Nested loops in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested loops with break

What is the output of this C++ code snippet?

C++

#include <iostream>
int main() {
    for (int i = 1; i <= 3; ++i) {
        for (int j = 1; j <= 3; ++j) {
            if (j == 2) break;
            std::cout << i << j << " ";
        }
    }
    return 0;
}

A 12 22 32

B 11 12 13 21 22 23 31 32 33

C 11 21 31

D 11 12 21 22 31 32

💡 Hint

Look at where the break statement stops the inner loop.

The inner loop breaks when j == 2, so only j == 1 prints for each i. So output is 11 21 31.

❓ Predict Output

intermediate

Counting iterations in nested loops

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

C++

#include <iostream>
int main() {
    int count = 0;
    for (int i = 0; i < 4; ++i) {
        for (int j = i; j < 4; ++j) {
            ++count;
        }
    }
    std::cout << count;
    return 0;
}

A 10

B 16

C 6

D 12

💡 Hint

Count how many times j runs for each i.

For i=0, j=0..3 (4 times); i=1, j=1..3 (3 times); i=2, j=2..3 (2 times); i=3, j=3..3 (1 time). Total 4+3+2+1=10.

🔧 Debug

advanced

Identify the error in nested loops

What error will this code produce when compiled?

C++

#include <iostream>
int main() {
    for (int i = 0; i < 3; i++)
        for (int j = 0; j < 3; j++)
            std::cout << i << j << std::endl;
    std::cout << "Done" << std::endl;
    return 0;
}

A Compilation error: undeclared variable

B Compilation error: missing braces

C Runtime error: segmentation fault

D Runs and prints pairs then 'Done'

💡 Hint

Check how indentation affects loops in C++.

The code runs fine. The inner loop prints pairs, then the last cout prints 'Done'. No braces needed for single statements.

❓ Predict Output

advanced

Output of nested loops with continue

What is the output of this code?

C++

#include <iostream>
int main() {
    for (int i = 1; i <= 3; ++i) {
        for (int j = 1; j <= 3; ++j) {
            if (i == j) continue;
            std::cout << i << j << " ";
        }
    }
    return 0;
}

A 11 22 33

B 12 13 21 23 31 32

C 12 13 23 31 32

D 21 23 31 32

💡 Hint

The continue skips printing when i equals j.

Pairs where i == j (11, 22, 33) are skipped. All others print in order.

🧠 Conceptual

expert

Number of iterations in nested loops with variable limits

Consider this nested loop code. How many times does the innermost statement execute?

C++

#include <iostream>
int main() {
    int count = 0;
    for (int i = 1; i <= 5; ++i) {
        for (int j = 1; j <= i; ++j) {
            for (int k = 1; k <= j; ++k) {
                ++count;
            }
        }
    }
    std::cout << count;
    return 0;
}

A 35

B 55

C 50

D 45

💡 Hint

Sum the counts: for each i, sum j=1 to i, then sum k=1 to j.

The innermost loop runs sum_{i=1}^5 sum_{j=1}^i j = sum_{i=1}^5 (i(i+1)/2) = 35.