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I have an array a = [[1,1],[2,2],[3,3]]. How could I reduce it to b = [1,2,3]?

Adath Skyderson
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    Are the values in the inner lists always identical? i.e. `[x, x]`? If yes, why do you even have a nested list like that? – Aran-Fey Oct 21 '18 at 17:47
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    How exactly do you want it to be reduced? Say you had a list `a = [[1, 2], [3, 4], [5, 6]]`, or perhaps `[[1, 2], [3], [5, 4]]`. Would you want out of these `[1, 3, 5]`, or something else? –  Oct 21 '18 at 17:47
  • ...if so, then the [list comprehension](https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions) `[inner_list[0] for inner_list in a]` will do the trick, but I don't know if that is indeed what you want. –  Oct 21 '18 at 17:48
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    the elements are always the same – Adath Skyderson Oct 21 '18 at 17:48
  • a = [a[i][0] for i in range(len(a))] – Rarblack Oct 21 '18 at 18:16
  • `np.array(a)[:, 0]` – Shivid Oct 22 '18 at 08:47

1 Answers1

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If the inner array is always two of the same values.

a = [[1,1],[2,2],[3,3]]
b = [i for i,j in a]

This produces:

b = [1,2,3]
Christian Sloper
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    And if the inner lists can be of different lengths (but can't be empty): `[head for head, *tail in a]`. This is especially useful for lists of generic iterables. If the elements are indexable, just `[sub[0] for sub in a]` should do. – timgeb Oct 21 '18 at 17:54
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    I like that @timgeb – Christian Sloper Oct 21 '18 at 17:55