Position built-in function with "test" instead of "pattern matching"?

Question

I read the doc about Position built-in function:

"Position[expr, pattern] gives a list of the positions at which objects matching pattern appear in expr."

For instance:

Position[{"a", "b", "A", "a", "B", "c", "b"}, "b"]

returns

{{2}, {7}}

However, I am desperately looking for a built-in function that works with a test instead of a pattern matching. To be clear I would like a built-in function that behaves as follow:

(* does not work, for illustration purpose *)
Position[{"a", "b", "A", "a", "B", "c", "b"},UpperCaseQ]

would return:

{{3}, {5}}

I am aware of the Select function,

Select[{"a", "b", "A", "a", "B", "c", "b"}, UpperCaseQ]

but it returns values and not positions (and I want positions!):

{"A", "B"}

Question: Do I miss read the doc, maybe such built-in function exist?

Picaud Vincent · Answer 1 · 2017-11-20T21:52:31.230

6

I just found this:

Position[{"a", "b", "A", "a", "B", "c", "b"}, _String?UpperCaseQ]

is working.

Update: Albert Retey, just rightly commented that it works without the head check.

For instance:

Position[Range[10], _?OddQ]

is working.

edited Nov 20 '17 at 21:52

answered Nov 20 '17 at 16:30

Picaud Vincent

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that's something along the lines of what I was going to suggest. :) – rcollyer Nov 20 '17 at 16:33
@rcollyer, thanks. On my side it was not on purpose. I was annoyed by this and just found the solution I posted. – Picaud Vincent Nov 20 '17 at 16:37
1

this would work equally well without the head-check, e.g.: Position[{0.,-1,5,-5.3},_?Negative]. That is probably just relevant for those cases where your entries do not all have the same head but I often use it without checking for the head, the typical test function would return false for non-matching entries anyway, cf.: OddQ["a"] – Albert Retey Nov 20 '17 at 21:35
@AlbertRetey thanks for the constructive comment, I have updated my answer. – Picaud Vincent Nov 20 '17 at 21:53

score 4 · Accepted Answer · answered Nov 20 '17 at 16:33

4

Position[UpperCaseQ /@ vars, True]

answered Nov 20 '17 at 16:33

Alucard

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score 1 · Answer 3 · answered Nov 20 '17 at 16:29

1

You could try something like this.

vars = {"a", "b", "A", "a", "B", "c", "b"};

Position[Table[UpperCaseQ[vars[[i]]], {i, Length[vars]}], True]

(*{{3},{5}}*)

answered Nov 20 '17 at 16:29

Bob Brooks

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Thanks, for your answer. I just found something that seems to work ( Position[{"a", "b", "A", "a", "B", "c", "b"}, _String?UpperCaseQ] ) – Picaud Vincent Nov 20 '17 at 16:31
@Picaud Vincent Nice! – Bob Brooks Nov 20 '17 at 16:33
Thanks, I only hope this can work in all cases, the syntax _String?UpperCaseQ is not the most natural one. – Picaud Vincent Nov 20 '17 at 16:34
why? i find it elegant: it select all the string elements and test them with UpperCaseQ – Alucard Nov 20 '17 at 16:37
@Alucard for instance Position[Range[10], _Number?OddQ] returns {}... I certainly miss something... – Picaud Vincent Nov 20 '17 at 16:42
1

@PicaudVincent try Position[Range[10], _Integer?OddQ] – Alucard Nov 20 '17 at 16:43
@Alucard, good catch, that works. Thanks. My "Number" was something stupid, this is not the Head of "numbers"... – Picaud Vincent Nov 20 '17 at 16:46
@PicaudVincent i suggest you to wait a little longer before accepting an answer, the experts may want to add their solution – Alucard Nov 20 '17 at 16:51
@alucard thanks, I will do that next time... It is just that I am not very proud of this duh "question" :-/ – Picaud Vincent Nov 20 '17 at 16:53

Position built-in function with "test" instead of "pattern matching"?

3 Answers3