[SciPy-User] Picking a random element of an array with conditions

[Keith Goodman]

On Fri, Jun 11, 2010 at 11:21 AM, Keith Goodman <kwgoodman at gmail.com> wrote:
> On Fri, Jun 11, 2010 at 11:16 AM, Keith Goodman <kwgoodman at gmail.com> wrote:
>> On Fri, Jun 11, 2010 at 11:12 AM, R. Padraic Springuel
>> <R.Springuel at umit.maine.edu> wrote:
>>> I'd like to pick the random element of an array from those elements
>>> which meet a certain condition.  I.e. pick an element of a for which a
>>> == value is True.
>>>
>>> Without the condition, I'd phrase the command like this:
>>> a[random.randint(len(a))]
>>>
>>> Is there some similar thing that I can do to pick with the condition in
>>> an efficient manner?  So far all I've come up with involves looping over
>>> the array to construct an array of indecies so that I can write:
>>> a[indecies[random.randint(len(indecies))]]
>>
>> How about:
>>
>>>> a = np.random.rand(10)
>>>> idx = a > 0.5
>>>> a[idx[np.random.randint(10)]]
>>   0.58803647603961251
>
> Oh, sorry, that doesn't work since idx is bool. How about:
>
>>> a = np.random.rand(10)
>>> idx = np.where(a > 0.5)[0]
>>> a[idx[np.random.randint(idx.size)]]
>   0.94308730304099841

And here's the nd case:

>> a = np.random.rand(10,10)
>> idx = np.where(a.flat > 0.5)[0]
>> a.flat[idx[np.random.randint(idx.size)]]
   0.6073571170281532