[FIXED] Use of loops (example, for) in the conditional arguments of numpy.where

Issue

I have a matrix "x" and an array "index". I just want to find the position (row number) of the "index" array in the matrix "x". Example:

x = np.array([[0, 0],
       [1, 0],
       [2, 0],
       [0, 1],
       [1, 1],
       [2, 1],
       [0, 2],
       [1, 2],
       [2, 2],
       [0, 3],
       [1, 3],
       [2, 3],])

index = [2,1]

Here if I use the code: np.where((x[:,0]==index[0]) & (x[:,1]==index[1]))[0] it is working.

But if I have the matrix "x" with N number of columns (instead of 2), I have to use loop inside the np.where arguments. I tried this: np.where((for b in range(2):(x[:,b]==index[b]) & (x[:,b]==index[b])))[0]

Then it shows "invalid syntax" error. Can you please help me regarding this? Thanks in advance.

Solution

The where is only as good as its argument, which is evaluated in full before being passed to the where function:

In [292]: np.where((x[:,0]==index[0]) & (x[:,1]==index[1]))[0]
Out[292]: array([5], dtype=int64)

The condition is a boolean array:

In [293]: (x[:,0]==index[0]) & (x[:,1]==index[1])
Out[293]: 
array([False, False, False, False, False,  True, False, False, False,
       False, False, False])

Looks like you tried to create a for loop:

for b in range(2):
      (x[:,b]==index[b]) & (x[:,b]==index[b])

Using that as argument is not valid python. You could create a function that does

def foo(x,index):
    res = []
    for b in ....
         res.append(...)
    return res

But a simpler syntax is list comprehension:

In [294]: [x[:,i]==index[i] for i in range(2)]
Out[294]: 
[array([False, False,  True, False, False,  True, False, False,  True,
        False, False,  True]),
 array([False, False, False,  True,  True,  True, False, False, False,
        False, False, False])]

and the arrays can be combined with a np.all:

In [295]: np.all([x[:,i]==index[i] for i in range(2)], axis=0)
Out[295]: 
array([False, False, False, False, False,  True, False, False, False,
       False, False, False])

But as others show you don't need to iterate. Let the (n,2) x broadcast against the (2,) index:

In [296]: x==index
Out[296]: 
array([[False, False],
       [False, False],
       [ True, False],
       [False,  True],
       [False,  True],
       [ True,  True],
       [False, False],
       [False, False],
       [ True, False],
       [False, False],
       [False, False],
       [ True, False]])

In [297]: (x==index).all(axis=1)
Out[297]: 
array([False, False, False, False, False,  True, False, False, False,
       False, False, False])

Answered By - hpaulj