Nested for loop in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested for loop with multiplication table

What is the output of the following Java code?

Java

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 2; i++) {
            for (int j = 1; j <= 3; j++) {
                System.out.print(i * j + " ");
            }
            System.out.println();
        }
    }
}

A

1 2 3 
2 4 6 

B

1 2 3 
1 2 3 

C

1 1 1 
2 2 2 

D

1 3 6 
2 5 9 

💡 Hint

Think about how the outer and inner loops multiply their counters.

The outer loop runs for i=1 and i=2. The inner loop runs j=1 to 3. Each print outputs i*j followed by a space. Then a newline after inner loop ends.

❓ Predict Output

intermediate

Number of iterations in nested loops

How many times will the innermost statement execute in this code?

Java

for (int i = 0; i < 4; i++) {
    for (int j = 0; j < 5; j++) {
        System.out.println(i + "," + j);
    }
}

A 10

B 25

C 20

D 9

💡 Hint

Multiply the number of iterations of outer and inner loops.

Outer loop runs 4 times (0 to 3), inner loop runs 5 times (0 to 4). Total iterations = 4*5 = 20.

🔧 Debug

advanced

Identify the error in nested for loop

What error will this code produce when compiled?

Java

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

A SyntaxError: missing semicolon

B SyntaxError: unexpected '}'

C No error, runs fine

D RuntimeException: NullPointerException

💡 Hint

Check the braces and semicolons carefully.

The closing brace '}' after the print statement is unmatched because the opening brace for the outer loop is missing.

❓ Predict Output

advanced

Output of nested loops with break statement

What is the output of this Java code?

Java

public class Main {
    public static void main(String[] args) {
        for (int i = 1; i <= 3; i++) {
            for (int j = 1; j <= 3; j++) {
                if (j == 2) break;
                System.out.print(i * j + " ");
            }
            System.out.println();
        }
    }
}

A

1 
2 
3 

B

1 2 
2 4 
3 6 

C

1 2 3 
1 2 3 
1 2 3 

D

1 
2 
3 
4 

💡 Hint

The break stops the inner loop when j equals 2.

When j==2, the inner loop breaks, so only j=1 runs each time. The output is i*1 for i=1 to 3.

🧠 Conceptual

expert

Effect of modifying loop variable inside nested loop

Consider this code snippet. What will be the value of i after the loops finish?

Java

int i = 0;
for (; i < 3; i++) {
    for (int j = 0; j < 2; j++) {
        if (j == 1) {
            i++;
        }
    }
}
System.out.println(i);

A 5

B 3

C 6

D 4

💡 Hint

Remember i is incremented both by the for loop and inside the inner loop.

i starts at 0. Outer loop runs while i<3. Each outer iteration increments i once normally, and once more inside inner loop when j==1. So i increments twice per outer iteration. After 2 iterations i=4 and loop stops.