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If I have a list, (“apple” “pear” “grape” “apricot”) how do I create a new list with elements starting with the letter “a”? The new list will be: (“apple” “apricot”).

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  • Are the words present together in the buffer, or are they scattered among other text? What do you mean by "copy" - copy all of the words together, to the kill-ring/clipboard, so you can paste them all together? The question really isn't clear (unless maybe someone knows just what that awk expression does).
    – Drew
    Sep 20 at 20:07
  • @Drew I added a sample list for the awk code. I was thinking a function that takes these words and returns only a list with words starting with the letter k.
    – zeynel
    Sep 20 at 20:35
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    This may help: emacs.stackexchange.com/help/how-to-ask
    – Drew
    Sep 21 at 0:23
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    @Drew Sorry, the question was not clear. I changed it. Does it make sense now?
    – zeynel
    Sep 21 at 3:52
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    @shynur Sorry about that. I changed the question. I hope it makes sense now.
    – zeynel
    Sep 21 at 3:54

2 Answers 2

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(seq-filter (apply-partially #'string-prefix-p "a")
            '("apple" "pear" "grape" "apricot"))

If you have other similar questions, consult the seq library, which provides some functions to manipulate sequence (including list).

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    Thanks. This is what I was trying to do. Functions listed here gnu.org/software/emacs/manual/html_node/elisp/… is the seq librarary, right?
    – zeynel
    Sep 21 at 7:48
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    @zeynel Just type C-h f seq- <tab> in Emacs
    – shynur
    Sep 21 at 7:57
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    This is the answer, IMO. Corresponds exactly to the question posed: nothing missing or hidden.
    – Drew
    Sep 21 at 15:17
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For this specific use-case, you can also reuse a builtin function related to text completion named all-completions:

(all-completions "a" '("apple" "pear" "grape" "apricot"))
;; => ("apple" "apricot")

For more details, see C-h f all-completions RET.

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  • This is so simple. I accepted this one. Is @shynur's answer more general?
    – zeynel
    Sep 21 at 14:41
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    I think the accepted answer should mention completion-regexp-list, since it's not necessarily true that all-completions uses basic (i.e., plain prefix) completion. See Basic Completion.
    – Drew
    Sep 21 at 15:06
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    IOW, this answer really hides what's needed and how it might be solved by all-completions: filtering according to a prefix string.
    – Drew
    Sep 21 at 15:12
  • @zeynel Yes, @shynur's answer is more general since seq-filter accepts arbitrary predicates as its first argument. Furthermore, it works on any type that satisfies the predicate seqp, whereas all-completions works on specific collection types (each with slightly different semantics) and is not extensible.
    – Basil
    Sep 21 at 20:37
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    @Basil: You're right, I shouldn't have mentioned completion-regexp-list, any more than arg PREDICATE - those are additional, optional tests made after matching. And yes, all-completions does use only literal-prefix comparison to do the matching. I'll leave my mistaken comments, for info. And I've just filed Emacs bug #66147, to have the doc specify that matching for functions such as all-completions means literal-prefix string matching.
    – Drew
    Sep 21 at 21:50

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