[Numpy-discussion] index of the first element fulfilling a condition?
Erin Sheldon
erin.sheldon@gmail....
Thu Aug 19 20:05:50 CDT 2010
Excerpts from Johann Bauer's message of Wed Aug 18 14:07:48 -0400 2010:
> Hi,
>
> is there a good method to find the index of the first element in a 1D
> array fulfilling a condition?
>
> The following does the job
>
> >>> import numpy
> >>> a = numpy.array([1,5,78,3,12,4])
> >>> numpy.where( a>10 )[0][0]
> 2
>
> but it first compares the entire array and then selects the first index.
> Is there a way (especially for big arrays) to stop the comparison after
> the first hit?
>
> Thanks for any help, Johann
If you are willing to use scipy.weave, it is straightforward:
import numpy
from scipy import weave
def get_first_greater(arr, value):
"""
Return index of first element in array that is greater than value.
"""
code = """
int64_t i=0, index=-1;
while (i < arr.size()) {
if ( arr(i) > value ) {
index = i;
break;
}
i++;
}
return_val = index;
"""
index = weave.inline(code, ['arr','value'],
type_converters = weave.converters.blitz)
return index
Caveats: In scipy 0.7 and gcc >= 4.3 this probably won't compile. It
does work for scipy 0.8.
Erin Scott Sheldon
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