Is there a concise emacs lisp equivalent of Python's [n:m] list slices?

Question

One thing I find myself missing in emacs lisp is, surprisingly, a particular bit of list manipulation. I miss Python's concise list slicing.

>>> mylist = ["foo", "bar", "baz", "qux", "frobnitz"]
>>> mylist[1:4]
['bar', 'baz', 'qux']

I see the functions butlast and nthcdr in the emacs documentation, which would give the same results from code like this:

(setq mylist '("foo" "bar" "baz" "qux" "frobnitz"))
(butlast (nthcdr 1 mylist) 1)
;; ("bar" "baz" "qux")

Is there a more concise way of getting a list slice than combining butlast and nthcdr?

related: http://stackoverflow.com/questions/108169/how-do-i-take-a-slice-of-a-list-a-sublist-in-scheme - the third answer recommends subseq which is a slice; no idea if it is available in emacs lisp though... — l4mpi, Nov 01 '12 at 20:02
Ahah! That's exactly what I was looking for, thank you. `subseq` is, like many other Nice Things, available through emacs's `clmacs.el`. Also, calling it "subseq" is probably why I came up blank Googling for this - I've had terminology issues like that more than once with emacs. Want to make an answer so I can accept it? — Brighid McDonnell, Nov 01 '12 at 20:14
Don't forget that Python `list`s are arrays, while Lisp's are linked lists, so the performance characteristics are very different. If you need to do lots of indexing and subsequencing, especially if the lists can be long, you should be using a different type in Lisp. (For example, a Python slice `mylist[x:y]` is `O(y-x)`, while a Lisp equivalent is `O(y)`: for `mylist[50000,50005]` this means List will be 10000x slower…) — abarnert, Nov 01 '12 at 20:27
Thanks for that clarification, abarnert. Fortunately the problem at hand has lists of manageable size. I'd probably try changing them to vectors if they got larger. — Brighid McDonnell, Nov 01 '12 at 20:55
@SeanM `subseq` is not bad Emacs terminology, it's how Common Lisp calls the function. Common Lisp sequences are ordered collections of elements, which can be implemented as either lists of vectors, analogous to the abstraction that Python also calls sequence. — user4815162342, Nov 01 '12 at 21:49
@user4815162342: I didn't mean that it was bad. By "terminology issues" I meant that I've had several incidents where I was looking for a concept under one name and emacs/lisp knows that concept by another name. — Brighid McDonnell, Nov 01 '12 at 22:25

score 23 · Accepted Answer · edited Sep 11 '18 at 04:45

23

Sure there is:

(require 'cl-lib)
(setq mylist '("foo" "bar" "baz" "qux" "frobnitz"))
(cl-subseq mylist 1 4)
;; ("bar" "baz" "qux")

In modern Emacs, please note cl is deprecated see In Emacs, what does this error mean? "Warning: cl package required at runtime"

edited Sep 11 '18 at 04:45

CT Zhu

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answered Nov 01 '12 at 22:27

Bozhidar Batsov

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Great! The correct, helpful answer is now on Stack Overflow where Google can find it easily: all is right with the World. :) – Brighid McDonnell Nov 01 '12 at 22:29
1

Glad to be of service to the World :-) – Bozhidar Batsov Nov 01 '12 at 22:30
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Emacs, starting with version 25, has [`seq-subseq`](https://www.gnu.org/software/emacs/manual/html_node/elisp/Sequence-Functions.html#index-seq_002dsubseq). – Tobias May 22 '21 at 05:19

score 5 · Answer 2 · answered Aug 03 '14 at 16:51

Common Lisp library is great, but if your codebase becomes large and you want to write concise code in functional style, I endorse dash.el library, which provides enormous amount of functions for list and tree manipulations. There is a function -slice that behaves just like Python's slicing:

(-slice (number-sequence 1 10) 1 7 2) ; (2 4 6)

Arguments are in order: list, start, (optional) stop, (optional) step.

Is there a concise emacs lisp equivalent of Python's [n:m] list slices?

2 Answers2