Loops in C++

A loop is a control structure that repeats a block of code multiple times. Instead of writing the same line 100 times, a loop does it automatically. C++ provides three main types of loops: for, while, and do-while.

The for Loop

The for loop is the most commonly used loop when the number of iterations is known in advance. It combines initialization, condition checking, and updating in one line.

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

#include <iostream>
using namespace std;

int main() {
    for (int i = 1; i <= 5; i++) {
        cout << "Count: " << i << endl;
    }
    return 0;
}

Output:

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5

How it works:

1. int i = 1 — Start with i = 1
2. i <= 5 — Keep looping while i is 5 or less
3. i++ — After each loop, add 1 to i

Counting Downward with for

for (int i = 5; i >= 1; i--) {
    cout << i << " ";
}
cout << endl;

Output:

5 4 3 2 1

The while Loop

The while loop repeats as long as the condition is true. It is best used when the number of repetitions is not known in advance.

while (condition) {
    // code to repeat
}

int num = 1;
while (num <= 5) {
    cout << num << " ";
    num++;
}

Output:

1 2 3 4 5

Practical Example — Sum of digits:

int n = 12345;
int sum = 0;
while (n > 0) {
    sum += n % 10;   // extract last digit
    n /= 10;         // remove last digit
}
cout << "Sum of digits: " << sum << endl;

Output:

Sum of digits: 15

The do-while Loop

The do-while loop is like the while loop, but with one key difference — it always runs at least once, because the condition is checked after the loop body executes.

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

int x = 1;
do {
    cout << "x = " << x << endl;
    x++;
} while (x <= 3);

Output:

x = 1
x = 2
x = 3

When do-while runs even with false condition:

int val = 10;
do {
    cout << "This runs once." << endl;
} while (val < 5);   // false, but body already ran

Output:

This runs once.

Nested Loops

A loop inside another loop is called a nested loop. Each iteration of the outer loop runs the inner loop completely.

for (int row = 1; row <= 3; row++) {
    for (int col = 1; col <= 3; col++) {
        cout << row << "x" << col << "=" << row*col << "	";
    }
    cout << endl;
}

Output:

1x1=1   1x2=2   1x3=3
2x1=2   2x2=4   2x3=6
3x1=3   3x2=6   3x3=9

Loop Control Statements

break — Exit the loop immediately

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

continue — Skip current iteration and move to next

for (int i = 1; i <= 6; i++) {
    if (i == 3) continue;
    cout << i << " ";
}
// Output: 1 2 4 5 6

Range-Based for Loop (C++11)

A cleaner way to iterate over arrays or collections:

int numbers[] = {10, 20, 30, 40, 50};

for (int n : numbers) {
    cout << n << " ";
}

Output:

10 20 30 40 50

Infinite Loops

When the condition of a loop never becomes false, it runs forever. This is sometimes intentional (like a server waiting for requests), but usually a bug.

// Intentional infinite loop example — always provide a break condition!
while (true) {
    cout << "Running..." << endl;
    break;  // without this, it would never stop
}

Comparison of Loops

Key Takeaways

• Loops repeat code automatically based on a condition.
• for is best when the count is known.
• while checks the condition before entering the loop.
• do-while always runs the loop body at least once.
• break exits a loop; continue skips to the next iteration.