Question

I often use to memorize all matrixes in a single vector, because my book says it’s faster to use a single vector.And the access to a matrix is slower in time.
If I have a code like this one:

int main(int argc, char **argv)
{
    int mat[10][10],i;
    for(i=0;i<10;i++)
        mat[i][0]=99;
    int *ptr=&mat[0][0];
    for(i=0;i<10;i++)
    {
        printf("%d\n",*ptr);
        ptr+=10;
    }
    return 0;
}

I tried to run it 4/5 times and all times prints 10 times 99.
So also matrixes are memorized in contigous positions of memory? Always?
If yes, why the access to a vector is faster?

Answer 1

Well, arrays (in C) are stored in contiguous memory, and since that your mat is array of arrays, it also stored in a contiguous memory. I think that dereferencing by one index (when you have some separating 1D arrays) may be a little faster than dereferencing by two indexes (in matrix), but the difference is too small to worry about.