Using Cases for lists containing Rule ( -> ) [duplicate]

Question

I am trying to extract an expression of given pattern from the list of the form

list = {a -> b, c -> d} (* typical output of Solve *)

For example, all the expressions which look like x -> 0.

For obvious reasons, Cases[list, _ -> 0] does not work. What should I do instead?

I'd say 90% methods there make it a good duplicate? Do you agree? 65892, another related: 3916 — Kuba, Commented Jul 3, 2017 at 6:11
This is literally the only example listed under "Possible issues" for Cases. Close as "easily found in the documentation". — LLlAMnYP, Commented Jul 3, 2017 at 6:34
@LLlAMnYP I agree that this is easily found in the documentation for Cases but I think this question is more useful to the community marked as a duplicate than simply closed, so I did that. I do not mean any disrespect to you by this action. If you feel that my action was inappropriate please tell me. — Mr.Wizard, Commented Jul 4, 2017 at 5:00
@Mr.Wizard yes, duplication is an important mechanism, if only for the fact that so many don't bother reading the documentation. On a meta-note, I got a feeling that this QA is rep-farming: trivial Q, immediate textbook A... I'd make a meta post, but applying Hanlon's razor is easier. — LLlAMnYP, Commented Jul 4, 2017 at 6:58

mavzolej · Accepted Answer · 2017-07-03 03:29:36Z

9

HoldPattern[] solves the problem!

Cases[list, HoldPattern[_ -> 0]]

answered Jul 3, 2017 at 3:29

mavzolej

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$\begingroup$ You can also use Pattern: Cases[{a -> 0, b -> 1}, s : (_ -> 0)]. -- And Verbatim[Rule][_, 0], etc. $\endgroup$
– Michael E2
Commented Jul 3, 2017 at 5:12
1

$\begingroup$ I think the answer should be Verbatim[Rule][_,0] as that is the construct especially made for such cases. The others are just working for this particular case "by accident" but are meant for different use cases... $\endgroup$
– Albert Retey
Commented Jul 3, 2017 at 8:54
3

$\begingroup$ "The others are just working for this particular case "by accident" but are meant for different use cases". How do you conclude that? The documentation for HoldPattern specifically mentions the use case Cases[{a -> b, c -> d}, HoldPattern[a -> _]]. $\endgroup$
– Sjoerd Smit
Commented Jul 3, 2017 at 9:49

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