivalent in spirit to:

Java

while (true) {
    System.out.println("Hello");
}

Usually you will see this pattern with an explicit exit condition:

Java

for (;;) {
    // work

    if (shouldStop) {
        break;
    }
}

34. Scope of the `for` Variable#

Consider:

Java

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

The variable:

Java

exists within the scope of the for statement and its body.

This is not valid:

Java

for (int i = 0; i < 5; i++) {
    System.out.println(i);
}

System.out.println(i);

because i is out of scope after the loop.

35. Declaring the Counter Outside#

If you need the counter afterward:

Java

int i;

for (i = 0; i < 5; i++) {
    System.out.println(i);
}

System.out.println("After loop: " + i);

Output:

Output

0
1
2
3
4
After loop: 5

36. Nested Loops#

A loop inside another loop is called a:

Output

nested loop

Example:

Java

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        System.out.println(i + " " + j);
    }
}

Output:

Output

37. How Nested Loops Work#

The outer loop controls:

Output

The inner loop completes all of its iterations for every outer iteration.

Think:

Output

i = 1
    j = 1
    j = 2
    j = 3

i = 2
    j = 1
    j = 2
    j = 3

i = 3
    j = 1
    j = 2
    j = 3

If the outer loop runs 3 times and the inner loop runs 3 times each time:

Output

3 × 3 = 9

inner iterations occur.

38. Nested Loop — Multiplication Table#

Java

for (int i = 1; i <= 10; i++) {
    for (int j = 1; j <= 10; j++) {
        System.out.print((i * j) + "\t");
    }

    System.out.println();
}

This produces a multiplication table.

39. Nested Loop — Rectangle Pattern#

Java

for (int row = 1; row <= 4; row++) {
    for (int col = 1; col <= 5; col++) {
        System.out.print("* ");
    }

    System.out.println();
}

Output:

Output

* * * * *
* * * * *
* * * * *
* * * * *

The outer loop controls rows.

The inner loop controls columns.

40. Nested Loop — Triangle Pattern#

Java

for (int row = 1; row <= 5; row++) {
    for (int col = 1; col <= row; col++) {
        System.out.print("* ");
    }

    System.out.println();
}

Output:

Output

*
* *
* * *
* * * *
* * * * *

Notice that the number of inner iterations depends on the outer loop variable.

41. Nested Loop — Number Triangle#

Java

for (int row = 1; row <= 5; row++) {
    for (int col = 1; col <= row; col++) {
        System.out.print(col + " ");
    }

    System.out.println();
}

Output:

Output

42. Nested Loop — Same Number Pattern#

Java

for (int row = 1; row <= 5; row++) {
    for (int col = 1; col <= row; col++) {
        System.out.print(row + " ");
    }

    System.out.println();
}

Output:

Output

43. Loop Control — `break`#

break immediately exits the nearest loop.

Example:

Java

for (int i = 1; i <= 10; i++) {
    if (i == 5) {
        break;
    }

    System.out.println(i);
}

Output:

Output

When:

Java

i == 5

becomes true, the loop stops.

44. Flow of `break`#

Loop starts
   ↓
Iteration
   ↓
break condition?
   ↓
yes ───────► Exit loop
   │
   no
   ↓
Continue

break exits the nearest enclosing loop or switch statement.

45. `break` in `while`#

Java

int i = 1;

while (i <= 10) {
    if (i == 5) {
        break;
    }

    System.out.println(i);
    i++;
}

Output:

Output

46. `break` in Nested Loops#

Consider:

Java

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (j == 2) {
            break;
        }

        System.out.println(i + " " + j);
    }
}

Output:

Output

1 1
2 1
3 1

Why?

break exits only the nearest loop:

Output

inner loop

It does not automatically stop the outer loop.

47. Labeled `break`#

Java allows labeled statements.

Example:

Java

outer:
for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (i == 2 && j == 2) {
            break outer;
        }

        System.out.println(i + " " + j);
    }
}

Output:

Output

When:

Output

i = 2
j = 2

the labeled break exits the outer loop.

Labeled breaks are legal but should be used carefully because simpler control flow is often easier to understand.

48. `continue`#

continue skips the remaining body of the current iteration and moves to the next iteration.

Example:

Java

for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        continue;
    }

    System.out.println(i);
}

Output:

Output

The iteration where i == 3 is skipped.

49. `break` vs `continue`#

This difference is extremely important.

Diagram

break
  ↓
stop the loop completely

continue
  ↓
skip current iteration
then continue loop

Example:

Java

for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        break;
    }

    System.out.println(i);
}

Output:

Output

1
2

But:

Java

for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        continue;
    }

    System.out.println(i);
}

Output:

Output

50. `continue` in a `while` Loop#

Be careful with continue in a while loop.

Bad:

Java

int i = 1;

while (i <= 5) {
    if (i == 3) {
        continue;
    }

    System.out.println(i);
    i++;
}

When i becomes 3, continue jumps back to the condition without executing:

Java

i++;

So i remains 3 forever.

This creates an infinite loop.

Correct:

Java

int i = 1;

while (i <= 5) {
    if (i == 3) {
        i++;
        continue;
    }

    System.out.println(i);
    i++;
}

Or structure the loop differently.

51. Enhanced `for` Loop#

Java provides an enhanced for loop, also called the:

Output

for-each loop

It is especially useful for arrays and collections.

Syntax:

Java

for (Type variable : collectionOrArray) {
    // use variable
}

Example:

Java

int[] numbers = {10, 20, 30, 40};

for (int number : numbers) {
    System.out.println(number);
}

Output:

Output

52. Why Use Enhanced `for`?#

Suppose you only want to process every element of an array.

Traditional loop:

Java

int[] numbers = {10, 20, 30, 40};

for (int i = 0; i < numbers.length; i++) {
    System.out.println(numbers[i]);
}

Enhanced loop:

Java

for (int number : numbers) {
    System.out.println(number);
}

The second version is simpler when you don't need the index.

53. Understanding the Enhanced `for`#

For:

Java

for (int number : numbers) {
    System.out.println(number);
}

read it as:

Output

For each int number in numbers:
    print number

If:

Output

numbers = [10, 20, 30, 40]

iterations are:

Output

number = 10
number = 20
number = 30
number = 40

54. Enhanced `for` with Strings#

Java

String[] languages = {
    "Java",
    "Python",
    "C++"
};

for (String language : languages) {
    System.out.println(language);
}

Output:

Output

Java
Python
C++

55. Enhanced `for` with `char[]`#

Java

char[] letters = {'J', 'a', 'v', 'a'};

for (char ch : letters) {
    System.out.println(ch);
}

Output:

Output

J
a
v
a

56. Enhanced `for` and Index#

The enhanced for loop does not directly provide the current index.

If you need the index, use a traditional loop:

Java

for (int i = 0; i < numbers.length; i++) {
    System.out.println(i + ": " + numbers[i]);
}

If you only need the values:

Java

for (int number : numbers) {
    System.out.println(number);
}

57. Enhanced `for` and Modification#

Suppose:

Java

int[] numbers = {1, 2, 3};

for (int number : numbers) {
    number = number * 2;
}

This does not modify the array elements.

Why?

number is a local loop variable holding the current element value.

The array remains:

Output

1 2 3

To modify elements, use an index:

Java

for (int i = 0; i < numbers.length; i++) {
    numbers[i] *= 2;
}

Now the array becomes:

Output

2 4 6

58. Enhanced `for` with Object References#

Later, when you study OOP, you will see:

Java

Student[] students = ...;

for (Student student : students) {
    student.display();
}

The loop variable contains a reference to each object.

This becomes extremely useful when processing collections of objects.

59. Looping Through a String#

A String is not directly an array, but you can access its characters.

Example:

Java

String text = "Java";

for (int i = 0; i < text.length(); i++) {
    System.out.println(text.charAt(i));
}

Output:

Output

J
a
v
a

60. Counting Characters#

Java

String text = "banana";
int count = 0;

for (int i = 0; i < text.length(); i++) {
    if (text.charAt(i) == 'a') {
        count++;
    }
}

System.out.println("a count = " + count);

Output:

Output

a count = 3

This demonstrates:

Output

loop
+
condition
+
counter

61. Sum of Numbers#

Calculate:

Output

1 + 2 + 3 + ... + 10

Code:

Java

int sum = 0;

for (int i = 1; i <= 10; i++) {
    sum += i;
}

System.out.println(sum);

Output:

Output

62. Understanding the Sum#

Initially:

Output

sum = 0

Iteration:

Output

i = 1 → sum = 1
i = 2 → sum = 3
i = 3 → sum = 6
i = 4 → sum = 10
...
i = 10 → sum = 55

This is called an accumulator pattern.

63. Accumulator Pattern#

A common loop structure is:

Java

result = initialValue;

for (...) {
    result = result + currentValue;
}

Examples:

Java

sum = sum + number;

or:

Java

product = product * number;

or:

Java

total += price;

This pattern appears everywhere in programming.

64. Product of Numbers#

Calculate:

Output

1 × 2 × 3 × 4 × 5

Code:

Java

int product = 1;

for (int i = 1; i <= 5; i++) {
    product *= i;
}

System.out.println(product);

Output:

Output

This is also the basis of factorial.

65. Factorial#

Mathematically:

Output

5! = 5 × 4 × 3 × 2 × 1
   = 120

Java:

Java

int n = 5;
long factorial = 1;

for (int i = 1; i <= n; i++) {
    factorial *= i;
}

System.out.println(factorial);

Output:

Output

For large values, even long can overflow. Later chapters will discuss numeric limits and safer approaches.

66. Factorial with User Input#

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter n: ");
        int n = sc.nextInt();

        long factorial = 1;

        if (n < 0) {
            System.out.println("Factorial is not defined for negative integers.");
        } else {
            for (int i = 1; i <= n; i++) {
                factorial *= i;
            }

            System.out.println("Factorial = " + factorial);
        }

        sc.close();
    }
}

67. Multiplication Table#

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("Enter number: ");
        int n = sc.nextInt();

        for (int i = 1; i <= 10; i++) {
            System.out.println(n + " × " + i + " = " + (n * i));
        }

        sc.close();
    }
}

Input:

Output

Output:

Output

5 × 1 = 5
5 × 2 = 10
5 × 3 = 15
5 × 4 = 20
5 × 5 = 25
5 × 6 = 30
5 × 7 = 35
5 × 8 = 40
5 × 9 = 45
5 × 10 = 50

68. Counting Digits#

Suppose:

Output

number = 12345

We want:

Output

5 digits

One common approach:

Java

int number = 12345;
int count = 0;

while (number != 0) {
    number /= 10;
    count++;
}

System.out.println(count);

Output:

Output

69. How Digit Counting Works#

Start:

Output

Divide by 10:

Output

Again:

Output

Again:

Output

Again:

Output

Again:

Output

Number of divisions:

Output

Therefore:

Output

5 digits

70. Special Case — Zero#

The previous method gives:

Output

0 digits

for:

Java

number = 0;

But zero has one digit.

Handle it separately:

Java

int number = 0;
int count;

if (number == 0) {
    count = 1;
} else {
    count = 0;

    while (number != 0) {
        number /= 10;
        count++;
    }
}

System.out.println(count);

71. Reverse a Number#

Example:

Output

Input: 1234
Output: 4321

Code:

Java

int number = 1234;
int reverse = 0;

while (number != 0) {
    int digit = number % 10;
    reverse = reverse * 10 + digit;
    number /= 10;
}

System.out.println(reverse);

Output:

Output

72. Reverse Number — Step by Step#

Start:

Output

number = 1234
reverse = 0

First digit:

Output

1234 % 10 = 4

Then:

Output

reverse = 0 × 10 + 4
        = 4

Remove digit:

Output

1234 / 10 = 123

Output

123 % 10 = 3
reverse = 4 × 10 + 3
        = 43

Output

2
reverse = 432

Output

1
reverse = 4321

Finally:

Output

number = 0

Loop stops.

73. Palindrome Number#

A palindrome reads the same forward and backward.

Examples:

Output

121
1331
1221

Not palindromes:

Output

123
1234

We can reverse the number and compare.

Java

int number = 121;
int original = number;
int reverse = 0;

while (number != 0) {
    int digit = number % 10;
    reverse = reverse * 10 + digit;
    number /= 10;
}

if (original == reverse) {
    System.out.println("Palindrome");
} else {
    System.out.println("Not palindrome");
}

Output:

Output

Palindrome

74. Sum of Digits#

For:

Output

sum:

Output

1 + 2 + 3 + 4 = 10

Program:

Java

int number = 1234;
int sum = 0;

while (number != 0) {
    int digit = number % 10;
    sum += digit;
    number /= 10;
}

System.out.println(sum);

Output:

Output

75. Count Even Numbers#

Java

int[] numbers = {10, 15, 20, 25, 30};

int count = 0;

for (int number : numbers) {
    if (number % 2 == 0) {
        count++;
    }
}

System.out.println("Even numbers = " + count);

Output:

Output

Even numbers = 3

76. Find Maximum#

Java

int[] numbers = {10, 50, 20, 90, 30};

int max = numbers[0];

for (int number : numbers) {
    if (number > max) {
        max = number;
    }
}

System.out.println("Maximum = " + max);

Output:

Output

Maximum = 90

This is a very important algorithmic pattern.

77. Find Minimum#

Java

int[] numbers = {10, 50, 20, 90, 30};

int min = numbers[0];

for (int number : numbers) {
    if (number < min) {
        min = number;
    }
}

System.out.println("Minimum = " + min);

Output:

Output

Minimum = 10

78. Search for a Value#

Java

int[] numbers = {10, 20, 30, 40, 50};

int target = 30;
boolean found = false;

for (int number : numbers) {
    if (number == target) {
        found = true;
        break;
    }
}

if (found) {
    System.out.println("Found");
} else {
    System.out.println("Not found");
}

Output:

Output

Found

Notice how break avoids unnecessary additional searching after the target is found.

79. Linear Search#

The previous example is a basic:

Output

linear search

The algorithm checks elements one by one.

For:

Output

[10, 20, 30, 40, 50]

searching for 40:

Output

10 → no
20 → no
30 → no
40 → yes

Then we can stop.

This is an important foundation for later data structures and algorithms.

80. Loop with Input#

You can use loops to process repeated user input.

Example:

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int sum = 0;

        for (int i = 1; i <= 5; i++) {
            System.out.print("Enter number " + i + ": ");
            int number = sc.nextInt();

            sum += number;
        }

        System.out.println("Sum = " + sum);

        sc.close();
    }
}

Example:

Output

Enter number 1: 10
Enter number 2: 20
Enter number 3: 30
Enter number 4: 40
Enter number 5: 50
Sum = 150

81. Sentinel-Controlled Loop#

Sometimes you do not know how many values the user will enter.

You can use a special value called a:

Output

sentinel

Example:

Output

Enter numbers.
Enter -1 to stop.

Code:

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int sum = 0;

        while (true) {
            System.out.print("Enter number (-1 to stop): ");
            int number = sc.nextInt();

            if (number == -1) {
                break;
            }

            sum += number;
        }

        System.out.println("Sum = " + sum);

        sc.close();
    }
}

The sentinel is:

Output

-1

82. Validation Loop#

Loops are excellent for repeatedly asking for valid input.

Example:

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int age;

        while (true) {
            System.out.print("Enter age between 1 and 120: ");

            age = sc.nextInt();

            if (age >= 1 && age <= 120) {
                break;
            }

            System.out.println("Invalid age. Try again.");
        }

        System.out.println("Accepted age = " + age);

        sc.close();
    }
}

This pattern is used constantly in real applications.

83. `while` for Unknown Number of Iterations#

Use while when you don't know exactly how many times the loop will run.

Example:

Java

while (userHasNotExited) {
    // process command
}

The loop may run:

Output

1 time
10 times
100 times
0 times

depending on the input.

84. `for` for Known Iterations#

If you know the number of repetitions:

Java

for (int i = 0; i < 10; i++) {
    ...
}

This clearly expresses:

Output

repeat 10 times

This is one reason for is so common.

85. `do-while` for At-Least-Once Logic#

If the body must execute at least once:

Java

do {
    ...
} while (...);

Examples:

Output

menus
user prompts
retry operations

86. Choosing the Right Loop#

A useful guideline:

Known/countable repetitions
        ↓
       for

Condition-controlled repetition
        ↓
      while

Must execute at least once
        ↓
    do-while

Process every element of array/collection
        ↓
   enhanced for

These are guidelines, not strict rules.

87. Nested Loops and Complexity#

Suppose:

Java

for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
        // work
    }
}

The inner body executes approximately:

Output

n × n

times:

Output

n²

This leads to the concept of algorithmic complexity.

For example:

Output

n = 10
→ 100 iterations

n = 100
→ 10,000 iterations

n = 1000
→ 1,000,000 iterations

You will study algorithm analysis more deeply in data structures and algorithms.

88. Three Nested Loops#

You can nest more than two loops.

Example:

Java

for (int i = 1; i <= 2; i++) {
    for (int j = 1; j <= 3; j++) {
        for (int k = 1; k <= 4; k++) {
            System.out.println(i + " " + j + " " + k);
        }
    }
}

Total inner executions:

Output

2 × 3 × 4 = 24

Nested loops can become expensive quickly.

89. Avoid Unnecessary Nested Loops#

Sometimes a nested loop is required.

But do not use one automatically.

For example, searching for a value in an array can often use one loop:

Java

for (int number : numbers) {
    if (number == target) {
        ...
    }
}

A second loop may be unnecessary.

Good programming is not just about making code work; it is also about choosing an appropriate algorithm.

90. Loop Invariants — Basic Idea#

A useful algorithmic concept is a:

Output

loop invariant

It is something that remains true at a particular point during every iteration.

For example, in:

Java

int sum = 0;

for (int i = 1; i <= n; i++) {
    sum += i;
}

After each iteration, sum represents the sum of all integers processed so far.

Understanding invariants helps you prove that loops are correct.

You do not need advanced mathematical proof yet, but learning to ask:

Output

What does my variable mean after every iteration?

is an excellent habit.

91. Dry Run a Loop#

A dry run means manually tracing the values.

Example:

Java

int sum = 0;

for (int i = 1; i <= 4; i++) {
    sum += i;
}

Trace:

Iteration	`i`	`sum`
Start	—	0
1	1	1
2	2	3
3	3	6
4	4	10

Final:

Output

sum = 10

Dry runs are extremely useful when learning loops.

92. Common Loop Mistake — Off-by-One#

Consider:

Java

for (int i = 0; i <= 10; i++) {
    System.out.println(i);
}

This prints:

Output

0 through 10

That is:

Output

11 values

If you wanted exactly 10 values:

Java

for (int i = 0; i < 10; i++) {
    System.out.println(i);
}

prints:

Output

0 through 9

This is called an:

Output

off-by-one error

93. Common Loop Mistake — Wrong Update Direction#

This is dangerous:

Java

for (int i = 10; i >= 1; i++) {
    System.out.println(i);
}

The condition requires i to decrease, but the code increases it.

Correct:

Java

for (int i = 10; i >= 1; i--) {
    System.out.println(i);
}

Always make sure the update moves toward the stopping condition.

94. Common Loop Mistake — Forgetting Update#

Bad:

Java

int i = 1;

while (i <= 10) {
    System.out.println(i);
}

i never changes.

Correct:

Java

int i = 1;

while (i <= 10) {
    System.out.println(i);
    i++;
}

95. Common Loop Mistake — Updating the Wrong Variable#

Consider:

Java

int i = 1;
int j = 10;

while (i <= 5) {
    System.out.println(i);
    j++;
}

The condition depends on:

Java

but the code updates:

Java

Therefore i never changes and the loop does not terminate normally.

96. Common Loop Mistake — Accumulator Initialization#

For a sum:

Java

int sum = 0;

is normally appropriate.

For a product:

Java

int product = 1;

is normally appropriate.

Why not:

Java

int product = 0;

Because:

Output

0 × anything = 0

The accumulator's initial value must match the operation.

97. Common Loop Mistake — Modifying the Loop Counter Inside the Body#

Example:

Java

for (int i = 0; i < 10; i++) {
    i++;
    System.out.println(i);
}

This is legal but makes the number of iterations less obvious.

It can be appropriate in special cases, but beginners should generally keep the counter update in one clear place.

Prefer:

Java

for (int i = 0; i < 10; i++) {
    System.out.println(i);
}

when no special behavior is needed.

98. Common Loop Mistake — Floating-Point Loop Counters#

Avoid using floating-point values as loop counters when possible.

For example:

Java

for (double x = 0; x <= 1; x += 0.1) {
    ...
}

Floating-point representation can cause surprising boundary behavior.

Prefer integer counters:

Java

for (int i = 0; i <= 10; i++) {
    double x = i / 10.0;
}

This gives more predictable control.

99. `break` vs Boolean Flag#

Instead of:

Java

while (true) {
    ...
    if (done) {
        break;
    }
}

you can sometimes use:

Java

while (!done) {
    ...
}

Both can be valid.

Use the form that makes the termination logic easiest to understand.

100. `continue` and Readability#

Although continue can be useful, too many early jumps can make a loop difficult to follow.

Example:

Java

for (int number : numbers) {
    if (number < 0) {
        continue;
    }

    if (number == 0) {
        continue;
    }

    ...
}

Sometimes this is clearer as:

Java

for (int number : numbers) {
    if (number > 0) {
        ...
    }
}

Do not avoid continue completely; just use it when it makes the logic clearer.

101. Looping Through Arrays#

Arrays will be covered in detail in Chapter 9, but loops are essential for using arrays.

Example:

Java

int[] marks = {80, 90, 70, 85};

for (int i = 0; i < marks.length; i++) {
    System.out.println(marks[i]);
}

Output:

Output

Notice:

Java

marks.length

gives the number of elements.

102. Enhanced Loop with Array#

The same task can be written:

Java

int[] marks = {80, 90, 70, 85};

for (int mark : marks) {
    System.out.println(mark);
}

This is often cleaner when the index is unnecessary.

103. Sum of Array Elements#

Java

int[] numbers = {10, 20, 30, 40};

int sum = 0;

for (int number : numbers) {
    sum += number;
}

System.out.println("Sum = " + sum);

Output:

Output

Sum = 100

104. Average of Array Elements#

Java

int[] numbers = {10, 20, 30, 40};

int sum = 0;

for (int number : numbers) {
    sum += number;
}

double average = (double) sum / numbers.length;

System.out.println("Average = " + average);

Output:

Output

Average = 25.0

The cast:

Java

(double) sum

ensures floating-point division.

105. Count Values Above a Limit#

Java

int[] marks = {45, 80, 90, 30, 70};

int count = 0;

for (int mark : marks) {
    if (mark >= 50) {
        count++;
    }
}

System.out.println("Passing students = " + count);

Output:

Output

Passing students = 3

106. Search and Stop#

Java

int[] numbers = {10, 20, 30, 40, 50};

int target = 40;

for (int number : numbers) {
    if (number == target) {
        System.out.println("Found");
        break;
    }
}

The break prevents unnecessary work after the value is found.

107. Loop Through a 2D Array#

Two-dimensional arrays are covered later, but the loop pattern is important.

Java

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6}
};

for (int row = 0; row < matrix.length; row++) {
    for (int col = 0; col < matrix[row].length; col++) {
        System.out.print(matrix[row][col] + " ");
    }

    System.out.println();
}

Output:

Output

1 2 3
4 5 6

108. Nested Enhanced `for`#

You can also use enhanced loops:

Java

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6}
};

for (int[] row : matrix) {
    for (int value : row) {
        System.out.print(value + " ");
    }

    System.out.println();
}

Output:

Output

1 2 3
4 5 6

109. Looping Through a String — Reverse#

Java

String text = "Java";

for (int i = text.length() - 1; i >= 0; i--) {
    System.out.print(text.charAt(i));
}

Output:

Output

avaJ

110. Count Vowels#

Java

String text = "programming";
int count = 0;

for (int i = 0; i < text.length(); i++) {
    char ch = text.charAt(i);

    if (ch == 'a' || ch == 'e' ||
        ch == 'i' || ch == 'o' ||
        ch == 'u') {
        count++;
    }
}

System.out.println("Vowels = " + count);

Output:

Output

Vowels = 3

This is a good example of:

Output

String
+
loop
+
condition
+
counter

111. Find First Matching Character#

Java

String text = "programming";

for (int i = 0; i < text.length(); i++) {
    if (text.charAt(i) == 'g') {
        System.out.println("First g at index " + i);
        break;
    }
}

Output:

Output

First g at index 3

112. Print Only Odd Numbers#

Java

for (int i = 1; i <= 20; i++) {
    if (i % 2 == 0) {
        continue;
    }

    System.out.println(i);
}

Output:

Output

Here continue skips even numbers.

113. Print Multiples#

Java

for (int i = 1; i <= 50; i++) {
    if (i % 5 == 0) {
        System.out.println(i);
    }
}

Output:

Output

114. Prime Number Check#

A prime number has exactly two positive divisors:

Output

1
itself

Examples:

Output

A simple approach:

Java

int n = 29;
boolean prime = true;

if (n < 2) {
    prime = false;
} else {
    for (int i = 2; i < n; i++) {
        if (n % i == 0) {
            prime = false;
            break;
        }
    }
}

if (prime) {
    System.out.println("Prime");
} else {
    System.out.println("Not prime");
}

Output:

Output

Prime

115. Better Prime Check#

You don't need to test all numbers below n.

If a number has a divisor larger than its square root, it must also have a corresponding divisor smaller than the square root.

So you can test:

Java

for (int i = 2; i * i <= n; i++) {
    if (n % i == 0) {
        prime = false;
        break;
    }
}

Complete example:

Java

int n = 29;
boolean prime = n >= 2;

for (int i = 2; i * i <= n && prime; i++) {
    if (n % i == 0) {
        prime = false;
    }
}

System.out.println(prime ? "Prime" : "Not prime");

This is more efficient than checking every number below n.

116. Print Prime Numbers#

Java

for (int n = 2; n <= 50; n++) {
    boolean prime = true;

    for (int i = 2; i * i <= n; i++) {
        if (n % i == 0) {
            prime = false;
            break;
        }
    }

    if (prime) {
        System.out.print(n + " ");
    }
}

Output:

Output

2 3 5 7 11 13 17 19 23 29 31 37 41 43 47

This demonstrates nested loops.

117. Fibonacci Sequence#

A common beginner problem is Fibonacci numbers:

Output

0 1 1 2 3 5 8 13 21 ...

Each number is generally the sum of the previous two.

Program:

Java

int n = 10;

int a = 0;
int b = 1;

for (int i = 1; i <= n; i++) {
    System.out.print(a + " ");

    int next = a + b;
    a = b;
    b = next;
}

Output:

Output

0 1 1 2 3 5 8 13 21 34

118. GCD Using a Loop#

A simple method for finding the greatest common divisor:

Java

int a = 48;
int b = 18;

int gcd = 1;

for (int i = 1; i <= Math.min(a, b); i++) {
    if (a % i == 0 && b % i == 0) {
        gcd = i;
    }
}

System.out.println("GCD = " + gcd);

Output:

Output

GCD = 6

There are more efficient algorithms, such as Euclid's algorithm, which you may encounter later.

119. Practical Program — Sum Until Zero#

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int sum = 0;

        while (true) {
            System.out.print("Enter number (0 to stop): ");
            int n = sc.nextInt();

            if (n == 0) {
                break;
            }

            sum += n;
        }

        System.out.println("Sum = " + sum);

        sc.close();
    }
}

120. Practical Program — Count Positive and Negative Values#

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int positive = 0;
        int negative = 0;
        int zero = 0;

        for (int i = 1; i <= 5; i++) {
            System.out.print("Enter number " + i + ": ");
            int n = sc.nextInt();

            if (n > 0) {
                positive++;
            } else if (n < 0) {
                negative++;
            } else {
                zero++;
            }
        }

        System.out.println("Positive: " + positive);
        System.out.println("Negative: " + negative);
        System.out.println("Zero: " + zero);

        sc.close();
    }
}

121. Practical Program — Find Largest Input#

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        System.out.print("How many numbers? ");
        int n = sc.nextInt();

        if (n <= 0) {
            System.out.println("Number of values must be positive.");
            sc.close();
            return;
        }

        System.out.print("Enter number 1: ");
        int max = sc.nextInt();

        for (int i = 2; i <= n; i++) {
            System.out.print("Enter number " + i + ": ");
            int value = sc.nextInt();

            if (value > max) {
                max = value;
            }
        }

        System.out.println("Maximum = " + max);

        sc.close();
    }
}

122. Practical Program — Guessing Game#

This is a simple loop-based game.

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int secret = 42;
        int guess;

        do {
            System.out.print("Guess the number: ");
            guess = sc.nextInt();

            if (guess < secret) {
                System.out.println("Too low");
            } else if (guess > secret) {
                System.out.println("Too high");
            } else {
                System.out.println("Correct!");
            }

        } while (guess != secret);

        sc.close();
    }
}

This combines:

Output

do-while
if-else
input
comparison

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int choice;

        do {
            System.out.println();
            System.out.println("1. Add");
            System.out.println("2. Multiply");
            System.out.println("3. Exit");
            System.out.print("Choice: ");

            choice = sc.nextInt();

            switch (choice) {
                case 1:
                    System.out.println("Addition selected");
                    break;

                case 2:
                    System.out.println("Multiplication selected");
                    break;

                case 3:
                    System.out.println("Goodbye");
                    break;

                default:
                    System.out.println("Invalid choice");
            }

        } while (choice != 3);

        sc.close();
    }
}

This pattern is the foundation of many command-line applications.

124. Practical Program — ATM Loop#

Java

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        double balance = 10000;
        int choice;

        do {
            System.out.println();
            System.out.println("===== ATM =====");
            System.out.println("1. Check Balance");
            System.out.println("2. Deposit");
            System.out.println("3. Withdraw");
            System.out.println("4. Exit");

            System.out.print("Enter choice: ");
            choice = sc.nextInt();

            switch (choice) {
                case 1:
                    System.out.printf("Balance = %.2f%n", balance);
                    break;

                case 2:
                    System.out.print("Enter deposit amount: ");
                    double deposit = sc.nextDouble();

                    if (deposit > 0) {
                        balance += deposit;
                        System.out.println("Deposit successful");
                    } else {
                        System.out.println("Invalid amount");
                    }
                    break;

                case 3:
                    System.out.print("Enter withdrawal amount: ");
                    double withdrawal = sc.nextDouble();

                    if (withdrawal <= 0) {
                        System.out.println("Invalid amount");
                    } else if (withdrawal > balance) {
                        System.out.println("Insufficient balance");
                    } else {
                        balance -= withdrawal;
                        System.out.println("Withdrawal successful");
                    }
                    break;

                case 4:
                    System.out.println("Thank you");
                    break;

                default:
                    System.out.println("Invalid choice");
            }

        } while (choice != 4);

        sc.close();
    }
}

This combines many concepts learned so far.

125. Loop Patterns#

Pattern problems are useful for learning nested loops.

The general structure is:

Java

for (int row = ...; ...; ...) {

    for (int col = ...; ...; ...) {
        // print something
    }

    System.out.println();
}

Think:

Output

outer loop
→ rows

inner loop
→ columns

126. Square Pattern#

Java

for (int row = 1; row <= 4; row++) {
    for (int col = 1; col <= 4; col++) {
        System.out.print("* ");
    }

    System.out.println();
}

Output:

Output

* * * *
* * * *
* * * *
* * * *

127. Increasing Triangle#

Java

for (int row = 1; row <= 5; row++) {
    for (int col = 1; col <= row; col++) {
        System.out.print("* ");
    }

    System.out.println();
}

Output:

Output

*
* *
* * *
* * * *
* * * * *

128. Decreasing Triangle#

Java

for (int row = 5; row >= 1; row--) {
    for (int col = 1; col <= row; col++) {
        System.out.print("* ");
    }

    System.out.println();
}

Output:

Output

* * * * *
* * * *
* * *
* *
*

129. Right-Aligned Triangle#

Java

int n = 5;

for (int row = 1; row <= n; row++) {
    for (int space = 1; space <= n - row; space++) {
        System.out.print("  ");
    }

    for (int star = 1; star <= row; star++) {
        System.out.print("* ");
    }

    System.out.println();
}

Output:

Output

        *
      * *
    * * *
  * * * *
* * * * *

This teaches you to use multiple inner loops for one row.

130. Pyramid Pattern#

Java

int n = 5;

for (int row = 1; row <= n; row++) {
    for (int space = 1; space <= n - row; space++) {
        System.out.print(" ");
    }

    for (int star = 1; star <= 2 * row - 1; star++) {
        System.out.print("*");
    }

    System.out.println();
}

Output:

Output

    *
   ***
  *****
 *******
*********

The formula:

Output

2 * row - 1

produces:

Output

stars.

131. Number Pattern#

Java

for (int row = 1; row <= 5; row++) {
    for (int col = 1; col <= row; col++) {
        System.out.print(col);
    }

    System.out.println();
}

Output:

Output

132. Repeated Number Pattern#

Java

for (int row = 1; row <= 5; row++) {
    for (int col = 1; col <= row; col++) {
        System.out.print(row);
    }

    System.out.println();
}

Output:

Output

133. `break` and `continue` in Nested Loops#

Remember:

Java

break;

affects the nearest enclosing loop.

Example:

Java

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (j == 2) {
            break;
        }

        System.out.println(i + "," + j);
    }
}

Only the inner loop stops each time.

For the outer loop too, use a labeled break if appropriate:

Java

outer:
for (...) {
    for (...) {
        if (...) {
            break outer;
        }
    }
}

134. `continue` in Nested Loops#

A continue applies to the nearest loop.

Example:

Java

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (j == 2) {
            continue;
        }

        System.out.println(i + "," + j);
    }
}

For every row, the j == 2 iteration is skipped.

135. Labeled `continue`#

Java also supports labeled continue.

Example:

Java

outer:
for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        if (j == 2) {
            continue outer;
        }

        System.out.println(i + "," + j);
    }
}

When j == 2, the program skips to the next iteration of the outer loop.

This is advanced control flow. Use it only when it makes the algorithm clearer.

136. Loop and Method Interaction#

A loop can call a method.

Example:

Java

public class Main {

    static void printSquare(int n) {
        System.out.println(n * n);
    }

    public static void main(String[] args) {
        for (int i = 1; i <= 5; i++) {
            printSquare(i);
        }
    }
}

Output:

Output

Methods are covered deeply in Chapter 8.

137. Loop and `return`#

return exits the current method, not just the loop.

Example:

Java

public class Main {

    static void findFirst(int[] numbers, int target) {
        for (int number : numbers) {
            if (number == target) {
                System.out.println("Found");
                return;
            }
        }

        System.out.println("Not found");
    }

    public static void main(String[] args) {
        int[] numbers = {10, 20, 30};

        findFirst(numbers, 20);
    }
}

Output:

Output

Found

Once return executes, the method ends.

138. `break` vs `return`#

Output

break
→ exits loop

return
→ exits method

Example:

Java

for (...) {
    if (...) {
        break;
    }
}

The method continues after the loop.

But:

Java

for (...) {
    if (...) {
        return;
    }
}

the current method ends.

139. Loop and Exceptions#

You may eventually have code like:

Java

try {
    // operation
} catch (...) {
    // handle error
}

inside a loop.

Or a loop may continue after handling an invalid input.

Exception handling is covered in Chapter 25, so for now focus on the loop mechanics.

140. Loop Performance Basics#

For simple loops:

Java

for (int i = 0; i < n; i++) {
    ...
}

the body runs approximately:

Output

n times

Nested:

Java

for (int i = 0; i < n; i++) {
    for (int j = 0; j < n; j++) {
        ...
    }
}

approximately:

Output

n² times

Three levels:

Output

n³

Understanding this helps you write efficient programs.

141. Avoid Repeating Expensive Work#

Suppose:

Java

for (int i = 0; i < array.length; i++) {
    expensiveCalculation();
}

If the calculation does not depend on i, doing it every iteration may be wasteful.

Prefer:

Java

var result = expensiveCalculation();

for (int i = 0; i < array.length; i++) {
    use(result);
}

The exact optimization depends on the program, but the general principle is:

Do not repeatedly calculate something that does not need to change.

Do not optimize blindly; first make the code correct and readable.

142. Infinite Loop Checklist#

If your program is stuck in a loop, ask:

Output

1. What condition controls the loop?
2. Which variable changes?
3. Does that variable move toward termination?
4. Can the condition ever become false?
5. Did I accidentally use the wrong update?
6. Did continue skip the update?
7. Did I accidentally use <= instead of <?

These questions solve many beginner loop bugs.

143. Loop Debugging Example#

Suppose:

Java

int i = 1;

while (i <= 5) {
    System.out.println(i);
    i += 2;
}

Trace:

Output

i = 1
print 1
i = 3

print 3
i = 5

print 5
i = 7

7 <= 5 → false

Output:

Output

1
3
5

Dry-running like this makes loop behavior predictable.

144. Another Dry Run#

Java

int x = 0;

for (int i = 1; i <= 3; i++) {
    x += i * 2;
}

Trace:

Output

Start x = 0

i = 1
x = 0 + 2
x = 2

i = 2
x = 2 + 4
x = 6

i = 3
x = 6 + 6
x = 12

Final:

Output

x = 12

145. Loop Design Formula#

When creating a loop, ask four questions:

Output

1. What is the starting state?
2. What condition means "continue"?
3. What work happens each iteration?
4. How does the state change?

Example:

Java

for (int i = 1; i <= 10; i++) {
    System.out.println(i);
}

Answers:

Output

Starting state → i = 1
Continue       → i <= 10
Work           → print i
Update         → i++

If you can answer these four questions, you can design most basic loops.

146. Loop Types Summary#

`while`#

Java

while (condition) {
    ...
}

Condition checked first.

May execute zero times.

`do-while`#

Java

do {
    ...
} while (condition);

Body executes first.

Executes at least once.

`for`#

Java

for (initialization; condition; update) {
    ...
}

Good for counter-controlled loops.

Enhanced `for`#

Java

for (Type value : arrayOrCollection) {
    ...
}

Good for processing each element.

147. Comparison Table#

Loop	Condition checked	Minimum executions	Common use
`while`	before body	0	unknown repetitions
`do-while`	after body	1	menu/retry
`for`	before body	0	known/countable repetitions
enhanced `for`	internally per element	0	arrays/collections

148. Important Loop Keywords#

Output

break
continue

break:

Output

exit loop

continue:

Output

skip current iteration

return:

Output

exit current method

149. Practice Set — Basic Loops#

Write programs to:

Print numbers from 1 to 10.
Print numbers from 10 to 1.
Print even numbers from 1 to 100.
Print odd numbers from 1 to 100.
Print multiples of 5 up to 100.
Print numbers from 1 to n.
Print numbers from n to 1.
Find the sum from 1 to n.
Find the product from 1 to n.
Calculate factorial.
Print a multiplication table.
Count digits.
Find sum of digits.
Reverse a number.
Check palindrome.

150. Practice Set — Conditions + Loops#

Write programs to:

Count positive and negative numbers.
Count even and odd numbers.
Find the largest number.
Find the smallest number.
Check whether a number is prime.
Print all primes from 1 to 100.
Print all factors of a number.
Find GCD.
Find LCM.
Check Armstrong number.
Generate Fibonacci numbers.
Count vowels in a String.
Count digits in a String.
Find the first occurrence of a character.
Find the number of times a character appears.

151. Practice Set — Nested Loops#

Create:

Output

*
**
***
****
*****

Then:

Output

*****
****
***
**
*

Then:

Output

Then:

Output

Then:

Output

    *
   ***
  *****
 *******
*********

152. Practice Set — `break`#

Write programs that:

Print 1–100 but stop at 50.
Search for a number in an array and stop when found.
Ask for numbers until the user enters 0.
Print numbers until the first negative value.
Stop a menu when the user chooses Exit.

153. Practice Set — `continue`#

Write programs that:

Print 1–100 but skip even numbers.
Print 1–100 but skip multiples of 5.
Process only positive numbers.
Skip zero values.
Print only numbers divisible by 3.

154. Practice Set — Enhanced `for`#

Given:

Java

int[] numbers = {10, 20, 30, 40, 50};

Use an enhanced for loop to:

Print every element.
Calculate the sum.
Calculate the average.
Find the maximum.
Find the minimum.
Count even values.
Count values greater than 25.
Search for 40.

155. Mini Project — Number Analyzer#

Create a program that repeatedly asks for numbers until the user enters 0.

At the end display:

Output

Count
Sum
Average
Largest
Smallest
Positive count
Negative count
Even count
Odd count

This project combines nearly everything from Chapters 3–7.

156. Mini Project — Multiplication Table Generator#

Ask:

Output

Enter starting number:
Enter ending number:

Then generate tables for every number in that range.

For example:

Output

Start = 2
End = 4

Output:

Output

2 × 1 = 2
...
2 × 10 = 20

3 × 1 = 3
...
3 × 10 = 30

4 × 1 = 4
...
4 × 10 = 40

Use nested loops.

157. Mini Project — Simple Quiz#

Create a quiz with several questions.

For each question:

Output

display question
read answer
check answer
update score

At the end:

Output

Score = X / N

Use loops to avoid repeating the same overall structure manually.

158. Mini Project — ATM#

Improve the ATM program so that:

Output

1 → Balance
2 → Deposit
3 → Withdraw
4 → Exit

continues until the user selects:

Output

Use:

Output

do-while
switch
if-else

159. Mini Project — Guessing Game#

Create a number guessing game.

Rules:

Output

Secret number = 1–100

User guesses

If guess is too low:
    print "Too low"

If guess is too high:
    print "Too high"

If correct:
    print "Correct"
    stop

Bonus:

Output

count attempts

At the end:

Output

You guessed it in 7 attempts.

160. Interview Questions#

Q1. What is a loop?#

A loop repeatedly executes a block of code while a condition or iteration rule allows it to continue.

Q2. What types of loops does Java provide?#

The main looping constructs are:

Output

while
do-while
for
enhanced for

Q3. Difference between `while` and `do-while`?#

while checks the condition before executing the body.

do-while executes the body first and checks the condition afterward.

Therefore, a do-while executes at least once.

Q4. When should you use a `for` loop?#

A for loop is often appropriate when initialization, termination condition, and update naturally form a counter-controlled loop.

Q5. What is an enhanced `for` loop?#

It is a loop designed to iterate through elements of arrays and other iterable structures without manually managing an index.

Q6. What does `break` do?#

It exits the nearest enclosing loop or switch statement.

Q7. What does `continue` do?#

It skips the remainder of the current loop iteration and proceeds with the next iteration.

Q8. Difference between `break` and `continue`?#

Output

break    → stop loop
continue → skip current iteration

Q9. What is an infinite loop?#

A loop that does not terminate because its continuation condition never becomes false or it otherwise lacks a reachable exit.

Q10. Can an infinite loop be intentional?#

Yes. Event-processing systems, servers, and interactive programs can intentionally run continuously until an external or internal termination condition occurs.

Q11. What is a nested loop?#

A loop inside another loop.

Q12. How many times does a nested loop execute?#

If the outer loop runs n times and the inner loop runs m times for every outer iteration, the inner body executes:

Output

n × m

times.

Q13. What is an off-by-one error?#

An error where a loop executes one iteration too many or one too few, often caused by incorrect use of < and <=.

Q14. What happens if you forget to update a loop counter?#

If the loop condition depends on that counter and it never changes toward termination, the loop may become infinite.

Q15. Can a `for` loop have no condition?#

Yes.

Example:

Java

for (;;) {
}

This creates an infinite loop unless something exits it.

Q16. Can you use `break` in a `while` loop?#

Yes.

Q17. Can you use `continue` in a `for` loop?#

Yes.

Q18. Does `break` exit all nested loops?#

No. An ordinary break exits only the nearest enclosing loop.

A labeled break can target a labeled outer loop.

Q19. Does `continue` exit the loop?#

No. It skips the current iteration and continues with the next iteration.

Q20. What is a sentinel?#

A special input value used to indicate that processing should stop.

Example:

Output

-1

can be chosen as a sentinel in a program that accepts non-negative values.

161. Interview Output Questions#

Question 1#

What is printed?

Java

for (int i = 1; i <= 5; i++) {
    System.out.print(i + " ");
}

Answer:

Output

1 2 3 4 5

Question 2#

Java

for (int i = 0; i < 5; i++) {
    System.out.print(i + " ");
}

Answer:

Output

0 1 2 3 4

Question 3#

Java

int i = 5;

while (i > 0) {
    System.out.print(i + " ");
    i--;
}

Answer:

Output

5 4 3 2 1

Question 4#

Java

int i = 10;

do {
    System.out.println(i);
} while (i < 5);

Answer:

Output

The body executes once before the condition is tested.

Question 5#

Java

for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        break;
    }

    System.out.print(i + " ");
}

Answer:

Output

1 2

Question 6#

Java

for (int i = 1; i <= 5; i++) {
    if (i == 3) {
        continue;
    }

    System.out.print(i + " ");
}

Answer:

Output

1 2 4 5

Question 7#

Java

int sum = 0;

for (int i = 1; i <= 4; i++) {
    sum += i;
}

System.out.println(sum);

Answer:

Output

Question 8#

Java

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 2; j++) {
        System.out.print("*");
    }

    System.out.println();
}

Answer:

Output

**
**
**

162. Advanced Thinking: Loop Correctness#

When solving a loop problem, don't just ask:

Output

"What code should I write?"

Ask:

Output

"What should be true after each iteration?"

Example:

Java

int sum = 0;

for (int i = 1; i <= n; i++) {
    sum += i;
}

At the end of iteration i, the intended meaning is:

Output

sum = 1 + 2 + ... + i

This way of thinking becomes extremely useful when you later study algorithms, recursion, data structures, and problem solving.

163. Loop Problem-Solving Strategy#

When given a loop problem:

Step 1 — Understand what repeats#

Ask:

Output

What operation is repeated?

Step 2 — Identify the changing value#

Examples:

Output

counter
index
input
remaining digits

Step 3 — Define the stopping condition#

Ask:

Output

When should repetition stop?

Step 4 — Decide how the state changes#

Examples:

Java

i++
i--
number /= 10

Step 5 — Dry run#

Test with a small example.

This process is more valuable than memorizing loop syntax.

164. Loop Templates#

Template 1 — Count Up#

Java

for (int i = start; i <= end; i++) {
    // work
}

Template 2 — Count Down#

Java

for (int i = start; i >= end; i--) {
    // work
}

Template 3 — While#

Java

while (condition) {
    // work
    // update state
}

Template 4 — Do-While#

Java

do {
    // work
} while (condition);

Template 5 — Enhanced For#

Java

for (Type value : values) {
    // process value
}

Template 6 — Search#

Java

for (Type value : values) {
    if (value == target) {
        // found
        break;
    }
}

Template 7 — Accumulator#

Java

int result = 0;

for (...) {
    result += value;
}

165. Final Chapter Summary#

Loops allow Java programs to repeat work.

The main loop constructs are:

Java

while
do-while
for
enhanced for

A while loop:

Java

while (condition) {
    ...
}

checks its condition before every iteration.

A do-while loop:

Java

do {
    ...
} while (condition);

runs its body at least once.

A for loop:

Java

for (initialization; condition; update) {
    ...
}

is excellent for counter-controlled repetition.

An enhanced for loop:

Java

for (Type value : values) {
    ...
}

is convenient when processing every element of an array or iterable structure.

Loop control:

Output

break
→ exit the nearest loop

continue
→ skip the current iteration

return
→ exit the current method

Nested loops allow loops inside loops:

outer loop
    ↓
inner loop

They are useful for:

Output

patterns
matrices
tables
pair comparisons
multi-dimensional data

The most important loop-design questions are:

Output

Where do I start?
What condition means "continue"?
What work happens?
How does the state change?
When does the loop stop?

If you can answer these questions, loops become much easier.

166. What You Should Be Able to Do Now#

Before moving to Chapter 8, you should be comfortable writing:

Output

1. for loops
2. while loops
3. do-while loops
4. enhanced for loops
5. nested loops
6. break
7. continue
8. counters
9. accumulators
10. searches
11. validation loops
12. number problems
13. String loops
14. array loops
15. basic patterns

You should also understand the difference between:

Output

0 < 5
0 <= 5

and be able to explain why a loop stops.

167. Course Progress#

You have now completed:

Output

01  Java Introduction
02  Setup & First Program
03  Variables & Data Types
04  Operators
05  Input & Output
06  Conditions
07  Loops  ← YOU ARE HERE

The next chapter is:

Chapter 8 — Methods#

You will learn:

Output

What is a method?
Why methods?
Method syntax
Parameters
Arguments
Return values
void
return
Method overloading
Variable scope
Local variables
Static methods
Calling methods
Pass-by-value
Recursion
Method design
Practical programs
Interview questions

Methods are extremely important because they teach you how to break a large program into small, reusable pieces.

Later, this idea becomes the foundation for classes and objects:

Methods
   ↓
Classes
   ↓
Objects
   ↓
Encapsulation
   ↓
Inheritance
   ↓
Polymorphism
   ↓
Abstraction
   ↓
Complete OOP

1. What Is a Loop?#

2. Why Do We Need Loops?#

3. Real-Life Example#

4. Main Loop Types in Java#

5. while Loop#

6. How while Works#

7. Important Parts of a Loop#

8. Initialization#

9. Condition#

10. Update#

11. Infinite Loop#

12. Intentional Infinite Loops#

13. Counter-Controlled Loop#

14. Counting Backwards#

15. Different Step Sizes#

16. do-while Loop#

17. while vs do-while#

18. Important Difference#

19. When Is do-while Useful?#

20. Menu Example with do-while#

21. for Loop#

22. Anatomy of a for Loop#

23. Exact Execution Order of for#

24. for Loop vs while Loop#

25. Counting from 0#

26. Number of Iterations#

27. < vs <=#

28. Counting Down with for#

29. Counting by 2#

30. Reverse Counting by 2#

31. Multiple Variables in a for Loop#

32. Omitting Parts of for#

33. Infinite for Loop#

34. Scope of the for Variable#

35. Declaring the Counter Outside#

36. Nested Loops#

37. How Nested Loops Work#

38. Nested Loop — Multiplication Table#

39. Nested Loop — Rectangle Pattern#

40. Nested Loop — Triangle Pattern#

41. Nested Loop — Number Triangle#

42. Nested Loop — Same Number Pattern#

43. Loop Control — break#

44. Flow of break#

45. break in while#

46. break in Nested Loops#

47. Labeled break#

48. continue#

49. break vs continue#

50. continue in a while Loop#

51. Enhanced for Loop#

52. Why Use Enhanced for?#

53. Understanding the Enhanced for#

54. Enhanced for with Strings#

55. Enhanced for with char[]#

56. Enhanced for and Index#

57. Enhanced for and Modification#

58. Enhanced for with Object References#

59. Looping Through a String#

60. Counting Characters#

61. Sum of Numbers#

62. Understanding the Sum#

63. Accumulator Pattern#

64. Product of Numbers#

65. Factorial#

66. Factorial with User Input#

67. Multiplication Table#

68. Counting Digits#

69. How Digit Counting Works#

70. Special Case — Zero#

71. Reverse a Number#

72. Reverse Number — Step by Step#

73. Palindrome Number#

74. Sum of Digits#

75. Count Even Numbers#

76. Find Maximum#

77. Find Minimum#

78. Search for a Value#

79. Linear Search#

80. Loop with Input#

5. `while` Loop#

6. How `while` Works#

16. `do-while` Loop#

17. `while` vs `do-while`#

19. When Is `do-while` Useful?#

20. Menu Example with `do-while`#

21. `for` Loop#

22. Anatomy of a `for` Loop#

23. Exact Execution Order of `for`#

24. `for` Loop vs `while` Loop#

27. `<` vs `<=`#

28. Counting Down with `for`#

31. Multiple Variables in a `for` Loop#

32. Omitting Parts of `for`#

33. Infinite `for` Loop#

34. Scope of the `for` Variable#

43. Loop Control — `break`#

44. Flow of `break`#

45. `break` in `while`#

46. `break` in Nested Loops#

47. Labeled `break`#

48. `continue`#

49. `break` vs `continue`#

50. `continue` in a `while` Loop#

51. Enhanced `for` Loop#

52. Why Use Enhanced `for`?#

53. Understanding the Enhanced `for`#

54. Enhanced `for` with Strings#

55. Enhanced `for` with `char[]`#

56. Enhanced `for` and Index#

57. Enhanced `for` and Modification#

58. Enhanced `for` with Object References#

83. `while` for Unknown Number of Iterations#

84. `for` for Known Iterations#

85. `do-while` for At-Least-Once Logic#

99. `break` vs Boolean Flag#

100. `continue` and Readability#

108. Nested Enhanced `for`#

133. `break` and `continue` in Nested Loops#

134. `continue` in Nested Loops#

135. Labeled `continue`#

137. Loop and `return`#

138. `break` vs `return`#

`while`#

`do-while`#

`for`#