Loops in Java (for, while, do-while)

What Is It?

What Are Loops?

A loop is a control flow structure that repeats a block of code multiple times. Instead of writing the same code 100 times, you write it once inside a loop and let the loop execute it repeatedly until a specified condition becomes false.

Consider printing numbers 1 to 1000. Without loops, you would need 1000 print statements. With a loop, you need 3 lines of code. Loops are one of the three fundamental building blocks of all programs (alongside sequence and conditional statements).

Java provides four types of loops:

1. for loop -- when you know the number of iterations in advance
2. while loop -- when you want to repeat while a condition is true (condition checked before each iteration)
3. do-while loop -- like while, but the body executes at least once (condition checked after each iteration)
4. enhanced for-each loop -- for iterating over arrays and collections without an index

Why Does It Matter?

Why Are Loops Essential?

1. Eliminating Repetition

Any task that involves repetition requires loops. Printing a multiplication table, processing each element of an array, reading lines from a file, handling multiple user requests in a server -- all are impossible (or impractical) without loops. They are the foundation of automation in programming.

2. Processing Collections of Data

Real programs work with collections: arrays, lists, database rows, API responses. Loops let you iterate over each element, apply transformations, filter results, compute aggregates (sum, average, min, max), and search for specific values. Without loops, you cannot work with data of variable size.

3. Algorithm Implementation

Almost every algorithm uses loops. Searching (linear search, binary search), sorting (bubble sort, selection sort), matrix operations, string processing, mathematical computations (factorial, Fibonacci, prime checking) -- all require loops. For placement preparation, mastering loops is non-negotiable.

4. User Interaction

Interactive programs often use loops to keep running until the user decides to exit. A menu-driven program shows options, processes the user's choice, and repeats. Without loops, the program would end after handling one request.

Detailed Explanation

Detailed Explanation

1. The for Loop

The for loop is the most commonly used loop when you know (or can calculate) the number of iterations beforehand. It has three parts in the header: initialization, condition, and update.

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

Execution flow:

1. Initialization executes once, at the very beginning.
2. Condition is checked before each iteration. If true, the body executes. If false, the loop ends.
3. Update executes after each iteration of the body.

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

This prints "Iteration: 1" through "Iteration: 5". The variable i is declared inside the loop and is not accessible outside it (block scope).

Variations of for Loop

• Counting down: for (int i = 10; i >= 1; i--)
• Step by 2: for (int i = 0; i <= 20; i += 2)
• Multiple variables: for (int i = 0, j = 10; i < j; i++, j--)
• Empty parts: Any of the three parts can be omitted: for (;;) is an infinite loop.

2. The while Loop

The while loop repeats as long as its condition is true. The condition is checked before each iteration, so if the condition is false from the start, the body never executes.

while (condition) {
    // body
}

Use while when you do not know the number of iterations in advance, such as reading input until the user types "quit" or searching for a value in a data stream.

int count = 1;
while (count <= 5) {
    System.out.println("Count: " + count);
    count++;
}

If you forget count++, the condition count <= 5 is always true, causing an infinite loop. The program hangs and must be terminated manually.

3. The do-while Loop

The do-while loop is similar to while, but the condition is checked after the body executes. This guarantees that the body runs at least once, even if the condition is false from the start.

do {
    // body
} while (condition);

Note the semicolon after while(condition); -- it is required.

int num = 10;
do {
    System.out.println("num = " + num);
    num++;
} while (num <= 5);

Even though 10 <= 5 is false, the body executes once and prints "num = 10". The condition is checked after the first execution.

When to Use do-while

Use do-while when the body must execute at least once. Common use cases: menu-driven programs (show the menu at least once), input validation (ask for input at least once), and game loops (play at least one round).

4. The Enhanced for-each Loop

The for-each loop (introduced in Java 5) provides a cleaner way to iterate over arrays and collections without managing an index variable.

for (type variable : arrayOrCollection) {
    // body
}

int[] nums = {10, 20, 30, 40, 50};
for (int n : nums) {
    System.out.println(n);
}

The variable n takes each element's value in order. You cannot modify the array through n (it is a copy, not a reference to the element). You also cannot access the index directly. For index-based operations, use a regular for loop.

5. Infinite Loops

An infinite loop runs forever (or until externally stopped). These are sometimes intentional (server event loops) but usually bugs (forgot to update the loop variable).

// Intentional infinite loops:
for (;;) { /* ... break when done ... */ }
while (true) { /* ... break when done ... */ }

Use break inside an infinite loop to exit when a condition is met.

6. Nested Loops

A loop inside another loop is called a nested loop. The inner loop runs completely for each iteration of the outer loop. If the outer loop runs N times and the inner loop runs M times, the innermost body executes N * M times.

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

This produces a 3x3 grid. Nested loops are used for matrix operations, pattern printing, and generating combinations.

7. Common Loop Patterns

Sum / Accumulator Pattern

int sum = 0;
for (int i = 1; i <= 100; i++) {
    sum += i;
}
System.out.println("Sum: " + sum); // 5050

Counting Pattern

int count = 0;
for (int i = 1; i <= 100; i++) {
    if (i % 2 == 0) count++;
}
System.out.println("Even numbers: " + count); // 50

Search Pattern

int[] arr = {5, 12, 8, 3, 17};
int target = 8;
boolean found = false;
for (int i = 0; i < arr.length; i++) {
    if (arr[i] == target) {
        System.out.println("Found at index " + i);
        found = true;
        break;
    }
}
if (!found) System.out.println("Not found");

Min/Max Pattern

int[] arr = {5, 12, 8, 3, 17};
int min = arr[0], max = arr[0];
for (int i = 1; i < arr.length; i++) {
    if (arr[i] < min) min = arr[i];
    if (arr[i] > max) max = arr[i];
}
System.out.println("Min: " + min + ", Max: " + max);

Code Examples

import java.util.Scanner;

public class MultiTable {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter a number: ");
        int num = sc.nextInt();

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

        sc.close();
    }
}

import java.util.Scanner;

public class SumUntilZero {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int sum = 0;
        int count = 0;

        System.out.println("Enter numbers (0 to stop):");
        int num = sc.nextInt();

        while (num != 0) {
            sum += num;
            count++;
            num = sc.nextInt();
        }

        System.out.println("Sum: " + sum);
        System.out.println("Count: " + count);
        if (count > 0) {
            System.out.println("Average: " + (double) sum / count);
        }

        sc.close();
    }
}

import java.util.Scanner;

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

        do {
            System.out.println("\n--- Menu ---");
            System.out.println("1. Say Hello");
            System.out.println("2. Say Goodbye");
            System.out.println("3. Exit");
            System.out.print("Choice: ");
            choice = sc.nextInt();

            switch (choice) {
                case 1: System.out.println("Hello!"); break;
                case 2: System.out.println("Goodbye!"); break;
                case 3: System.out.println("Exiting..."); break;
                default: System.out.println("Invalid choice.");
            }
        } while (choice != 3);

        sc.close();
    }
}

public class ForEachDemo {
    public static void main(String[] args) {
        String[] names = {"Aarav", "Neha", "Rohan", "Priya", "Karan"};

        System.out.println("Students:");
        for (String name : names) {
            System.out.println("  - " + name);
        }

        // Sum using for-each
        int[] marks = {85, 92, 78, 95, 88};
        int total = 0;
        for (int m : marks) {
            total += m;
        }
        System.out.println("Total marks: " + total);
        System.out.println("Average: " + (double) total / marks.length);
    }
}

public class MultiGrid {
    public static void main(String[] args) {
        System.out.println("Multiplication Table (1-5):");
        System.out.print("\t");
        for (int i = 1; i <= 5; i++) {
            System.out.print(i + "\t");
        }
        System.out.println();
        System.out.println("-------------------------------");

        for (int i = 1; i <= 5; i++) {
            System.out.print(i + " |\t");
            for (int j = 1; j <= 5; j++) {
                System.out.print((i * j) + "\t");
            }
            System.out.println();
        }
    }
}

public class WhileVsDoWhile {
    public static void main(String[] args) {
        int x = 10;

        // while: condition checked FIRST
        System.out.println("while loop:");
        while (x < 5) {
            System.out.println("x = " + x);
            x++;
        }
        System.out.println("(nothing printed, condition was false)\n");

        // do-while: body runs FIRST
        x = 10;
        System.out.println("do-while loop:");
        do {
            System.out.println("x = " + x);
            x++;
        } while (x < 5);
        System.out.println("(printed once, then condition was false)");
    }
}

public class ArrayLoops {
    public static void main(String[] args) {
        int[] scores = {78, 92, 65, 88, 45, 92, 73, 88, 92};

        // Count occurrences of 92
        int count = 0;
        for (int score : scores) {
            if (score == 92) {
                count++;
            }
        }
        System.out.println("92 appears " + count + " times.");

        // Find index of first score below 50
        int lowIndex = -1;
        for (int i = 0; i < scores.length; i++) {
            if (scores[i] < 50) {
                lowIndex = i;
                break;
            }
        }
        if (lowIndex != -1) {
            System.out.println("First low score at index: " + lowIndex);
        }

        // Sum, min, max
        int sum = 0, min = scores[0], max = scores[0];
        for (int score : scores) {
            sum += score;
            if (score < min) min = score;
            if (score > max) max = score;
        }
        System.out.println("Sum: " + sum);
        System.out.println("Min: " + min + ", Max: " + max);
        System.out.printf("Average: %.1f%n", (double) sum / scores.length);
    }
}

public class MultiVarLoop {
    public static void main(String[] args) {
        // Two variables moving toward each other
        for (int left = 0, right = 10; left < right; left++, right--) {
            System.out.println("left=" + left + ", right=" + right);
        }

        System.out.println();

        // Reverse an array in-place
        int[] arr = {1, 2, 3, 4, 5};
        for (int i = 0, j = arr.length - 1; i < j; i++, j--) {
            int temp = arr[i];
            arr[i] = arr[j];
            arr[j] = temp;
        }

        System.out.print("Reversed: ");
        for (int n : arr) {
            System.out.print(n + " ");
        }
        System.out.println();
    }
}

Common Mistakes

// Intended: print 1 to 5
for (int i = 1; i < 5; i++) {
    System.out.println(i);
}
// Only prints 1, 2, 3, 4 (misses 5)

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

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

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

Scanner sc = new Scanner(System.in);
String input = "";
while (input != "quit") {  // Bug: == compares references
    input = sc.nextLine();
    System.out.println("You typed: " + input);
}

Scanner sc = new Scanner(System.in);
String input = "";
while (!input.equals("quit")) {
    input = sc.nextLine();
    System.out.println("You typed: " + input);
}

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

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

int[] arr = {1, 2, 3, 4, 5};
for (int n : arr) {
    n = n * 2;  // Does NOT modify the array!
}
// arr is still {1, 2, 3, 4, 5}

int[] arr = {1, 2, 3, 4, 5};
for (int i = 0; i < arr.length; i++) {
    arr[i] = arr[i] * 2;  // Modifies the array
}
// arr is now {2, 4, 6, 8, 10}

Summary