Access several columns in an array in Python

Question

I'm new to python and numpy. Suppose I have an array A = np.random.rand(10,100) , how do i access some of its columns easily?

In Matlab I would write something like B = A(:,[2,4,6:10,50:80]). What is the equivalent of this line of code in Python?

I've looked at the examples in http://scipy.github.io/old-wiki/pages/NumPy_for_Matlab_Users.html but none of them answer my question.

See docs.scipy.org/doc/numpy/reference/arrays.indexing.html, and numpy.r_[] — Benjamin
– Benjamin, Commented Jun 22, 2016 at 19:51
Well, one way would be to use range to simulate the colon operation and then just index into cols : A[:,np.hstack((2,4,range(6,10+1),range(50,80+1)))]. — Divakar
– Divakar, Commented Jun 22, 2016 at 19:53
@Divakar I've actually also tried this way, but I believed there must be some method simpler than it :) — F.S.
– F.S., Commented Jun 22, 2016 at 21:46

hpaulj · Accepted Answer · 2016-06-22 20:21:47Z

2

As Benjamin suggested, np.r_ handles an expression like this nicely

In [1329]: np.r_[2,4,6:10,50:80]
Out[1329]: 
array([ 2,  4,  6,  7,  8,  9, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60,
       61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77,
       78, 79])

which can be used as:

B = A[:, np.r_[2,4,6:10,50:80]]

r_ expands the slices into ranges and concatenates everything together, as in Divakar's comment.

answered Jun 22, 2016 at 20:21

hpaulj

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3 Comments

Divakar Over a year ago

Ah didn't know that colon could be used within np.r_[], nice!

hpaulj Over a year ago

There are more features in r_ (and c_) than most of us need. I've never had to use the first argument string. The j step is just a cute way of using linspace instead of arange. If features are useful, great. But if you don't understand them, don't worry.

F.S. Over a year ago

This is exactly the information I'm looking for. I believe the link Benjamin provides is a good knowledge base. For now it seems overwhelming to me. I'm still trying to understand r_[] and c_[]

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