# [Numpy-discussion] Multiplicity of an entry

josef.pktd@gmai... josef.pktd@gmai...
Mon Oct 26 13:26:12 CDT 2009

```On Mon, Oct 26, 2009 at 2:12 PM, Christopher Barker
<Chris.Barker@noaa.gov> wrote:
> Alan G Isaac wrote:
>> On 10/26/2009 4:04 AM, Nils Wagner wrote:
>>> how can I obtain the multiplicity of an entry in a list
>>> a = ['abc','def','abc','ghij']
>>
>> That's a Python question, not a NumPy question.
>
> but we can make it a numpy question!
>
> In [15]: a = np.array(['abc','def','abc','ghij'])
>
>
> In [16]: a
> Out[16]:
> array(['abc', 'def', 'abc', 'ghij'],
>       dtype='|S4')
>
> In [17]: for item in set(a):
>     print item, (a == item).sum()

It's *very* slow, when there are a large number of items.
numpy creates the full boolean array for each item.

Josef

>
> abc 2
> ghij 1
> def 1
>
> I'll leave pro=filing to the OP.
>
> -Chris
>
>
>
> --
> Christopher Barker, Ph.D.
> Oceanographer
>
> Emergency Response Division
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>
> Chris.Barker@noaa.gov
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```