The for Loop

Most loops that count have the same three parts: a variable created before the loop, a condition, and a change at the end of the block:

int number = 1;            // start
while (number <= 5) {      // condition
    printf("%d\n", number);
    number++;              // step
}

Counting like this is so common that C has a loop that puts all three parts on one line, for:

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

Both loops print the same:

1
2
3
4
5

The parentheses after for hold the three parts, separated by semicolons (;):

1. int i = 1 is the start. It creates the loop variable and runs only once, before everything else.
2. i <= 5 is the condition. It is checked before each iteration. When it's false, the loop ends.
3. i++ is the step. It runs after each iteration, when the block has finished.

So the order is: start, condition, block, step, condition, block, step, and so on. In our loop, i is 1, 1 <= 5 is true, the block prints 1, the step makes i 2, 2 <= 5 is true, the block prints 2… After printing 5, the step makes i 6, 6 <= 5 is false, and the loop ends.

The loop variable is often called i, but any name works. It exists only inside the loop. If you use i after the loop, the compiler doesn't know it:

#include <stdio.h>

int main(void) {
    for (int i = 1; i <= 3; i++) {
        printf("%d\n", i);
    }
    printf("%d\n", i);
    return 0;
}

main.c:7:20: error: use of undeclared identifier 'i'
    7 |     printf("%d\n", i);
      |                    ^
1 error generated.

An undeclared identifier is a name the compiler doesn't know at that point. If you need the value after the loop, use a while loop with a variable created before it, as in the previous exercises. On the other hand, each loop can have its own i, because every i disappears when its loop ends.

Separate the three parts with semicolons, not commas. With commas, for (int i = 1, i <= 3, i++), the compiler gets confused and reports several errors at once. The first one, error: invalid '<=' at end of declaration; did you mean '='?, doesn't help much. When an error points at a line with for, check the semicolons first.

for or while? Use for when you know how many times the loop will run: from 1 to 10, or 7 times for the days of a week. Use while when you don't know it in advance, for example when you double a number until it is greater than 1000:

int number = 1;
int doublings = 0;
while (number <= 1000) {
    number = number * 2;
    doublings++;
}
printf("Doublings: %d\n", doublings);
printf("Number: %d\n", number);

Doublings: 10
Number: 1024