Which method for accessing a 2D array in C is most effective? (Using Pointers)

Question

If you have a function that takes the pointer of an array, would there be any use in using pointers to each element in the array, or is it redundant? For example:

int sum(int(*arr)[3]) {
    int i,j, sum = 0;
    for (i =0; i < ROW ; i ++) {
        for (j =0; j < COL ; j ++) {
            sum = sum + *(*( arr +i )+j);
        }
    }
}

Would using arr[i][j] be the same in this case?

bolov · Accepted Answer · 2022-02-23 20:35:00Z

1

arr[i] is required by the standard to be equivalent to *(arr + i) so no, there is no point in not using arr[i] which is more readable.

Speeding up 2d array access is a complex topic. There are some techniques for performance improvement, but those take hardware architecture (e.g. cache) into account are are related to access patterns rather than access syntax.

answered Feb 23, 2022 at 20:35

bolov

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2 Comments

Ailany Over a year ago

Correct, but would using *(arr+i) take up fewer resources? Or is the answer much more complicated as you noted?

bolov Over a year ago

@Ailany what resources? The answer is simple regarding the array index operator vs pointer dereference: it's 100% equivalent. It generates identical asm code as you can see here: godbolt.org/z/dYWz3bcof

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