Make subset of array, based on values of two other arrays in Python

Question

I am using Python. How to make a subselection of a vector, based on the values of two other vectors with the same length?

For example this three vectors

c1 = np.array([1,9,3,5])
c2 = np.array([2,2,3,2])
c3 = np.array([2,3,2,3])

c2==2
array([ True,  True, False,  True], dtype=bool)
c3==3
array([False,  True, False,  True], dtype=bool)

I want to do something like this:

elem  = (c2==2 and c3==3)
c1sel = c1[elem]

But the first statement results in an error:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: The truth value of an array with more than one element is ambiguous. 
Use a.any() or a.all()

In Matlab, I would use:

elem  = find(c2==2 & c3==3);
c1sel = c1(elem);

How to do this in Python?

Ashwini Chaudhary · Accepted Answer · 2014-04-16 12:25:47Z

5

You can use numpy.logical_and:

>>> c1[np.logical_and(c2==2, c3==3)]
array([9, 5])

answered Apr 16, 2014 at 12:25

Ashwini Chaudhary

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1 Comment

vincentv Over a year ago

As far as I can see, this does not work for more than two conditions. For three or more, I use the solution of mskimm.

emesday · Accepted Answer · 2014-04-16 12:52:51Z

3

Alternatively, try

>>> c1[(c2==2) & (c3==3)]
array([9, 5])

cf) By Python Operator Precedence, the priority of & is upper than ==. See the follow results.

>>> 1 == 1 & 2 == 2
False

>>> (1 == 1) & (2 == 2)
True

edited Apr 16, 2014 at 12:52

answered Apr 16, 2014 at 12:32

emesday

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2 Comments

vincentv Over a year ago

I still don't understand the difference between 'and' and '&'. I will read more about it.

emesday Over a year ago

see stackoverflow.com/questions/3845018/…

logc · Accepted Answer · 2014-04-16 12:33:20Z

1

You have to keep each of your conditions inside parenthesis:

In []: c1[(c2 == 2) & (c3 == 3)]
Out[]: array([9, 5])

answered Apr 16, 2014 at 12:33

logc

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