Why loops are needed in C++ - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print numbers from 1 to 5 using a 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 < prints only 1 to 4.

Using > or >= causes the loop to never run or run incorrectly.

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 sum numbers from 1 to 10 using a loop.

C++

#include <iostream>

int main() {
    int sum = 0;
    for (int num = 1; num [1] 10; num++) {
        sum += num;
    }
    std::cout << sum << 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 < excludes 10 from the sum.

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

The loop should include 10, so use <= to sum numbers from 1 to 10.

3fill in blank

hard

Fix the error in the loop condition to print numbers from 0 to 4.

C++

#include <iostream>

int main() {
    for (int i = 0; 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 <= prints 0 to 5 instead of 0 to 4.

Using > or >= causes the loop to not run or run incorrectly.

The loop should run while i is less than 5 to print 0 through 4.

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 i = [1]; i [2] 10; i += 2) {
        std::cout << i << std::endl;
    }
    return 0;
}

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

A2

B1

C<=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Starting at 1 prints odd numbers.

Using < excludes 10 from the output.

Start at 2 and run the loop while i is less than or equal to 10 to print even numbers.

5fill in blank

hard

Fill all three blanks to create a loop that prints the squares of numbers from 1 to 5.

C++

#include <iostream>

int main() {
    for (int [1] = 1; [2] [3] 5; [1]++) {
        std::cout << [1] * [1] << std::endl;
    }
    return 0;
}

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

Ai

C<=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in initialization and condition causes errors.

Using < excludes 5 from the output.

Use i as the loop variable, and run the loop while i is less than or equal to 5 to print squares from 1 to 5.

Practice

1. Why do we use loops in C++ programming?

easy

A. To repeat a set of instructions multiple times without writing them again

B. To store multiple values in a single variable

C. To make the program run faster by skipping code

D. To create new variables automatically

Solution

1. Step 1: Understand the purpose of loops

   Loops allow repeating the same instructions many times without rewriting the code.

2. Step 2: Compare other options

   Options A, C, and D describe other programming concepts, not loops.

3. Final Answer:

   To repeat a set of instructions multiple times without writing them again -> Option A

4. Quick Check:

   Loops repeat actions [OK]

Hint: Loops repeat code to save writing it many times [OK]

Common Mistakes:

• Confusing loops with variables
• Thinking loops speed up code by skipping
• Mixing loops with data storage

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

easy

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

B. for i = 0 to 5 { /* code */ }

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

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

Solution

1. Step 1: Recall correct for loop syntax

   The correct syntax uses: for (initialization; condition; update) { code }

2. Step 2: Check each option

   for (int i = 0; i < 5; i++) { /* code */ } matches the correct syntax. Options A, B, and C have syntax errors.

3. Final Answer:

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

4. Quick Check:

   Standard for loop syntax [OK]

Hint: For loops use three parts inside parentheses separated by semicolons [OK]

Common Mistakes:

• Using 'for i = 0 to 5' like other languages
• Writing 'loop' instead of 'for'
• Mixing order of initialization and condition

3. What will be the output of this C++ code?

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

medium

A. 0 1 2

B. 1 2 3

C. 1 2 3 4

D. 3 2 1

Solution

1. Step 1: Analyze loop range and output

   The loop starts at i=1 and runs while i <= 3, printing i each time.

2. Step 2: List printed values

   Values printed are 1, 2, and 3, each followed by a space.

3. Final Answer:

   1 2 3 -> Option B

4. Quick Check:

   Loop prints 1 to 3 [OK]

Hint: Check loop start, end, and print inside loop [OK]

Common Mistakes:

• Starting loop at 0 instead of 1
• Including extra number beyond 3
• Printing in reverse order

4. Identify the error in this C++ loop code:

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

medium

A. Missing semicolon after std::cout

B. Variable i is not declared

C. Incorrect while loop syntax

D. No update to i inside the loop causing infinite loop

Solution

1. Step 1: Check loop structure

   The while loop condition is correct and variable i is declared.

2. Step 2: Look for loop update

   There is no statement changing i inside the loop, so i stays 0 forever, causing infinite loop.

3. Final Answer:

   No update to i inside the loop causing infinite loop -> Option D

4. Quick Check:

   Loops need variable update to end [OK]

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

Common Mistakes:

• Forgetting to increment loop variable
• Assuming loop ends automatically
• Misreading syntax errors

5. You want to print all even numbers from 2 to 10 using a loop in C++. Which loop is best and why?

hard

A. Use a for loop with i starting at 2, incrementing by 2 until 10

B. Use a while loop starting at 1, incrementing by 1, and print if even

C. Use a do-while loop starting at 10, decrementing by 2 until 2

D. Use a for loop starting at 0, incrementing by 1, and print if even

Solution

1. Step 1: Consider loop control and efficiency

   Starting at 2 and incrementing by 2 prints only even numbers directly.

2. Step 2: Compare other options

   Use a while loop starting at 1, incrementing by 1, and print if even checks each number if even, less efficient. Use a do-while loop starting at 10, decrementing by 2 until 2 counts backward, which is okay but less natural here. Use a for loop starting at 0, incrementing by 1, and print if even starts at 0 and prints even numbers but includes 0 and extra checks.

3. Final Answer:

   Use a for loop with i starting at 2, incrementing by 2 until 10 -> Option A

4. Quick Check:

   Directly loop over even numbers [OK]

Hint: Increment by 2 to print even numbers efficiently [OK]

Common Mistakes:

• Using loops that check every number unnecessarily
• Starting loop at wrong number
• Confusing increment direction