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.

Practice

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

easy

A. Two loops running one after another

B. A loop inside another loop

C. A loop that runs only once

D. A loop that never ends

Solution

1. Step 1: Understand the code structure

   The code shows one for loop inside another for loop, which means nested loops.

2. Step 2: Define nested loops

   Nested loops mean one loop runs inside another loop's block of code.

3. Final Answer:

   A loop inside another loop -> Option B

4. Quick Check:

   Nested loop = loop inside loop [OK]

Hint: Look for one loop inside another's braces [OK]

Common Mistakes:

• Thinking loops run one after another, not inside
• Confusing nested loops with single loops
• Assuming nested loops run only once

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

easy

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

B. for i in range(3): for j in range(2): /* code */

C. while(i<3) { while(j<2) /* code */ }

D. loop(i=0; i<3; i++) { loop(j=0; j<2; j++) { /* code */ } }

Solution

1. Step 1: Identify C++ loop syntax

   C++ uses for(initialization; condition; increment) syntax for loops.

2. Step 2: Check each option

   for i in range(3): for j in range(2): /* code */ uses Python syntax, invalid in C++.
   for(int i=0; i<3; i++) { for(int j=0; j<2; j++) { /* code */ } } uses correct C++ syntax with nested for loops.
   while(i<3) { while(j<2) /* code */ } uses while loops missing initializations and increments.
   loop(i=0; i<3; i++) { loop(j=0; j<2; j++) { /* code */ } } uses invalid loop keyword.

3. Final Answer:

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

4. Quick Check:

   C++ nested loops use for(init; cond; inc) [OK]

Hint: Remember C++ loops use for(init; cond; inc) syntax [OK]

Common Mistakes:

• Using Python or other language syntax in C++
• Missing semicolons in for loop header
• Using wrong keywords like 'loop'

3. What is the output of this C++ code?

for(int i = 1; i <= 2; i++) {
  for(int j = 1; j <= 3; j++) {
    std::cout << i*j << " ";
  }
  std::cout << std::endl;
}

medium

A. 3 6 9 \n 6 12 18

B. 1 2 3 4 5 6

C. 1 2 3 \n 2 4 6

D. 1 1 1 \n 2 2 2

Solution

1. Step 1: Trace outer loop values

   Outer loop runs i=1 and i=2.

2. Step 2: Calculate inner loop outputs

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

3. Final Answer:

   1 2 3 2 4 6 -> Option C

4. Quick Check:

   Multiply i and j in nested loops [OK]

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

Common Mistakes:

• Ignoring the newline after inner loop
• Adding all numbers instead of printing products
• Confusing loop ranges

4. Find the error in this nested loop code:

for(int i = 0; i < 3; i++) {
  for(int j = 0; j < 3; i++) {
    std::cout << i+j << " ";
  }
  std::cout << std::endl;
}

medium

A. Inner loop increments i instead of j

B. Outer loop condition is wrong

C. Missing semicolon after std::cout

D. No error, code runs fine

Solution

1. Step 1: Check inner loop increment

   Inner loop header uses i++ instead of j++, causing infinite loop or wrong behavior.

2. Step 2: Identify correct increment variable

   Inner loop should increment j to progress through its range.

3. Final Answer:

   Inner loop increments i instead of j -> Option A

4. Quick Check:

   Inner loop must increment its own variable [OK]

Hint: Check loop increments match loop variables [OK]

Common Mistakes:

• Incrementing wrong variable in inner loop
• Assuming code runs without checking increments
• Confusing outer and inner loop variables

5. You want to print a 4x4 multiplication table using nested loops in C++. Which code snippet correctly achieves this?

hard

A. for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i-j << " "; } std::cout << std::endl; }

B. for(int i=0; i<4; i++) { for(int j=0; j<4; j++) { std::cout << i+j << " "; } std::cout << std::endl; }

C. for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i+j << " "; } std::cout << std::endl; }

D. for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i*j << " "; } std::cout << std::endl; }

Solution

1. Step 1: Understand multiplication table logic

   Each cell should print the product of row and column numbers (i*j).

2. Step 2: Check each option's output logic

   for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i*j << " "; } std::cout << std::endl; } prints i*j correctly.
   for(int i=0; i<4; i++) { for(int j=0; j<4; j++) { std::cout << i+j << " "; } std::cout << std::endl; } prints sum with zero-based indices.
   for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i+j << " "; } std::cout << std::endl; } prints sum with one-based indices.
   for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i-j << " "; } std::cout << std::endl; } prints difference.

3. Final Answer:

   for(int i=1; i<=4; i++) { for(int j=1; j<=4; j++) { std::cout << i*j << " "; } std::cout << std::endl; } -> Option D

4. Quick Check:

   Multiplication table = print i*j in nested loops [OK]

Hint: Multiply loop counters for multiplication table [OK]

Common Mistakes:

• Using addition or subtraction instead of multiplication
• Starting loops at 0 instead of 1 for table
• Not printing newline after each row