Pass list of list from Python to C as uint64_t matrix

Question

How can a list of list of numbers (uint64_t in C) for example: a = [[1, 5, 9], [4, 6, 7], [8, 2, 3]] be passed as an argument of a C program (via argv) and then be printed (or be able to access that data) in the C program as a 2D matrix?

In my C program I'm working with non-predefined size matrices of unint64_t type ( uint64_t matrix[nrows][ncols] )

Answer 1

There's many ways to do it, but the easiest is probably just flattening the matrix first:

def flatten(a):
    yield len(a)
    cols = max(len(i) for i in a)
    yield cols
    for i in a:
        for j in i:
            yield j
        for j in range(0, cols - len(i)):
            yield 0

Then you can use the result of that to feed the parameters to your C program, and from that you can build the matrix (C99 required):

#include <stdlib.h>
#include <inttypes.h>

void load_matrix(uint64_t *matrix, char **p, int alen) {
    while (alen-- > 0)
        *matrix++ = atoi(*p++);
}

void do_work(uint64_t matrix[nrows][ncols], int nrows, int ncols) {
    ...
}

int main(int argc, char **argv) {
    int nrows = atoi(argv[1]), ncols = atoi(argv[2]);

    // check ncols and nrows and argc
    ...

    uint64_t *matrix;
    matrix = malloc(nrows * ncols * sizeof(*matrix));

    // check matrix isn't NULL
    ...

    load_matrix(matrix, argv + 3, nrows * ncols);
    do_work(matrix, nrows, ncols);
    ...
}

Keep in mind it's just an illustration, ie things like atoi aren't appropriate.

If you want to use C89 you need to cope with a flat array, in the end it's the same thing.

However, if the matrix is large, you don't really want that; you would serialize it in a similar way (perhaps with a more efficient implementation), but you would want to deal with binary data directly, and pass that through shared memory, a file, a socket, or a pipe to your C program, so there's no need for further processing.

If you were working with a very large data set, then you definitely want to work with the same representation in both Python and C, and you probably want to use shared memory or files.