Filter an array in Python with 2 conditions

Question

How to filter an array A according to two conditions ?

A = array([1, 2.3, 4.3, 10, 23, 42, 23, 12, 1, 1])
B = array([1, 7, 21, 5, 9, 12, 14, 22, 12, 0])
print A[(B < 13)]      # here we get all items A[i] with i such that B[i] < 13
print A[(B > 5) and (B < 13)]   # here it doesn't work
                                # how to get all items A[i] such that B[i] < 13 
                                # AND B[i] > 5 ?

The Error is:

ValueError: The truth value of an array with more than one element is ambiguous.

Acorbe · Accepted Answer · 2013-11-19 15:23:56Z

5

You should use the bitwise (thanks @askewchan) version of the operator and which is &.

i.e.

 print A[(B > 5) & (B < 13)]

edited Nov 19, 2013 at 15:23

answered Nov 19, 2013 at 12:51

Acorbe

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2 Comments

askewchan Over a year ago

In numpy (and python in general), & isn't a set operator, actually it's a bitwise operator. Here, it works only because we're comparing two boolean arrays as a string of bits with 1-bit elements. It is the equivalent to saying np.bitwise_and, not np.logical_and as @larsmans has answered. You might find the behavior unexpected if you do something like np.array([0,1,2]) & np.array([True, True, True])

Acorbe Over a year ago

@askewchan, thanks for your explanation. I corrected my answer.

Fred Foo · Accepted Answer · 2013-11-19 15:17:22Z

5

>>> A[np.logical_and(B > 5, B < 13)]
array([  2.3,  23. ,  42. ,   1. ])

edited Nov 19, 2013 at 15:17

answered Nov 19, 2013 at 12:51

Fred Foo

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