Loop execution flow in C++

Introduction

Loops help repeat actions many times without writing the same code again and again.

When you want to print numbers from 1 to 10.

When you need to check each item in a list one by one.

When you want to keep asking a user for input until they give a correct answer.

When you want to repeat a task until a certain condition is met.

Syntax

C++

for (initialization; condition; update) {
    // code to repeat
}

while (condition) {
    // code to repeat
}

do {
    // code to repeat
} while (condition);

The for loop is good when you know how many times to repeat.

The while loop repeats as long as the condition is true.

Examples

This prints numbers 1 to 5 using a for loop.

C++

for (int i = 1; i <= 5; i++) {
    std::cout << i << " ";
}

This prints numbers 1 to 5 using a while loop.

C++

int i = 1;
while (i <= 5) {
    std::cout << i << " ";
    i++;
}

This prints numbers 1 to 5 using a do-while loop, which runs the code at least once.

C++

int i = 1;
do {
    std::cout << i << " ";
    i++;
} while (i <= 5);

Sample Program

This program shows how each loop type repeats code to print numbers 1 to 3.

C++

#include <iostream>

int main() {
    std::cout << "For loop output: ";
    for (int i = 1; i <= 3; i++) {
        std::cout << i << " ";
    }
    std::cout << "\n";

    std::cout << "While loop output: ";
    int j = 1;
    while (j <= 3) {
        std::cout << j << " ";
        j++;
    }
    std::cout << "\n";

    std::cout << "Do-while loop output: ";
    int k = 1;
    do {
        std::cout << k << " ";
        k++;
    } while (k <= 3);
    std::cout << "\n";

    return 0;
}

Important Notes

Loops run the code inside their block repeatedly until the condition is false.

Be careful to update variables inside loops to avoid infinite loops.

Summary

Loops help repeat tasks easily and clearly.

Use for when you know how many times to repeat.

Use while or do-while when repeating depends on a condition.

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