selecting specific part of array

Question

Lets say I have an array

a = [[[1, 2], [3, 4]],
     [[5, 6], [7, 8]], 
     [[9, 10], [11, 12]],
     [[13, 14], [15, 16]],
     [[17, 18], [19, 20]]]

At the end I would like an array

b = [[[5, 6]],[[9, 10]]]

I thought that

b = a[1:3][0:1]

would work. But from that i get

b=[[[5, 6], [7, 8]]]

If you're going to be doing a lot of matrix manipulation, you should consider using `numpy`, which lets you do slices like this much more naturally — Patrick Haugh, Nov 16 '17 at 22:26
Welcome to SO! Please consider marking the correct answer as correct. Best of luck! — alexisdevarennes, Nov 16 '17 at 22:30
I do also use `numpy` . But how would it change things in this example? — user3527668, Nov 16 '17 at 22:45
@user3527668 because you don't have a any arrays here, only `list` objects. `list` objects do not have dimensions, what you essentially want is multi-dimensional slicing, which `numpy.ndarray` supports. — juanpa.arrivillaga, Nov 16 '17 at 22:48
So note, if you did `import numpy; arr = numpy.array(a); print(arr[1:3,0:1])` — juanpa.arrivillaga, Nov 16 '17 at 22:49

score 0 · Accepted Answer · edited Nov 17 '17 at 00:02

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so you want [[a[1][0]],[a[2][0]]? Well, for one, I'd figure out another way of writing your arrays to make them easier to parse (visually and mechanically.) But, in response to your question, I'd use a list comprehension. [a[i][0] for i in [1,2]] would work, I believe.

edited Nov 17 '17 at 00:02

eugenhu

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answered Nov 16 '17 at 22:24

Jakob Lovern

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For list comprehension, `[x[0] for x in a[1:3]]` could be simpler. – eugenhu Nov 16 '17 at 22:30

score 0 · Answer 2 · answered Nov 16 '17 at 22:25

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>>> b = [[a[1][0],a[2][0]]]
>>> print(b)
[[[5, 6], [9, 10]]]

answered Nov 16 '17 at 22:25

alexisdevarennes

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score 0 · Answer 3 · answered Nov 16 '17 at 22:40

You can also see the answer to this question about transposing lists of lists, and do something like:

at = list(map(list, zip(*a)))
b = at[0][1:3]

Though as someone has mentioned in the comments, if you're going to be doing a lot of matrix manipulation, you'd be better off using NumPy.

selecting specific part of array

3 Answers3