How does the following Python expression work?

Question

I have got the following python expression in my code

import numpy as np  
a = np.array([1,0,1,0])  
b = np.array([True,False,False,True])
print a[b],b[a]

The output i am getting looks like this:

[1 0] [False  True False  True]

I am not able to understand how this output is generated.
Can anybody explain it.

I would suggest you learn the basics of numpy. After that, you will figure out the answer to this problem. — coder.in.me
– coder.in.me, Commented Nov 20, 2016 at 14:32

blue note · Accepted Answer · 2016-11-20 15:36:18Z

5

They are two difference cases

a[b] is logical indexing. The index b must be a boolean array, the same size as a. Each boolean value of b mean take/leave this element of a. That is: for each pair of values in a_i, b_i in vectors a, b: If b_i == True, add a_i to the output vector, otherwise ignore it.

b[a] is indexing with multiple values. Just like in normal python you can do b[0] to take the first value of b, in numpy you can use an array as index, to take multiple elements. in your case, this produces the same result as [b[1], b[0], b[1], b[0]]

edited Nov 20, 2016 at 15:36

answered Nov 20, 2016 at 14:34

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2 Comments

aamer aamer Over a year ago

Thanks for the answer. I got the second case, but a[b] still demands some explanation. I didn't understand the meaning of your statement: 'Each boolean value of b mean take/leave this element of a'.

aamer aamer Over a year ago

Yes got it. Thanks for the explanation. Because i am new, u will have to wait for the upvote i already have done for ur answer. It needs me to have some more reputations to do that. So, jus wait. Ur upvote to my question may help me gain some more reputations. Thanks :)

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