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proposal: slices: add LastIndex and LastIndexFunc #63128

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dsnet opened this issue Sep 20, 2023 · 4 comments
Open

proposal: slices: add LastIndex and LastIndexFunc #63128

dsnet opened this issue Sep 20, 2023 · 4 comments
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@dsnet
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dsnet commented Sep 20, 2023

I propose the addition of:

// LastIndex returns the index of the last occurrence of v in s, or -1 if not present.
func LastIndex[S ~[]E, E comparable](s S, v E) int

// LastIndexFunc returns the last index i satisfying f(s[i]), or -1 if none do.
func LastIndexFunc[S ~[]E, E any](s S, f func(E) bool) int

which parallels the existence of:

  • strings.IndexXXX
  • strings.LastIndexXXX
  • bytes.IndexXXX
  • bytes.LastIndexXXX

In my use-case, I'm processing Unicode runes as a []rune and wanted to find the last occurrence of a given rune.
@dsnet dsnet added the Proposal label Sep 20, 2023
@gopherbot gopherbot added this to the Proposal milestone Sep 20, 2023
@dsnet dsnet changed the title proposal: slices: add IndexLast and IndexLastFunc proposal: slices: add LastIndex and LastIndexFunc Sep 20, 2023
@earthboundkid
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This makes a lot of sense, but devil's advocate, should we just tell people to do iterutils.Index(slices.Backwards(s), v) in a world where iterutils exists? I think the tricky of it would be that even with slices.Backwards, you run into issues with matching just the value vs. the index. Probably it's worth it anyway.

@jimmyfrasche
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index doesn't make sense with iterators like it does with slices.

@ianlancetaylor
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I can see a reasonable argument that the slices package should have a version of all relevant functions in the bytes and strings packages.

@ghost
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ghost commented Sep 21, 2023 •
edited by ghost

This makes a lot of sense, but devil's advocate, should we just tell people to do iterutils.Index(slices.Backwards(s), v) in a world where iterutils exists? I think the tricky of it would be that even with slices.Backwards, you run into issues with matching just the value vs. the index. Probably it's worth it anyway.

slices.Backwards doesn't exist:

https://cs.opensource.google/search?q=backwards&ss=go/go:src/slices/

and according to the quoted comment, neither does iterutils. so I am not sure how much value would be in a suggestion thats not even actionable. also the implementation is dead simple:

https://github.com/golang/go/blob/go1.21.1/src/bytes/bytes.go#L146

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@earthboundkid @jimmyfrasche @ianlancetaylor @dsnet @gopherbot