Loops and Iteration in C++: while, for, do-while

1Why 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.

Without a loop:

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

With a loop:

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

Loops are one of the most powerful tools in programming. They let you:

• Process lists of data (arrays, vectors, files)
• Repeat an action a specific number of times
• Keep running until a condition changes (like user input)
• Build complex patterns and structures

2The while loop

The while loop checks the condition before each iteration. If the condition is false initially, the loop body never runs.

Syntax:

while (condition) {
    // code block
}

Example – printing numbers 1 to 5:

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

3The for loop

A for loop is ideal when the number of iterations is known in advance. It combines initialization, condition, and update in one line.

Syntax:

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

Example – printing numbers 1 to 5:

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

The for loop does exactly the same thing as the while example but is more compact and is the preferred style when you know exactly how many times to iterate.

4The do-while loop

The do-while loop checks the condition after each iteration. This guarantees that the loop body runs at least once.

Syntax:

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

Example – printing numbers 1 to 5:

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

5Example: Sum of first N numbers

Here is a practical example that calculates the sum of numbers from 1 to N using a for loop.

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

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

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

This code adds 1 + 2 + 3 + 4 + 5 = 15. The sum += i shorthand is equivalent to sum = sum + i.

6break and continue

break exits the loop immediately, even if the condition is still true. continue skips the current iteration and jumps to the next one.

break example

for (int i = 1; i <= 10; i++) {
    if (i == 5) break;
    cout << i << " ";
}

continue example

for (int i = 1; i <= 5; i++) {
    if (i == 3) continue;
    cout << i << " ";
}

7Nested loops

You can put one loop inside another. This is useful for working with grids, tables, or any problem that requires two dimensions.

Example – multiplication table (5x5):

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

8Common loop patterns

1. Counting from 0 to N-1

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

2. Sum of even numbers (2 to 100)

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

3. Reverse loop (10 down to 1)

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

9Common mistakes to avoid

10Mini exercises

11Quiz yourself