arrayscpointers

C: handle pointer to subset of array as small array

I have a problem where I need to handle small slices of a big array of doubles.
Is there a way to define a pointer which lets me handle it as a small array?

Given:

double bigarr[10000] = {0.0};
// doing something to fill the array

double *smallarr[10] = &bigarr[297];

int i;
for (i=0; i<10; i++) {
    *smallarr[i] += 15.0;
}

This example should result in adding 15.0 to the elements from bigarr[297]..bigarr[306] (but this is just an example. I want to do something more complicated)

What's the easiest way to achieve this?

Solution

  • Is there a way to define a pointer which lets me handle it as a small array?

    A plain double * will serve just fine:

    double *smallarr = &bigarr[297];

    or

    double *smallarr = bigarr + 297;

    You can then use smallarr much like you would bigarr, with smallarr[0] equivalent to bigarr[297].

    On the other hand, your original

    double *smallarr[10] = &bigarr[297];

    is an array of pointers, not an array of double nor a pointer to an array. You can define a pointer to an array:

    double (*smallarr_ptr)[10] = (double (*)[10]) &bigarr[297];

    ... but the syntax for using that is not the same as the syntax for using bigarr. And you don't gain much from encoding the sub-array length into the pointer type, especially if it's constant.