[Numpy-discussion] Find indices of largest elements

Nikolaus Rath Nikolaus@rath....
Thu Apr 15 14:41:29 CDT 2010


Keith Goodman <kwgoodman@gmail.com> writes:
> On Wed, Apr 14, 2010 at 12:39 PM, Nikolaus Rath <Nikolaus@rath.org> wrote:
>> Keith Goodman <kwgoodman@gmail.com> writes:
>>> On Wed, Apr 14, 2010 at 8:49 AM, Keith Goodman <kwgoodman@gmail.com> wrote:
>>>> On Wed, Apr 14, 2010 at 8:16 AM, Nikolaus Rath <Nikolaus@rath.org> wrote:
>>>>> Hello,
>>>>>
>>>>> How do I best find out the indices of the largest x elements in an
>>>>> array?
>>>>>
>>>>> Example:
>>>>>
>>>>> a = [ [1,8,2], [2,1,3] ]
>>>>> magic_function(a, 2) == [ (0,1), (1,2) ]
>>>>>
>>>>> Since the largest 2 elements are at positions (0,1) and (1,2).
>>>>
>>>> Here's a quick way to rank the data if there are no ties and no NaNs:
>>>
>>> ...or if you need the indices in order:
>>>
>>>>> shape = (3,2)
>>>>> x = np.random.rand(*shape)
>>>>> x
>>> array([[ 0.52420123,  0.43231286],
>>>        [ 0.97995333,  0.87416228],
>>>        [ 0.71604075,  0.66018382]])
>>>>> r = x.reshape(-1).argsort().argsort()
>>
>> I don't understand why this works. Why do you call argsort() twice?
>> Doesn't that give you the indices of the sorted indices?
>
> It is confusing. Let's look at an example:
>
>>> x = np.random.rand(4)
>>> x
>    array([ 0.37412289,  0.68248559,  0.12935131,  0.42510212])
>
> If we call argsort once we get the index that will sort x:
>
>>> idx = x.argsort()
>>> idx
>    array([2, 0, 3, 1])
>>> x[idx]
>    array([ 0.12935131,  0.37412289,  0.42510212,  0.68248559])
>
> Notice that the first element of idx is 2. That's because element x[2]
> is the min of x. But that's not what we want.

I think that's exactly what I want, the index of the smallest element.
It also seems to work:

In [3]: x = np.random.rand(3,3)
In [4]: x
Out[4]: 
array([[ 0.49064281,  0.54989584,  0.05319183],
       [ 0.50510206,  0.39683101,  0.22801874],
       [ 0.04595144,  0.3329171 ,  0.61156205]])
In [5]: idx = x.reshape(-1).argsort()
In [6]: [ np.unravel_index(i, x.shape) for i in idx[-3:] ]
Out[6]: [(1, 0), (0, 1), (2, 2)]


So why the additional complication with the second argsort? I just don't
get it...

> We want the first
> element to be the rank of the first element of x.

I'm not quite sure why we want that...?


Best,

   -Nikolaus

-- 
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