Nested for loop in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print numbers from 1 to 3 using a for loop.

Java

for (int i = 1; i [1] 4; i++) {
    System.out.println(i);
}

Drag options to blanks, or click blank then click option'

A>

B>=

C<

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using > or >= will not run the loop as expected.

Using <= 3 is correct but not the option here.

The loop runs while i is less than 4, so it prints 1, 2, and 3.

2fill in blank

medium

Complete the code to print a 3x3 grid of stars using nested for loops.

Java

for (int i = 1; i <= 3; i++) {
    for (int j = 1; j [1] 4; j++) {
        System.out.print("*");
    }
    System.out.println();
}

Drag options to blanks, or click blank then click option'

A<

B<=

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= 3 would also work but is not the correct answer here.

Using > or >= will cause the loop not to run properly.

The inner loop runs while j is less than 4, printing 3 stars each row.

3fill in blank

hard

Fix the error in the nested loop to print numbers 1 to 3 in each row.

Java

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

Drag options to blanks, or click blank then click option'

A<=

B<

C>=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using < 3 will stop before printing 3.

Using > or >= will cause the loop not to run correctly.

The inner loop should run while j is less than or equal to 3 to print numbers 1, 2, and 3.

4fill in blank

hard

Fill both blanks to create a nested loop that prints a 4x4 grid of numbers.

Java

for (int i = 1; i [1] 5; i++) {
    for (int j = 1; j [2] 5; j++) {
        System.out.print(j + " ");
    }
    System.out.println();
}

Drag options to blanks, or click blank then click option'

A<

B<=

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= 5 would run 5 times, printing numbers up to 5.

Using > or >= will cause the loops not to run correctly.

Both loops run while the variable is less than 5, printing numbers 1 to 4 in each row and column.

5fill in blank

hard

Fill all three blanks to create a nested loop that prints a triangle of stars.

Java

for (int i = 1; i [1] 5; i++) {
    for (int j = 1; j [2] i; j[3]) {
        System.out.print("*");
    }
    System.out.println();
}

Drag options to blanks, or click blank then click option'

A<=

B<

C++

D--

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of <= will print one less star per row.

Using -- will cause an infinite loop.

The outer loop runs from 1 to 5 inclusive. The inner loop runs from 1 up to i inclusive, printing stars. The j variable increments by 1 each time.

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