Loops and Iteration in C++ | Count-Controlled & While Loops

What You’ll Learn

By the end of this post, you’ll understand:

• What loops are and why they are useful
• The differences between while, for, and do-while loops
• How to control loop behavior with break and continue
• Common patterns like counting, summing, and nested loops

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Why Use Loops?

Loops allow you to repeat a block of code multiple times. Instead of writing the same code again and again, you let the loop do the repetition for you.

Example (without loop):

cout << "Hello" << endl;
cout << "Hello" << endl;
cout << "Hello" << endl;

With a loop:

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

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The while Loop

The while loop checks the condition before each iteration.

Syntax:

while (condition) {
    // code block
}

Example:

int i = 1;
while (i <= 5) {
    cout << i << endl;
    i++;
}

Output:

1
2
3
4
5

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The for Loop

A for loop is often used when the number of iterations is known.

Syntax:

for (initialization; condition; update) {
    // code block
}

Example:

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

This does exactly the same thing as the while example above but is more compact.

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The do-while Loop

This loop runs the code block at least once, then checks the condition.

Syntax:

do {
    // code block
} while (condition);

Example:

int i = 1;
do {
    cout << i << endl;
    i++;
} while (i <= 5);

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Example: Sum of First N Numbers

int n;
cout << "Enter a number: ";
cin >> n;

int sum = 0;
for (int i = 1; i <= n; i++) {
    sum += i;
}

cout << "Sum: " << sum << endl;

Try this with n = 5. You’ll get 1 + 2 + 3 + 4 + 5 = 15.

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break and continue

break: Exits the loop immediately.

for (int i = 1; i <= 10; i++) {
    if (i == 5) break;
    cout << i << " ";
}
// Output: 1 2 3 4

continue: Skips the current iteration.

for (int i = 1; i <= 5; i++) {
    if (i == 3) continue;
    cout << i << " ";
}
// Output: 1 2 4 5

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Nested Loops

You can put one loop inside another. This is useful for 2D problems like grids or tables.

Example: Multiplication Table

for (int i = 1; i <= 5; i++) {
    for (int j = 1; j <= 5; j++) {
        cout << i * j << "\t";
    }
    cout << endl;
}

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Common Loop Patterns

1. Counting:

for (int i = 0; i < 10; i++) {
    cout << i << " ";
}

2. Sum of Even Numbers:

int sum = 0;
for (int i = 2; i <= 100; i += 2) {
    sum += i;
}
cout << sum;

3. Reverse Loop:

for (int i = 10; i >= 1; i--) {
    cout << i << " ";
}

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Mini Exercises

1. Print numbers from 1 to 20 using a while loop.
2. Find the factorial of a number using a for loop.
3. Use a do-while loop to ask the user for a number until they enter a positive number.
4. Create a pattern like this using nested loops:

*
**
***
****

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Common Mistakes to Avoid

• Forgetting to update the loop variable (can lead to infinite loops)
• Off-by-one errors (using <= when it should be <, or vice versa)
• Misplacing braces — always use {} even for one-line loops if you’re new

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Summary

In this post, you learned:

• How to use while, for, and do-while loops in C++
• The purpose of break and continue
• Loop patterns like counting, summing, and nested loops
• How to write clean, structured loops that avoid common pitfalls

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What’s Next?

In the next post, we’ll explore Arrays and Vectors — how to store and work with lists of data.