Loop execution flow 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 11 12 13 21 22 23 31 32 33

B 11 12 21 22 31 32

C 11 21 31

D 11 21 22 31 32

💡 Hint

The inner loop breaks when j equals 2, so only j=1 prints per outer loop iteration.

The inner loop prints when j=1, then breaks at j=2, so for each i (1 to 3), only i1 is printed.

❓ Predict Output

intermediate

While loop with continue behavior

What is the output of this C++ code?

C++

#include <iostream>
int main() {
    int i = 0;
    while (i < 5) {
        i++;
        if (i == 3) continue;
        std::cout << i << " ";
    }
    return 0;
}

A 1 2 4 5

B 1 2 3 4 5

C 1 2 3 4

D 2 3 4 5

💡 Hint

When i equals 3, the continue skips the print for that iteration.

The loop increments i first, then skips printing when i==3, so output excludes 3.

❓ Predict Output

advanced

For loop with multiple increments

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

C++

#include <iostream>
int main() {
    for (int i = 0; i < 5; i += 2) {
        std::cout << i << " ";
        i++;
    }
    return 0;
}

A 0 2 3 4

B 0 2 4

C 0 1 3

D 0 3

💡 Hint

The loop increments i twice each iteration: once in the body and once in the for statement.

i starts at 0, prints 0, increments to 1 inside loop, then increments by 2 to 3 in for, prints 3, increments to 4 inside loop, then increments by 2 to 6 in for, loop ends.

❓ Predict Output

advanced

Do-while loop execution count

How many times does the loop body execute in this C++ code?

C++

#include <iostream>
int main() {
    int count = 0;
    int i = 5;
    do {
        count++;
        i++;
    } while (i < 5);
    std::cout << count;
    return 0;
}

A 0

B 1

C 5

D Infinite loop

💡 Hint

A do-while loop runs the body at least once before checking the condition.

The loop runs once, increments i from 5 to 6, then checks condition i<5 which is false, so it stops after one iteration.

❓ Predict Output

expert

Complex nested loop with continue and break

What is the output of this C++ program?

C++

#include <iostream>
int main() {
    for (int i = 1; i <= 3; ++i) {
        for (int j = 1; j <= 3; ++j) {
            if (i == j) continue;
            if (i + j == 4) break;
            std::cout << i << j << " ";
        }
    }
    return 0;
}

A 12 13 21 31

B 12 21 31

C 12 13 21 23 31

D 11 22 33

💡 Hint

Pay attention to when continue skips printing and when break stops the inner loop.

For i=1: j=1 skipped (continue), j=2 prints 12, j=3 prints 13; for i=2: j=1 prints 21, j=2 skipped (continue), j=3 breaks (2+3=5, no break, but condition is i+j==4, so break at j=2 since 2+2=4), so j=3 not reached; for i=3: j=1 prints 31, j=2 breaks (3+2=5 no), j=3 skipped (continue). So output is 12 13 21 31.

Practice

1. What is the main purpose of a loop in C++ programming?

easy

A. To repeat a block of code multiple times

B. To store multiple values in a variable

C. To create a new data type

D. To stop the program execution immediately

Solution

1. Step 1: Understand the role of loops

   Loops are used to repeat a set of instructions multiple times without writing the code again and again.

2. Step 2: Identify the correct purpose

   Among the options, only repeating code matches the loop's purpose.

3. Final Answer:

   To repeat a block of code multiple times -> Option A

4. Quick Check:

   Loop purpose = repeat code [OK]

Hint: Loops repeat code blocks to save effort [OK]

Common Mistakes:

• Confusing loops with variables
• Thinking loops create new data types
• Believing loops stop program immediately

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

easy

A. for (int i = 0; i < 5; i++) /* code */

B. for int i = 0; i < 5; i++ { /* code */ }

C. for (int i = 0; i < 5; i++) { /* code */ }

D. for (int i = 0; i < 5; i++) [] { /* code */ }

Solution

1. Step 1: Recall the correct for loop syntax

   The for loop syntax requires parentheses around initialization, condition, and increment, followed by a block in braces.

2. Step 2: Check each option

   for (int i = 0; i < 5; i++) { /* code */ } matches the correct syntax exactly. Others miss parentheses, braces, or use wrong symbols.

3. Final Answer:

   for (int i = 0; i < 5; i++) { /* code */ } -> Option C

4. Quick Check:

   Correct for loop syntax = for (int i = 0; i < 5; i++) { /* code */ } [OK]

Hint: For loops need parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses around conditions
• Missing braces for loop body
• Using wrong symbols like [] instead of {}

3. What will be the output of the following code?

int count = 0;
for (int i = 1; i <= 3; i++) {
  count += i;
}
std::cout << count;

medium

A. Error

B. 6

C. 0

D. 3

Solution

1. Step 1: Trace the loop iterations

   The loop runs with i = 1, 2, 3. Each time, count adds i: 0+1=1, 1+2=3, 3+3=6.

2. Step 2: Determine final count value

   After the loop, count is 6, which is printed.

3. Final Answer:

   6 -> Option B

4. Quick Check:

   Sum 1+2+3 = 6 [OK]

Hint: Add loop variable each iteration to sum quickly [OK]

Common Mistakes:

• Confusing loop end condition
• Forgetting to add inside loop
• Assuming count resets inside loop

4. Identify the error in this loop code:

int i = 0;
while (i < 3) {
  std::cout << i << " ";
}

medium

A. Variable i should be declared inside the loop

B. Incorrect while loop syntax

C. Missing semicolon after std::cout

D. Missing increment of i inside the loop

Solution

1. Step 1: Analyze the while loop behavior

   The loop condition is i < 3, but i never changes inside the loop, causing an infinite loop.

2. Step 2: Identify the missing increment

   To progress, i must increase inside the loop, e.g., i++.

3. Final Answer:

   Missing increment of i inside the loop -> Option D

4. Quick Check:

   Loop needs variable update to avoid infinite loop [OK]

Hint: Always update loop variable inside while loops [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Thinking syntax error causes infinite loop
• Declaring variable inside loop unnecessarily

5. Consider this code snippet:

int sum = 0;
for (int i = 1; i <= 5; i++) {
  if (i % 2 == 0) continue;
  sum += i;
}
std::cout << sum;

What is the output and why?

hard

A. 9, because only odd numbers 1, 3, 5 are added

B. 15, because all numbers 1 to 5 are added

C. 10, because even numbers are added

D. 0, because continue skips all additions

Solution

1. Step 1: Understand the loop and condition

   The loop runs i from 1 to 5. If i is even, continue skips adding it to sum.

2. Step 2: Calculate sum of odd numbers

   Odd numbers are 1, 3, 5. Their sum is 1+3+5 = 9.

3. Final Answer:

   9, because only odd numbers 1, 3, 5 are added -> Option A

4. Quick Check:

   Sum odd 1+3+5 = 9 [OK]

Hint: Continue skips even numbers, sum only odds [OK]

Common Mistakes:

• Adding even numbers by mistake
• Ignoring continue statement effect
• Summing all numbers instead of filtered ones