Counting

The second pattern counts: how many numbers meet a condition? It looks like adding up, but instead of the value, you add 1, and only when an if says so.

Every seventh visitor to a museum gets a free postcard. How many postcards does the museum give away when 50 visitors come?

int postcards = 0;
for (int visitor = 1; visitor <= 50; visitor++) {
    if (visitor % 7 == 0) {
        postcards++;
    }
}
printf("Postcards: %d\n", postcards);

Postcards: 7

The counter postcards starts at 0, before the loop. The loop goes through every visitor, and the if decides whether to count them: visitor % 7 == 0 is true when the remainder after dividing by 7 is 0. postcards++ runs only for the visitors 7, 14, 21, 28, 35, 42 and 49, so at the end, the counter is 7. Again, you print the result after the loop.

Notice the two pairs of curly braces: postcards++ is in the block of the if, which is in the block of the loop. If postcards++ ends up outside the if (but still in the loop), the program counts every visitor and prints Postcards: 50.

One loop can also count several things at once, each in its own counter. With if and else, every visitor is counted exactly once, in one counter or the other:

int postcards = 0;
int without = 0;
for (int visitor = 1; visitor <= 50; visitor++) {
    if (visitor % 7 == 0) {
        postcards++;
    } else {
        without++;
    }
}
printf("Postcards: %d\n", postcards);
printf("Without a postcard: %d\n", without);

Postcards: 7
Without a postcard: 43

7 + 43 is 50, one for each visitor. Adding up the counters like this is a quick way to check that you counted right.