C arrays and memory explained

Arrays in memory

An array is several values of the same type, stored contiguously: one right after another, no gaps.

int scores[4] = {70, 85, 90, 64};

With 4-byte ints, that is one 16-byte block. If scores[0] starts at some address A, then scores[1] is at A + 4, scores[2] at A + 8, and so on. That is why indexing starts at 0: the index is "how many elements past the start". Getting element i is just one multiply and one add, which is why arrays are fast.

index:    [0]   [1]   [2]   [3]
value:     70    85    90    64
offset:    +0    +4    +8   +12   (bytes)

The size of an array is fixed when you declare it. sizeof on the array gives the total bytes, so this classic trick counts its elements:

int count = sizeof(scores) / sizeof(scores[0]);   // 16 / 4 = 4

That only works where the array was declared. Pass it to a function and it turns into a pointer (more on that in a later lesson), and sizeof would measure the pointer instead.

Now the scary part. C does not check array bounds. scores[4] or scores[-1] compile fine and are undefined behaviour: you might read a neighbouring variable, get garbage, corrupt something, or crash. Many real security bugs, buffer overflows, are exactly this. Keeping your index between 0 and count - 1 is your job, not the compiler's.

Try it yourself

Edit it. Break it. Run it again.

#include <stdio.h>

int main(void) {
    int scores[4] = {70, 85, 90, 64};
    int count = sizeof(scores) / sizeof(scores[0]);

    printf("the array uses %zu bytes for %d ints\n", sizeof(scores), count);
    for (int i = 0; i < count; i++) {
        printf("scores[%d] = %d, %d bytes from the start\n", i, scores[i], i * (int)sizeof(int));
    }
    return 0;
}

Your turn

Type it yourself. That is the whole trick.

1Sum and max

Given the array in the starter, loop over it and print its total and its largest value:

total: 291
max: 93

#include <stdio.h>

int main(void) {
    int temps[6] = {41, 17, 93, 56, 38, 46};
    int total = 0;
    int max = 0;

    printf("total: %d\n", total);
    printf("max: %d\n", max);
    return 0;
}

2Backwards

Print the elements of the array in reverse order, separated by single spaces, on one line, then a newline: 50 40 30 20 10 Use the sizeof trick to get the count instead of typing 5.

#include <stdio.h>

int main(void) {
    int nums[] = {10, 20, 30, 40, 50};
    int count = 0;

    for (int i = 0; i < count; i++) {
        printf("%d ", nums[i]);
    }
    printf("\n");
    return 0;
}