Select smaller array of arrays

Question

In python I have an array of arrays:

[[1,2,3,4,5,6,7,8,6,5,4,3],
 [3,4,5,6,7,8,9,5,3,2,4,2],
 [3,4,5,6,7,8,9,5,3,2,4,2],
 [3,4,5,6,7,8,9,5,3,2,4,2],
 [3,4,5,6,7,8,9,5,3,2,4,2]]

And I want to select for each array in array only firs n elements. For example for n == 4 I will get:

    [[1,2,3,4],
     [3,4,5,6],
     [3,4,5,6],
     [3,4,5,6],
     [3,4,5,6]]

What is the most short code in Python for this?

Your question history suggests you may be using NumPy. If so, please make that clear in your question. NumPy arrays and Python lists are very different, and the appropriate ways of working with them are almost nothing alike. — user2357112
– user2357112, Commented Nov 27, 2016 at 9:30

roman · Accepted Answer · 2016-11-27 09:27:40Z

1

If you have list of lists then pure python will do:

>>> a = [[1,2,3,4,5,6,7,8,6,5,4,3],
...  [3,4,5,6,7,8,9,5,3,2,4,2],
...  [3,4,5,6,7,8,9,5,3,2,4,2],
...  [3,4,5,6,7,8,9,5,3,2,4,2],
...  [3,4,5,6,7,8,9,5,3,2,4,2]]
>>> [x[:4] for x in a]
[[1, 2, 3, 4], [3, 4, 5, 6], [3, 4, 5, 6], [3, 4, 5, 6], [3, 4, 5, 6]]

If it's numpy arrays you can use numpy indexing:

>>> a
array([[1, 2, 3, 4, 5, 6, 7, 8, 6, 5, 4, 3],
       [3, 4, 5, 6, 7, 8, 9, 5, 3, 2, 4, 2],
       [3, 4, 5, 6, 7, 8, 9, 5, 3, 2, 4, 2],
       [3, 4, 5, 6, 7, 8, 9, 5, 3, 2, 4, 2],
       [3, 4, 5, 6, 7, 8, 9, 5, 3, 2, 4, 2]])
>>> a[:,:4]
array([[1, 2, 3, 4],
       [3, 4, 5, 6],
       [3, 4, 5, 6],
       [3, 4, 5, 6],
       [3, 4, 5, 6]])

answered Nov 27, 2016 at 9:27

roman

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Comments

neverwalkaloner · Accepted Answer · 2016-11-27 09:38:18Z

0

You can use list comprehension:

l = [[1,2,3,4,5,6,7,8,6,5,4,3],
[3,4,5,6,7,8,9,5,3,2,4,2],
[3,4,5,6,7,8,9,5,3,2,4,2],
[3,4,5,6,7,8,9,5,3,2,4,2],
[3,4,5,6,7,8,9,5,3,2,4,2]]

result = [el[:4] for el in l]

edited Nov 27, 2016 at 9:38

answered Nov 27, 2016 at 9:27

neverwalkaloner

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