I'm new to numpy, I don't understand how the following works:
np.array([range(i, i + 3) for i in [2, 4, 6]])
and the output is:
array([[2, 3, 4],[4, 5, 6],[6, 7, 8]])
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1 Answer
Do you understand list comprehensions? range?
In [12]: [range(i, i + 3) for i in [2, 4, 6]]
Out[12]: [range(2, 5), range(4, 7), range(6, 9)]
np.array converts the range objects to lists, and then builds the array.
In [13]: [list(range(i, i + 3)) for i in [2, 4, 6]]
Out[13]: [[2, 3, 4], [4, 5, 6], [6, 7, 8]]
In [14]: np.array([list(range(i, i + 3)) for i in [2, 4, 6]])
Out[14]:
array([[2, 3, 4],
[4, 5, 6],
[6, 7, 8]])
So basically it's just a variation on the textbook example of making an array from a list of lists:
In [15]: np.array([[1,2,3],[10,11,12]])
Out[15]:
array([[ 1, 2, 3],
[10, 11, 12]])
2 Comments
user3232844
I understand the In & Out [12]. What I don't understand is if range is (i, i+3) for i in [2, 4, 6], how is the array outputting [2, 3, 4], [4, 5, 6], [6, 7, 8]?
hpaulj
It's
range(i, i+3). So if i is 2, we get range(2,5), which expanded to a list is [2,3,4].