TeachMate | Interactive Learning Platform

What you'll learn

Explain why a loop replaces repeated statements.
Write a for, while, or do-while loop that terminates correctly.
Trace a loop and predict its output before running it.

Loops repeat a block of code multiple times. They are one of the most fundamental control structures in programming.

Problem

You must print the numbers 1 to 10. Writing them by hand looks like this:

Goal: Print 1 through 10 with a single repeating statement instead of ten separate lines.

cout << 1 << " ";
cout << 2 << " ";
cout << 3 << " ";
// ...seven more lines...
cout << 10 << " ";

Ten near-identical lines is slow and easy to get wrong. Look for the pattern:

Pseudocode

START
  set i = 1
  WHILE i <= 10
    print i
    i = i + 1
END

C++ expresses 'repeat' with a loop:

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

Now that the reason is clear, the rest of this note explains the syntax and the three loop types.

Why Loops Matter

Avoid repeating the same code for similar tasks
Process collections of data (arrays, strings, files)
Implement algorithms (searching, sorting, aggregation)
Create repetitive behaviors (animations, polling, retries)

Three Types of Loops

for loop: when you know the number of iterations
while loop: when you don't know iterations, check before each
do-while loop: execute at least once, then check

for Loop

for (int i = 0; i < 5; i++) {
    cout << i << " ";
}

while Loop

int i = 0;
while (i < 5) {
    cout << i << " ";
    i++;
}

do-while Loop

int i = 0;
do {
    cout << i << " ";
    i++;
} while (i < 5);

Infinite Loops

while (true) { ... }     // Runs forever (use break to exit)
for (;;) { ... }          // Same as while(true)

Complete Program: Three Ways to Count to 5

#include <iostream>

int main() {
    for (int i = 1; i <= 5; ++i) std::cout << i << ' ';
    std::cout << '\n';

    int j = 1;
    while (j <= 5) { std::cout << j << ' '; ++j; }
    std::cout << '\n';

    int k = 1;
    do { std::cout << k << ' '; ++k; } while (k <= 5);
    std::cout << '\n';

    return 0;
}

Common mistakes: forgetting the update step so a while loop never terminates, an off-by-one bound (i <= n vs i < n) that runs one iteration too many, and dropping the semicolon after do-while(...).

Key takeaway: Choose the right loop type — for when you know the count, while for unknown iterations, and do-while when you need at least one execution.

Try it: Write a program that uses each of the three loop types (for, while, do-while) to print numbers 1 through 5.

Infinite Loops

while (true) { ... }     // Runs forever (use break to exit)
for (;;) { ... }          // Same as while(true)

Complete Program: Three Ways to Count to 5

#include <iostream>

int main() {
    for (int i = 1; i <= 5; ++i) std::cout << i << ' ';
    std::cout << '\n';

    int j = 1;
    while (j <= 5) { std::cout << j << ' '; ++j; }
    std::cout << '\n';

    int k = 1;
    do { std::cout << k << ' '; ++k; } while (k <= 5);
    std::cout << '\n';

    return 0;
}

Common mistakes: forgetting the update step so a while loop never terminates, an off-by-one bound (i <= n vs i < n) that runs one iteration too many, and dropping the semicolon after do-while(...).

Key takeaway: Choose the right loop type — for when you know the count, while for unknown iterations, and do-while when you need at least one execution.

Try it: Write a program that uses each of the three loop types (for, while, do-while) to print numbers 1 through 5.