[Numpy-discussion] cross

[Lisandro Dalcin]

On 3/3/08, Revaz Yves <yves.revaz@obspm.fr> wrote:
>  I'm computing the cross product of positions and velocities of n points
>  in a 3d space.
>  Using the numpy function "cross", this can be written as :
>  I compare the computation time needed with a C-api I wrote (dedicated to
>  this operation).
>  It appears that my api is in average 20 times faster than the cross
>  function (for n between 100 and 1000000),
>  making the latter useless for my purpose :-( .
>
>  Is it normal ? or I'm I using the "cross" function the wrong way ?

Wel, the numpy 'cross' function is (cleverly) implemented in Python.
However, it internall generate some teporary arrays (associated to
binary operation) with could be the cause of the slowdown.


>
>  yves
>
>
>
>
>  PS :Here after you can see some lines the of the C-api.
>
>
>
>           if (!PyArg_ParseTuple(args, "OO", &pos , &vel))
>               return NULL;
>
>           /* create a NumPy object similar to the input */
>           int   ld[2];
>           ld[0]=pos->dimensions[0];
>           ld[1]=pos->dimensions[1];
>           lxyz = (PyArrayObject *)
>  PyArray_FromDims(pos->nd,ld,pos->descr->type_num);
>
>
>             /* loops over all elements */
>             for (i = 0; i < pos->dimensions[0]; i++) {
>
>               x  = (float *) (pos->data + i*(pos->strides[0])
>    );
>               y  = (float *) (pos->data + i*(pos->strides[0]) +
>  1*pos->strides[1]);
>               z  = (float *) (pos->data + i*(pos->strides[0]) +
>  2*pos->strides[1]);
>
>               vx = (float *) (vel->data +
>  i*(vel->strides[0])                    );
>               vy = (float *) (vel->data + i*(vel->strides[0]) +
>  1*vel->strides[1]);
>               vz = (float *) (vel->data + i*(vel->strides[0]) +
>  2*vel->strides[1]);
>
>               lx = (*y * *vz - *z * *vy);
>               ly = (*z * *vx - *x * *vz);
>               lz = (*x * *vy - *y * *vx);
>
>               *(float *)(lxyz->data + i*(lxyz->strides[0]) +
>  0*lxyz->strides[1])  = lx;
>               *(float *)(lxyz->data + i*(lxyz->strides[0]) +
>  1*lxyz->strides[1])  = ly;
>               *(float *)(lxyz->data + i*(lxyz->strides[0]) +
>  2*lxyz->strides[1])  = lz;
>             }
>
>
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-- 
Lisandro Dalcín
---------------
Centro Internacional de Métodos Computacionales en Ingeniería (CIMEC)
Instituto de Desarrollo Tecnológico para la Industria Química (INTEC)
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