3 ms·

Unless you're passing arrays of pointers to things about, the individual elements in a C array should be contiguous.

tyingq 9y ago
>Unless you're passing arrays of pointers That seems to be the context from his example: for(i = 0; i < nrows; i++){ array[i] = malloc(ncolumns * sizeof(double)); } I suppose the counterpoint is that he's doing it wrong. But again, maybe physicists just don't want a tool with that much flexibility.
- sbmassey 9y ago
  Fair enough then, I suppose that in C you have to do something like that if you want both that your matrix size is decidable at runtime, and to be able to index it with m[x][y] rather than some function.
  - iamrecursion 9y ago
    That’s not strictly true. You can malloc an m*n sized array and then assign to another array the pointer to the head of each column, allowing you standard style indexing.
    - PeterisP 9y ago
      If you do that, every access will actually dereference that pointer and won't be able to optimize the standard style indexing to a multiplication and addition, so this still carries a significant performance cost.
  - Peaker 9y ago
    You can always use a macro (or inline function, if the array is a rich wrapper type): arr(m, x, y) Not quite as nice, but not too bad.
- ryanpepper 9y ago
  I was taught to do it like this in C: double *a = malloc(sizeof(double)*nx*ny*nz); int id; for(int k = 0; k < nz; k++) { for(int j = 0; j < ny; j++) { for(int i = 0; < i < nx; i++) { id = k*nx*ny + j*nx + i; a[id] = /* some mathematical operation */ } } } You can do it in other ways (arrays of pointes, etc), but I think this is probably the simplest conceptually, and the memory is contiguous.
- cowsandmilk 9y ago
  he's definitely doing it wrong there. As a scientific C programmer, I would never do that, I would malloc one contiguous array of nrows*ncolumns. And my freeing of the array would be as simple as his Fortran deallocate. Sure, C could make multi-dimensional array handling nicer, but I have macros that basically do his A[x,y,z] for me, admittedly a bit more verbosely. Many of his points about Fortran are true, but almost all his statements about C are false. Or nearly so, yes, copying an array of floats requires calling a function, memcpy, and not just using an equals sign, but that ain't rough and falls into his idea of "what actually happens ‘under the hood’ inside a computer". And others are easily handled by adopting things like the MKL (want to take the sin of everything in an array, see https://software.intel.com/en-us/mkl-developer-reference-c-trigonometric-functions https://software.intel.com/en-us/mkl-developer-reference-c-t... )

>Unless you're passing arrays of pointers That seems to be the context from his example: for(i = 0; i < nrows; i++){ array[i] = malloc(ncolumns * sizeof(double)); } I suppose the counterpoint is that he's doing it wrong. But again, maybe physicists just don't want a tool with that much flexibility.

sbmassey 9y ago
Fair enough then, I suppose that in C you have to do something like that if you want both that your matrix size is decidable at runtime, and to be able to index it with m[x][y] rather than some function.
- iamrecursion 9y ago
  That’s not strictly true. You can malloc an m*n sized array and then assign to another array the pointer to the head of each column, allowing you standard style indexing.
  - PeterisP 9y ago
    If you do that, every access will actually dereference that pointer and won't be able to optimize the standard style indexing to a multiplication and addition, so this still carries a significant performance cost.
- Peaker 9y ago
  You can always use a macro (or inline function, if the array is a rich wrapper type): arr(m, x, y) Not quite as nice, but not too bad.
ryanpepper 9y ago
I was taught to do it like this in C: double *a = malloc(sizeof(double)*nx*ny*nz); int id; for(int k = 0; k < nz; k++) { for(int j = 0; j < ny; j++) { for(int i = 0; < i < nx; i++) { id = k*nx*ny + j*nx + i; a[id] = /* some mathematical operation */ } } } You can do it in other ways (arrays of pointes, etc), but I think this is probably the simplest conceptually, and the memory is contiguous.
cowsandmilk 9y ago
he's definitely doing it wrong there. As a scientific C programmer, I would never do that, I would malloc one contiguous array of nrows*ncolumns. And my freeing of the array would be as simple as his Fortran deallocate. Sure, C could make multi-dimensional array handling nicer, but I have macros that basically do his A[x,y,z] for me, admittedly a bit more verbosely. Many of his points about Fortran are true, but almost all his statements about C are false. Or nearly so, yes, copying an array of floats requires calling a function, memcpy, and not just using an equals sign, but that ain't rough and falls into his idea of "what actually happens ‘under the hood’ inside a computer". And others are easily handled by adopting things like the MKL (want to take the sin of everything in an array, see https://software.intel.com/en-us/mkl-developer-reference-c-trigonometric-functions https://software.intel.com/en-us/mkl-developer-reference-c-t... )