Using memcpy to copy a range of elements from an 2d array

Question

Say we have two 2d arrays:

double matrix [64][100];
double array[64][32];

And we want to copy 32 elements from matrix[64][50:82] to array[64][32] using memcpy. Do you the solution?

`matrix[64][...]` is out of bounds. Show your attempt to use `memcpy`. — Ted Lyngmo, Jan 04 '21 at 23:33
So you want to copy elements from indexes 50..81 from each row of `matrix` to elements at indexes 0..31 of the the corresponding row of `array`? — Blastfurnace, Jan 04 '21 at 23:40
Yes, you are right. I want to copy elements from indexes 50..81 from each row of matrix to elements at indexes 0..31 of the the corresponding row of array. — user14940899, Jan 05 '21 at 00:01
I'm afraid you should first read some tutorial/book on pointer arithmetic and how 2D arrays correspond to 1D arrays and then at least try to provide your attempt towards the solution. — zkoza, Jan 05 '21 at 00:40

score 1 · Answer 1 · answered Jan 05 '21 at 00:53

double matrix[64][100];
double array[64][32];

for (int i = 0; i < 64; ++i) {
    memcpy(&array[i][0], &matrix[i][50], sizeof(double) * 32);
}

However, consider using std::copy() or std::copy_n() instead. They will use memcpy() internally when safe to do so:

#include <algorithm>

double matrix[64][100];
double array[64][32];

for (int i = 0; i < 64; ++i) {
    std::copy(&matrix[i][50], &matrix[i][82], &array[i][0]);
    or
    std::copy_n(&matrix[i][50], 32, &array[i][0]);
}

Using memcpy to copy a range of elements from an 2d array

1 Answers1