Recommended syntax for slicing arrays

[josephtoles]

I use Hy for data science. This requires a lot of cutting/slicing. In particular, I frequently have to cut a list or an array from an index until the end. In Python, the way to slice an array from an index to the end is to leave the second index argument of a slice empty as in arra[5:]. In the Hy stable release, I used cut with a single argument.

This no longer works in Hy's alpha. According to the Hy alpha's changelog, "(cut x n) gets the first n elements instead of everything after the first n."

What is the recommended method for getting everything after the first n elements?

[allison-casey]

cut was changed to have the same syntax as slice which is what python's : sugar compiles down to. So arr[5:] actually compiles down to arr[slice(5, None)]. Now that cut is equivalent all the following are identical.

(setv x [1 2 3 4 5 6 7])
(get x (slice 5 None))
;; => [6 7]
(cut x 5 None)
;; => [6 7]

x[5:]
# => [6, 7]
x[slice(5, None)]
# => [6, 7]

If you need more complex slicing mechanics (or more "Pythonic"), and you're willing to use the master branch Hy version, you can check out Hyrule's #: and ncut macros. Specifically ncut for you since it allows the sugared n-dimensional slicing that numpy and pandas use frequently

(get x #: 5:)
;; => [6 7]
(ncut x 5:)
;; => [6 7]

[josephtoles]

Thank you so much. I didn't realize I could add None as the second argument to slice. I also didn't know what ncut did since I only read the stable version's documentation.