Question

gprof is not working properly on my system (MinGW) so I’d like to know which one of the following snippets is more efficient, on average.

I’m aware that internally C compilers convert everything into pointers arithmetic, but nevertheless I’d like to know if any of the following snippets has any significant advantage over the others.

The array has been allocated dynamically in contiguous memory as 1d array and may be re-allocated at run time (its for a simple board game, in which the player is allowed to re-define the board’s size, as often as he wants to).

Please note that i & j must get calculated and passed into the function set_cell() in every loop iteration (gridType is a simple struct with a few ints and a pointer to another cell struct).

Thanks in advance!

Allocate memory

grid = calloc( (nrows * ncols), sizeof(gridType) );

Snippet #1 (parse sequentially as 1D)

gridType *gp = grid;
register int i=0 ,j=0;      // we need to pass those in set_cell()

if ( !grid )
return;

for (gp=grid; gp < grid+(nrows*ncols); gp++)
{
    set_cell( gp, i, j, !G_OPENED, !G_FOUND, value, NULL );

    if (j == ncols-1) {     // last col of current row has been reached
        j=0;
        i++;
    }
    else                    // last col of current row has NOT been reached
        j++;
}

Snippet #2 (parse as 2D array, using pointers only)

gridType *gp1, *gp2;

if ( !grid )
    return;

for (gp1=grid; gp1 < grid+nrows; gp1+=ncols)
    for (gp2=gp1; gp2 < gp1+ncols; gp2++)
        set_cell( gp2, (gp1-grid), (gp2-gp1), !G_OPENED, !G_FOUND, value, NULL );

Snippet #3 (parse as 2D, using counters only)

register int i,j;           // we need to pass those in set_cell()

for (i=0; i<nrows; i++)
    for (j=0; j<ncols; j++)
        set_cell( &grid[i * ncols + j], i, j, !G_OPENED, !G_FOUND, value, NULL);

Free memory

free( grid );

EDIT:
I fixed #2 form gp1++) to gp1+=ncols), in the 1st loop, after Paul’s correction (thx!)

Answer 1

This is the way I’d write it. IMHO it’s shorter, clearer and simpler than any of your ways.

int i, j;
gridType *gp = grid;

for (i = 0; i < nrows; i++)
    for (j = 0; j < ncols; j++)
        set_cell( gp++, i, j, !G_OPENED, !G_FOUND, value, NULL );