Loop execution flow in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print numbers from 1 to 5 using a for loop.

C++

#include <iostream>

int main() {
    for (int i = 1; i [1] 5; i++) {
        std::cout << i << std::endl;
    }
    return 0;
}

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

A<

B<=

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using < will stop before printing 5.

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

The loop should run while i is less than or equal to 5 to print numbers 1 through 5.

2fill in blank

medium

Complete the code to skip printing the number 3 inside the loop.

C++

#include <iostream>

int main() {
    for (int i = 1; i <= 5; i++) {
        if (i [1] 3) {
            continue;
        }
        std::cout << i << std::endl;
    }
    return 0;
}

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

A==

B!=

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using != will skip all numbers except 3.

Using > or < will skip more numbers than intended.

The continue statement should run only when i equals 3 to skip printing 3.

3fill in blank

hard

Fix the error in the while loop condition to stop the loop after printing numbers 1 to 5.

C++

#include <iostream>

int main() {
    int count = 1;
    while (count [1] 5) {
        std::cout << count << std::endl;
        count++;
    }
    return 0;
}

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

A>=

B<

C>

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using < will stop before printing 5.

Using > or >= will cause the loop to never run.

The loop should continue while count is less than or equal to 5 to print numbers 1 through 5.

4fill in blank

hard

Fill both blanks to create a loop that prints even numbers from 2 to 10.

C++

#include <iostream>

int main() {
    for (int num = 2; num [1] 10; num [2]) {
        std::cout << num << std::endl;
    }
    return 0;
}

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

A<=

B<

C++

D+= 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using < will exclude 10 from output.

Using ++ increments by 1, printing odd numbers too.

The loop runs while num is less than or equal to 10, and increments by 2 each time to print even numbers.

5fill in blank

hard

Fill all three blanks to create a loop that prints numbers from 10 down to 1.

C++

#include <iostream>

int main() {
    for (int i = [1]; i [2] 1; i [3]) {
        std::cout << i << std::endl;
    }
    return 0;
}

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

A1

B10

C>=

D>

E--

F++

Attempts:

3 left

💡 Hint

Common Mistakes

Starting from 1 will not count down.

Using ++ will increase i instead of decreasing.

Using > instead of >= will exclude 1.

The loop starts at 10, runs while i is greater than or equal to 1, and decrements by 1 each time.

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