Set a column in numpy array to zero

Question

I want to set a column in numpy array to zero at different times, in other words, I have numpy array M with size 5000x500. When I enter shape command the result is (5000,500), I think 5000 are rows and 500 are columns

shape(M)
(5000,500)

But the problem when I want to access one column like first column

Mcol=M[:][0]

Then I check by shape again with new matrix Mcol

shape(Mcol)
(500,)

I expected the results will be (5000,) as the first has 5000 rows. Even when changed the operation the result was the same

shape(M)
(5000,500)
Mcol=M[0][:]
shape(Mcol)
(500,)

Any help please in explaining what happens in my code and if the following operation is right to set one column to zero

M[:][0]=0

What about doing M[:,0] = 0

Chris
– Chris

2016-05-16 11:02:23 +00:00
Commented May 16, 2016 at 11:02 — Chris
– Chris, Commented May 16, 2016 at 11:02

John Zwinck · Accepted Answer · 2019-02-13 13:20:05Z

12

You're doing this:

M[:][0] = 0

But you should be doing this:

M[:,0] = 0

The first one is wrong because M[:] just gives you the entire array, like M. Then [0] gives you the first row.

Similarly, M[0][:] gives you the first row as well, because again [:] has no effect.

edited Feb 13, 2019 at 13:20

answered May 16, 2016 at 11:01

John Zwinck

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1 Comment

Tommy Over a year ago

this answer doesn't actually answer the title of this question, only partially when combined with OPs post.

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