Nested for loop in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nested for loop in Java?

A nested for loop is a for loop inside another for loop. It lets you repeat actions in a grid or table-like way, like rows and columns.

beginner

How many times does the inner loop run if the outer loop runs 3 times and the inner loop runs 4 times each time?

The inner loop runs 4 times for each of the 3 outer loop runs, so 3 × 4 = 12 times in total.

beginner

What is the output of this code?

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

The output is: 1-1 1-2 1-3 2-1 2-2 2-3 This shows the outer loop number paired with each inner loop number.

beginner

Why do we use nested for loops?

We use nested for loops to handle tasks that need multiple levels of repetition, like printing tables, grids, or working with multi-dimensional arrays.

intermediate

What happens if you swap the inner and outer loops in a nested for loop?

Swapping loops changes the order of repetition. The inner loop becomes the outer loop and vice versa, which can change the output pattern or how data is processed.

How many times will the inner loop run in this code?

for (int i = 0; i < 5; i++) {
  for (int j = 0; j < 2; j++) {
    // inner loop body
  }
}

A 5 times

B 2 times

C 10 times

D 7 times

The inner loop runs 2 times for each of the 5 outer loop runs, so 5 × 2 = 10 times.

What is the purpose of the inner loop in a nested for loop?

A To run once

B To stop the outer loop

C To declare variables

D To repeat actions multiple times for each outer loop iteration

The inner loop repeats its actions multiple times for each time the outer loop runs.

Which of these is a correct nested for loop structure in Java?

A for (int i=0; i<3; i++) { for (int j=0; j<2; j++) { /* code */ } }

B for (int i=0; i<3; i++) { int j=0; while(j<2) { /* code */ } }

C while (i<3) { for (int j=0; j<2; j++) { /* code */ } }

D for (int i=0; i<3; i++) { if (j<2) { /* code */ } }

Option A shows a proper nested for loop with one for loop inside another.

What will this code print?

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

A 1 2 2 4

B 1 1 2 2

C 2 4 1 2

D 1 2 3 4

The output is 1*1=1, 1*2=2, 2*1=2, 2*2=4 printed in sequence.

If the outer loop runs 4 times and the inner loop runs 3 times, how many total iterations happen?

A 7

B 12

C 1

D 3

Total iterations = outer loop count × inner loop count = 4 × 3 = 12.

Explain what a nested for loop is and give a simple example in Java.

Think about loops inside loops and how they repeat actions.

Describe a real-life situation where you might use a nested for loop.

Imagine rows and columns in a spreadsheet or seats in a theater.

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