[Numpy-discussion] seeking advice on a fast string->array conversion

Darren Dale dsdale24@gmail....
Tue Nov 16 09:31:31 CST 2010


On Tue, Nov 16, 2010 at 9:55 AM, Pauli Virtanen <pav@iki.fi> wrote:
> Tue, 16 Nov 2010 09:41:04 -0500, Darren Dale wrote:
> [clip]
>> That loop takes 0.33 seconds to execute, which is a good start. I need
>> some help converting this example to return an actual numpy array. Could
>> anyone please offer a suggestion?
>
> Easiest way is probably to use ndarray buffers and resize them when
> needed. For example:
>
> https://github.com/pv/scipy-work/blob/enh/interpnd-smooth/scipy/spatial/qhull.pyx#L980

Thank you Pauli. That makes it *incredibly* simple:

import time
cimport numpy as np
import numpy as np

cdef extern from 'stdlib.h':
    double atof(char*)


def test():
    py_string = '100'
    cdef char* c_string = py_string
    cdef int i, j
    cdef double val
    i = 0
    j = 2048*1200
    cdef np.ndarray[np.float64_t, ndim=1] ret

    ret_arr = np.empty((2048*1200,), dtype=np.float64)
    ret = ret_arr

    d = time.time()
    while i<j:
        c_string = py_string
        ret[i] = atof(c_string)
        i += 1
    ret_arr.shape = (1200, 2048)
    print ret_arr, ret_arr.shape, time.time()-d

The loop now takes only 0.11 seconds to execute. Thanks again.


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