[Numpy-discussion] searching a list of arrays

[Michiel Jan Laurens de Hoon]

This is because of how "==" is defined for arrays. For lists, list1==list2 if 
all elements are the same; a boolean value is returned:

 >>> x = [0,1,0]
 >>> x==[0,1,0]
True
 >>> x==[1,0,0]
False

For arrays, "==" does a element-wise comparison:

 >>> from Numeric import *
 >>> x = array([0,1,0])
 >>> x==array([0,1,0])
array([1, 1, 1])
 >>> x==array([1,0,0])
array([0, 0, 1])
 >>>

Now, when you count how often array([0,1,0]) appears in b, actually you evaluate 
element==array([0,1,0]) for each element in b, and count how often you get a 
True, with every array other than array([0,0,0]) regarded as True.

For list a, this happens to work because array([0,1]) and array([1,0]) have no 
elements in common. But in this case:

 >>> a = [array([0,0]),array([0,0]),array([0,1]),array([0,1])]
 >>> a
[array([0, 0]), array([0, 0]), array([0, 1]), array([0, 1])]
 >>> a.count(array([0,0]))
4

you also get the non-intuitive answer 4.

An easy way to get this to work is to use lists instead of arrays:

 >>> b = [[0,1,0], [0,1,0], [1,0,0], [1,0,0]]
 >>> b.count([0,1,0])
2

But I don't know if this solution is suitable for your application.

--Michiel.

Darren Dale wrote:

> Hi,
> 
> I have a list of numeric-23.8 arrays:
> 
> a = [array([0,1]),
>       array([0,1]),
>       array([1,0]),
>       array([1,0])]
> 
> b = [array([0,1,0]),
>       array([0,1,0]),
>       array([1,0,0]),
>       array([1,0,0])]
> 
> and I want to make a new list out of b:
> 
> c = [array([0,1,2]),
>       array([1,0,2])]
> 
> where the last index in each array is the result of
> 
> b.count([0,1,0]) # or [1,0,0]
> 
> 
> The problem is that the result of b.count(array([1,0,0])) is 4, not 2, and 
> b.remove(array([1,0,0])) indescriminantly removes arrays from the list. 
> a.count and a.remove work the way I expected.
> 
> Does anyone know why 1x2 arrays work, but 1x3 or larger arrays do not?
> 
> Thanks,
> Darren
> 
> 
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-- 
Michiel de Hoon, Assistant Professor
University of Tokyo, Institute of Medical Science
Human Genome Center
4-6-1 Shirokane-dai, Minato-ku
Tokyo 108-8639
Japan
http://bonsai.ims.u-tokyo.ac.jp/~mdehoon