Question

I have two arrays of chars, allocated as follows:

 unsigned char *arr1 = (unsigned char *)malloc((1024*1024) * sizeof(char));
 unsigned char *arr2 = (unsigned char *)malloc((768*768) * sizeof(char));

I would like to copy arr2 into arr1, but preserve the row/column structure. This means that only the first 768 bytes of each of the first 768 rows will be changed in arr1.

I wrote a for loop for this, but it’s not fast enough for my needs.

for (int x = 0; x < 768; x++) //copy each row
{
    memcpy(arr1+(1024*x),arr2+(768*x), nc);
}

Is there a better solution?

Answer 1

maybe get rid of the multiplications

size_t bigindex = 0, smallindex = 0;
for (int x = 0; x < 768; x++) //copy each row
{
    memcpy(arr1 + bigindex, arr2 + smallindex, nc);
    bigindex += 1024;
    smallindex += 768;
}

---

Edit d’oh! use the pointers!

unsigned char *a1 = arr1;
unsigned char *a2 = arr2;
for (int x = 0; x < 768; x++) //copy each row
{
    memcpy(a1, a2, nc);
    a1 += 1024;
    a2 += 768;
}