For Loops in C: Syntax, Examples and Common Mistakes

The for loop is one of the first tools you reach for in C. It runs a block of code a known number of times, which makes it perfect for counting, iterating over arrays and building tables.

The three parts of a for loop

A C for loop has three sections inside the parentheses, separated by semicolons:

for (initialization; condition; update) {
    // body runs while condition is true
}

• Initialization runs once, before the loop starts. It usually sets a counter, like int i = 0.
• Condition is checked before every pass. While it is true, the body runs.
• Update runs at the end of every pass, usually to change the counter.

A first example

This loop prints the numbers 1 through 5:

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 5; i++) {
        printf("%d\n", i);
    }
    return 0;
}

Read it as: start i at 1, keep going while i is less than or equal to 5, and add 1 to i after each pass. The output is the numbers 1 to 5 on separate lines.

Counting down

You are not limited to counting up. Swap the condition and the update to count down instead:

for (int i = 5; i >= 1; i--) {
    printf("%d ", i);
}
// prints: 5 4 3 2 1

Common mistakes beginners make

A few errors come up again and again when you are starting out:

1. The stray semicolon. Writing for (int i = 0; i < 5; i++); ends the loop with an empty body. The block after it then runs only once. Watch that semicolon right after the closing parenthesis.
2. Off-by-one errors. i < 5 runs five times (0 to 4); i <= 5 runs six times (0 to 5). Decide whether your last value should be included and pick < or <= to match.
3. Changing the counter inside the body. Modifying i both in the update section and inside the loop body makes the loop hard to follow. Keep counter changes in one place.

When to use a for loop

Reach for a for loop when you know, or can compute, how many times you need to repeat something — printing a multiplication table, summing the elements of an array, or drawing a pattern. When the number of repetitions depends on a condition you cannot count in advance, a while loop is usually clearer.

Practising loops by hand is the fastest way to build intuition. Try rewriting the count-up example as a count-down, then as a loop that prints only even numbers. Small variations like these teach you how the three parts work together.