Counting Down and in Steps

The step of a for loop doesn't have to be i++. A loop can count down, count by twos or by fives, or start at zero. You only change its three parts.

Counting down. Start at the highest number, continue while the variable is at least the lowest one, and decrease it with i--:

for (int i = 3; i >= 1; i--) {
    printf("%d\n", i);
}
printf("Liftoff!\n");

3
2
1
Liftoff!

Watch the condition: it is i >= 1, “greater than or equal”. If you copy i <= 1 from a loop that counts up, the condition is false right at the start (3 <= 1), and the loop doesn't run even once. For the compiler, that's not an error: the loop simply prints nothing.

Counting in steps. To count by more than one, use += or -= in the step. This loop counts by fives:

for (int i = 0; i <= 20; i += 5) {
    printf("%d\n", i);
}

0
5
10
15
20

The variable doesn't have to hit the limit exactly. With the condition i <= 22, the loop prints the same five numbers: after 20 comes 25, and 25 <= 22 is false.

Counting from zero. Programmers often start counting at 0. This loop runs 3 times, for i equal to 0, 1 and 2:

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

Round 0
Round 1
Round 2

A loop for (int i = 0; i < n; i++) runs exactly n times. You'll see it very often, because in the next lesson, on arrays, counting starts at 0.

The most common mistake with for is being off by one: the loop runs one time more or one time less than it should. i < 3 stops at 2, i <= 3 at 3. If you start at 0 and write i <= 3, the loop runs 4 times: for 0, 1, 2 and 3. When you aren't sure how many times a loop runs, write down the values its variable takes and count them.