CODEWITHSUNDEEP
cpointers
tail
tail

Reputation: 411

Pointer Arithmetic equivalence

I have got this function written in C.

Are them two statements equivalent?

void dot_prod(float *A, int m, float *B, float *C) {
    int i, j, z, k;
    for (i = 0; i < m; i++) {
        for (j = 0; j < m; j++) {
            for (k = 0; k < m; k++) {
                C[i * m + k] += A[i * m + j] * B[m * j + k];
                //*(C + i * m + k) += *(A + i * m + j) * (*(B + m * j + k)); // is equivalent?
            }
        }
    }
}

Is

C[i * m + k] += A[i * m + j] * B[m * j + k]

equivalent to

*(C + i * m + k) += *(A + i * m + j) * (*(B + m * j + k))?

Upvotes: 0

Views: 95

Answers (1)

user17732522
user17732522

Reputation: 76688

Almost.

C[i * m + k] += A[i * m + j] * B[m * j + k]

is equivalent to

*(C + (i * m + k)) += *(A + (i * m + j)) * (*(B + (m * j + k)))

This is different than without the extra parentheses, since the addition in the indices may overflow, in which case the pointer/integer addition is not necessarily associative, depending on the sizes of int and pointers.

Upvotes: 3

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