Error with multiple numpy.delete uses on array?

Question

So, I was trying to understand the numpy.delete function, and I came up with something weird. Here's the program:

>>>import numpy as np  
>>>a = np.arange(10)  
>>> a  
array([0, 1, 2, 3, 4, 6, 7, 9])  
>>> a[5]  
5  
>>> a=np.delete(a,[a[5]])  
>>> a  
array([0, 1, 2, 3, 4, 6, 7, 8, 9])  #so far so good  
>>> a[6]  
7  
>>> a=np.delete(a,[a[6]])  
>>> a  
array([0, 1, 2, 3, 4, 6, 7, 9])

So... When I put a=np.delete(a,[a[6]]), it should be expected to remove the number 7 from the array, right? Why was it the number 8 (the term a[7]) from the array) removed instead of the expected a[6]?
I also noticed that when I try to remove the a[0](=0) from the array after the first delete, I just can't. It always removes one term ahead. Any Idea how do I remove it?

Noam · Accepted Answer · 2016-08-04 00:07:12Z

1

The second argument should be the index of the element you want to delete, not the element itself.

a=np.delete(a,6)

In the first case, it only worked because a[5] happened to equal 5, so the index and the value were the same.

When you have:

a=np.delete(a,[a[6]])

You are deleting the 7th element since a[6] = 7 there.

answered Aug 4, 2016 at 0:07

Noam

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2 Comments

Wellington Júnior Over a year ago

Thank you very much. I'm new to this function and thought the a[i] was the parameter. But it makes sense now. Thanks!

hpaulj Over a year ago

np.delete is not the same as list remove.

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