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Comment by krapp

3 months ago

Yes, you can do that, it's fine as long as you stay within the bounds of the indexes. Under the hood, it's a single contiguous block of memory.

Although at least with 2d arrays I prefer to just use a 1d array and index it with [x * width + y], because one problem with multidimensional arrays in C is they need multiple allocations/frees.

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krapp

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sigsev_251  3 months ago

Why would you need multiple allocations?

edit: Isn't it just:

  float (*arr)[m][n] = malloc(sizeof(*arr));

  • krapp  3 months ago

    You have to allocate the size for m and n, because C arrays decay bare pointers.

    But it seems to work, which I didn't expect[0]. It also seems like you have to de-reference it to use it which is weird.

    [0]https://onlinegdb.com/HwX-WTL5t

    • uecker  3 months ago

      Double indirection arrays with multiple allocations are 25 years obsolete (ok, there are some use cases) but since C99 we prefer to do it like the parent.

      In your code link you over allocate memory, sizeof *arr is enough and you need to dereference like with (*arr)[i][j]. You need to dereference it because it is a pointer to an array, if you dereference you get an array. You can also let the first dimensions decay then it looks like:

        double (*arr)[m] = malloc(n * sizeof *arr);
  arr[i][j] = ...

      but this is not as safe because the bound of the outermost dimension is lost. (Edited)