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NON-HOMEWORK

I have list of floats x, and I want to turn it into list y, a list of all the tenth elements in x.

For my own reasons, I really want to do this in a tiny amount of lines. I came up with something like this:

i = 0
y = filter(lambda x: (++i)%10; x)

Theoretically this should work, i is already defined, and the ++i would typically add one to variable i, then go about the expression.

Unfortunately, ++ doesn't exist in Python.

Any Pythonic ways to go about this?

Another idea I had was to use a map, and have the expression push elements onto list y.

Let me know if I can be more clear.

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3 Answers 3

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15

Use Extended slices:

>>> x = range(1,101)
>>> y = x[9::10] # 9 -> start from 10th, 10: step
>>> y
[10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
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1 Comment

Beautiful. Hadn't seen this before, as, though I've been using Python for a while, I haven't fully explored a lot of the more Pythonic ways of doing things. Thanks!
3

What about [value for index, value in enumerate(list_of_floats) if index % 10 == 0]?

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It's worth noting that slicing is better in this case. However, this technique is worth knowing in cases where the filtering criterion is more complex.
0

Alternative that use itertools.count:

>>> x = range(1, 101)
>>> i = itertools.count(1)
>>> y = filter(lambda item: next(i) % 10 == 0, x)
>>> y
[10, 20, 30, 40, 50, 60, 70, 80, 90, 100]
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