[Numpy-discussion] an indexing question

Jaime Fernández del Río jaime.frio at gmail.com
Wed Jan 8 14:32:19 EST 2014


On Wed, Jan 8, 2014 at 11:12 AM, Neal Becker <ndbecker2 at gmail.com> wrote:

> I have a 1d vector d.  I want compute the means of subsets of this vector.
> The subsets are selected by looking at another vector s or same shape as d.
>
> This can be done as:
>
> [np.mean (d[s == i]) for i in range (size)]
>
> But I think this could be done directly with numpy addressing, without
> resorting
> to list comprehension?
>

You could get it done with np.bincount:

d_sums = np.bincount(s, weights=d)
d_counts = np.bincount(s)
d_means = d_sums / d_counts

Jaime

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