CODEWITHSUNDEEP
pythonlistpython-3.xnumpyslice
user71346
user71346

Reputation: 723

For an array a and b in Python, what is the code a[b[0:2]] is actually trying to do?

Could somebody please help on explaining the meaning of this error message?

There is this part of lines of code that I am trying to understand. So I experimented myself with an easier example.

I have 

a = array([[0, 1],  
       [2, 3],  
       [4, 5]])

and

b = [1,3,5,7]

I wrote 

a[b[0:2]]

and there is an error comment :

index 3 is out of bounds for axis 0 with size 3

I understand what does b[0:2] means, it means you take the element of b with index 0 to index 1, so you get [1,3]. But I don't quite understand what does it mean when you pass it to a?

I am trying to understand what is the code a[b[0:2]] is actually trying to do.

Could somebody please lend a help? Thanks!

Upvotes: 0

Views: 6010

Answers (1)

Yaron
Yaron

Reputation: 10450

>>> a = np.array([[0, 1],  [2,3],[4,5]])
>>> a2 = np.array([[0, 1],  [2,3],[4,5],[6,7]])

Two arrays:

a - with elements number 0 to 2

a2 - with elements number 0 to 3

>>> b[0:2]
[1, 3]

b[0:2] - means two elements (#1 and #3)

>>> a[b[0:2]]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: index 3 is out of bounds for axis 0 with size 3

means - you try to get elements #1 and #3 from a. But you don't have element #3 in a

>>> a2[b[0:2]]
array([[2, 3],
       [6, 7]])
>>>

means - you try to get elements #1 and #3 from a2. the result is: two elements #1 is [2,3] and #3 is [6,7]

Upvotes: 3

Related Questions