C For Loop

When you know how many times to repeat, the for loop is the clearest choice. It puts the three parts of a counting loop, the start, the condition and the step, on one line.

Syntax

for (start; condition; step) {
    // code to repeat
}

The start runs once. Before each round the condition is checked; if it is false the loop ends. After each round the step runs.

#include <stdio.h>

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

Round 1
Round 2
Round 3
Round 4
Round 5

The variable i declared in the loop exists only inside the loop. (Declaring it inside the for brackets needs C99 or later; very old code declares int i; before the loop.)

Counting in other steps

The step can be anything: i += 2 counts in twos, i-- counts down.

#include <stdio.h>

int main(void) {
    for (int i = 0; i <= 10; i += 2) {
        printf("%d ", i);
    }
    printf("\n");
    for (int i = 5; i >= 1; i--) {
        printf("%d ", i);
    }
    printf("\n");
    return 0;
}

0 2 4 6 8 10 
5 4 3 2 1 

A multiplication table

#include <stdio.h>

int main(void) {
    int n = 7;
    for (int i = 1; i <= 10; i++) {
        printf("%d x %2d = %2d\n", n, i, n * i);
    }
    return 0;
}

7 x  1 =  7
7 x  2 = 14
7 x  3 = 21
7 x  4 = 28
7 x  5 = 35
7 x  6 = 42
7 x  7 = 49
7 x  8 = 56
7 x  9 = 63
7 x 10 = 70

Nested loops

A loop inside a loop runs completely for each round of the outer loop. This prints patterns and tables:

#include <stdio.h>

int main(void) {
    for (int row = 1; row <= 4; row++) {
        for (int col = 1; col <= row; col++) {
            printf("*");
        }
        printf("\n");
    }
    return 0;
}

*
**
***
****

Off-by-one errors

The most common loop bug is running one time too many or too few. for (int i = 0; i < 5; i++) runs 5 times with i from 0 to 4, which is exactly what you need for an array of 5 elements. i <= 5 would run 6 times. Count the rounds on paper when unsure.

Practice exercises

1. Print the multiplication table of a number the user types.
2. Add up the numbers from 1 to 100 and print the total.
3. Print the triangle of stars upside down, from 4 stars down to 1.
4. Print a 5 by 5 square of # characters with nested loops.