Looping over an Array

You can't print a whole array at once. If you try printf("%d\n", scores);, the program doesn't compile:

main.c:5:20: error: format specifies type 'int' but the argument has type 'int *' [-Werror,-Wformat]

%d prints one whole number, but printf got the whole array scores. (C writes its type here as int *. You'll learn what the asterisk means in the lesson on pointers.) An array has to be printed element by element.

For five elements, five printfs would do. For a hundred, it would be a lot of typing. Luckily, an index doesn't have to be a number written in the code. It can also be a variable: if i is 2, then scores[i] is the same as scores[2]. And a for loop can take i through all the indexes, from 0 to the last one:

int scores[] = {80, 92, 75, 60, 88};
for (int i = 0; i < 5; i++) {
    printf("%d\n", scores[i]);
}

This prints:

80
92
75
60
88

In the first iteration of the loop, i is 0 and the loop prints scores[0]. In the second iteration, i is 1 and it prints scores[1], and so on up to scores[4]. The condition is i < 5, not i <= 5: the array has 5 elements, so the last index is 4. In the next exercise, you'll see what happens with index 5.

The number of elements often appears in several places in a program, for example in several loops. It's a good idea to store it in a constant, so that it's written only once:

int scores[] = {80, 92, 75, 60, 88};
const int COUNT = 5;
for (int i = 0; i < COUNT; i++) {
    printf("Test %d: %d points\n", i + 1, scores[i]);
}

This prints:

Test 1: 80 points
Test 2: 92 points
Test 3: 75 points
Test 4: 60 points
Test 5: 88 points

The test number is i + 1, because people count tests from 1, while indexes start at 0. The constant must match the array. If you add a sixth score to the array and forget to change COUNT, the loop still goes through only five elements, and the new score never gets printed.

A loop can also print the elements on one line, separated by commas. The comma goes before every element except the first one, the one with index 0:

int scores[] = {80, 92, 75, 60, 88};
const int COUNT = 5;
for (int i = 0; i < COUNT; i++) {
    if (i > 0) {
        printf(", ");
    }
    printf("%d", scores[i]);
}
printf("\n");

This prints:

80, 92, 75, 60, 88

The last printf("\n") is after the loop. It ends the line once all the elements are printed.