Nested for loop in Java

Introduction

A nested for loop lets you repeat actions inside another repeated action. It helps when you want to work with rows and columns or pairs of items.

When printing a table of numbers with rows and columns.

When checking every pair of items in a list.

When drawing patterns like squares or triangles with characters.

When working with grids, like a chessboard or pixel art.

Syntax

Java

for (initialization; condition; update) {
    for (initialization; condition; update) {
        // inner loop code
    }
    // outer loop code
}

The outer loop runs first, then the inner loop runs completely each time the outer loop runs once.

Use different variable names for each loop to avoid confusion.

Examples

This prints pairs of i and j values. The inner loop runs fully for each i.

Java

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

This prints a 4x4 square of stars.

Java

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

Sample Program

This program prints a 3x3 multiplication table. Each row shows multiples of the row number.

Java

public class NestedLoopExample {
    public static void main(String[] args) {
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                System.out.print(i * j + " ");
            }
            System.out.println();
        }
    }
}

Important Notes

Remember the inner loop finishes all its cycles before the outer loop moves to the next step.

Nested loops can slow down your program if they run many times, so use them wisely.

Summary

Nested for loops run one loop inside another to handle multi-level repetition.

They are useful for working with tables, grids, and pairs of data.

Always keep track of loop variables and their ranges to avoid mistakes.

Practice

1. What does a nested for loop in Java do?
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 2; j++) {
// code
}
}

easy

A. Runs one loop inside another to repeat code multiple times

B. Runs two loops one after another sequentially

C. Runs only the inner loop multiple times

D. Runs only the outer loop once

Solution

1. Step 1: Analyze nested loop iterations

   The outer loop runs 3 times, and for each outer loop iteration, the inner loop runs 2 times.

2. Step 2: Explain the repetition behavior

   This means the inner loop runs fully inside each outer loop iteration, repeating code multiple times in total.

3. Final Answer:

   Runs one loop inside another to repeat code multiple times -> Option A

4. Quick Check:

   Nested loops = inner loop inside outer loop [OK]

Hint: Nested loops repeat inner loop fully for each outer loop [OK]

Common Mistakes:

• Thinking loops run sequentially, not nested
• Confusing inner and outer loop counts
• Assuming only one loop runs multiple times

2. Which of the following is the correct syntax for a nested for loop in Java?

easy

A. for (int i = 0; i < 3; i++) { for (int j = 0; j < 2; j++) System.out.println(i + j); }

B. for int i = 0; i < 3; i++ { for int j = 0; j < 2; j++ { System.out.println(i + j); } }

C. for (int i = 0; i < 3; i++) for (int j = 0; j < 2; j++ { System.out.println(i + j); }

D. for (int i = 0; i < 3; i++) { for (int j = 0; j < 2; j++) System.out.println(i + j); } else {}

Solution

1. Step 1: Verify Java for loop syntax requirements

   Java for loops require parentheses around initialization, condition, and increment, and braces for blocks.

2. Step 2: Identify correct nested loop syntax

   for (int i = 0; i < 3; i++) { for (int j = 0; j < 2; j++) System.out.println(i + j); } correctly uses braces for outer loop without extra invalid keywords or else blocks.

3. Final Answer:

   for (int i = 0; i < 3; i++) { for (int j = 0; j < 2; j++) System.out.println(i + j); } -> Option A

4. Quick Check:

   Correct Java syntax = for (int i = 0; i < 3; i++) { for (int j = 0; j < 2; j++) System.out.println(i + j); } [OK]

Hint: Remember Java for loop needs parentheses and braces [OK]

Common Mistakes:

• Missing parentheses around for loop conditions
• Using else after for loop incorrectly
• Omitting braces for multiple statements

3. What is the output of this Java code?

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

medium

A. 2 4 6
4 8 12

B. 1 2 3 4 5 6

C. 1 2 3
1 2 3

D. 1 2 3
2 4 6

Solution

1. Step 1: Trace outer loop iterations

   Outer loop runs i=1 and i=2.

2. Step 2: Calculate inner loop outputs for each i

   For i=1: j=1,2,3 outputs 1*1=1, 1*2=2, 1*3=3.
   For i=2: j=1,2,3 outputs 2*1=2, 2*2=4, 2*3=6.

3. Final Answer:

   1 2 3 2 4 6 -> Option D

4. Quick Check:

   Multiply i and j in nested loops = 1 2 3 2 4 6 [OK]

Hint: Multiply outer and inner loop variables for output [OK]

Common Mistakes:

• Ignoring new line after inner loop
• Concatenating all numbers in one line
• Mixing up i and j values

4. Find the error in this nested for loop code:

for (int i = 0; i < 3; i++) {
  for (int j = 0; j < 3; j++)
    System.out.println(i + j);
  }
}

medium

A. Missing semicolon after inner loop

B. Inner loop condition should be j <= 3

C. Extra closing brace '}' after inner loop

D. Outer loop variable i is not initialized

Solution

1. Step 1: Check braces matching

   The inner loop lacks braces but has an extra closing brace '}' after it, causing syntax error.

2. Step 2: Identify correct brace placement

   Remove the extra '}' after inner loop or add braces properly to fix syntax.

3. Final Answer:

   Extra closing brace '}' after inner loop -> Option C

4. Quick Check:

   Brace mismatch error = Extra closing brace '}' after inner loop [OK]

Hint: Match opening and closing braces carefully [OK]

Common Mistakes:

• Adding extra braces after loops
• Omitting braces for multiple statements
• Incorrect loop conditions

5. How can you use nested for loops to print a 4x4 multiplication table in Java?

hard

A. Use a single loop from 1 to 16 and print i * i

B. Use outer loop for rows 1 to 4 and inner loop for columns 1 to 4, print product

C. Use nested loops but print only the outer loop variable

D. Use nested loops but skip printing inside inner loop

Solution

1. Step 1: Set outer loop for rows 1 to 4

   The outer loop controls the row number from 1 to 4.

2. Step 2: Set inner loop for columns 1 to 4 and print product

   The inner loop runs columns 1 to 4, printing the product of row and column numbers to form the table.

3. Final Answer:

   Use outer loop for rows 1 to 4 and inner loop for columns 1 to 4, print product -> Option B

4. Quick Check:

   Nested loops for rows and columns = Use outer loop for rows 1 to 4 and inner loop for columns 1 to 4, print product [OK]

Hint: Outer loop rows, inner loop columns, print product [OK]

Common Mistakes:

• Using single loop for 2D table
• Not printing product of both loop variables
• Skipping inner loop printing