I have a numpy array as:
groups=np.array([('Species1',), ('Species2', 'Species3')], dtype=object).
When I ask np.where(groups == ('Species2', 'Species3')) or even np.where(groups == groups[1]) I get an empty reply: (array([], dtype=int64),)
Why is this and how can I get the indexes for such an element?
The problem here is probably the way array.__contains__() is implemented.
See here. Basically the issue is that
print(('Species2', 'Species3') in groups)
prints False.
If you want to use the numpy.where function nonetheless, and not a for loop as the other answer suggests, it is probably best to somehow construct a suitable truth mask. For example
x = np.array(list(map(lambda x: x== ('Species2', 'Species3'), groups)))
print(np.where(x))
gives the correct result. There might be a more elegant way though.
It's not means search a tuple('Species2', 'Species3') from groups when you use
np.where(groups == ('Species2', 'Species3'))
it means search 'Species2' and 'Species3' separately if you have a Complete array like this
groups=np.array([('Species1',''), ('Species2', 'Species3')], dtype=object)
Yes you can search it but not with the np.where but with the hep of for loop and if-else
for index,var in enumerate(groups):
if var == ('Species2', 'Species3'):
print("('Species2', 'Species3') -->>", index)
else:
print("('Species1',) -->>", index)
Output
('Species1',) -->> 0
('Species2', 'Species3') -->> 1
Your array has two tuples:
In [53]: groups=np.array([('Species1',), ('Species2', 'Species3')], dtype=object)
In [54]: groups
Out[54]: array([('Species1',), ('Species2', 'Species3')], dtype=object)
In [55]: groups.shape
Out[55]: (2,)
But be careful with that kind of definition. If the tuples were all the same size, the array would have a different shape, and the elements would no longer be tuples.
In [56]: np.array([('Species1',), ('Species2',), ('Species3',)], dtype=object)
Out[56]:
array([['Species1'],
['Species2'],
['Species3']], dtype=object)
In [57]: _.shape
Out[57]: (3, 1)
Any use of where is only as good as the boolean array given to it. This where returns empty because the equality test produces all False:
In [58]: np.where(groups == groups[1])
Out[58]: (array([], dtype=int64),)
In [59]: groups == groups[1]
Out[59]: array([False, False])
If I use a list comprehension to compare the group elements:
In [60]: [g == groups[1] for g in groups]
Out[60]: [False, True]
In [61]: np.where([g == groups[1] for g in groups])
Out[61]: (array([1]),)
But for this sort of thing, a list would be just as good
In [66]: alist = [('Species1',), ('Species2', 'Species3')]
In [67]: alist.index(alist[1])
Out[67]: 1
In [68]: alist.index(('Species1',))
Out[68]: 0
In [69]: alist.index(('Species2',))
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-69-0b16b56ad28c> in <module>
----> 1 alist.index(('Species2',))
ValueError: ('Species2',) is not in list
