2

In another question here, I saw this

poincareMetric[u_?VectorQ, v_?VectorQ] := ...

Whilst I am familiar with the syntax label_Type, e.g. x_Real, I had not seen this form, but when I tried to search the MMA documentation I found that searching for _? returned no results and neither _ or ? on their own suggested to me a clear meaning for _? - especially since I would have written u_List for u_?VectorQ

So,

1/ What does _? actually mean, and

2/ What is this function definition doing that would not be done by e.g. u_List?

but most importantly,

3/ How does one search for help on such terse syntactic constructions effectively? (So as not to ask almost trivial questions here)

Julian Moore
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  • Almost all of these can be found by a documentation search. Your only mistake is assuming that _? is a single unit. It's _ and ? separately. You can search for both. – Szabolcs Oct 29 '16 at 08:59
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    Otherwise look at this post: http://mathematica.stackexchange.com/questions/18393/what-are-the-most-common-pitfalls-awaiting-new-users/25616#25616 – Szabolcs Oct 29 '16 at 08:59
  • Also see my comments on VectorQ here. – Szabolcs Oct 29 '16 at 09:02
  • Whilst searching for "_" produced only 1 result, searching for "?" produced 1055 hits. Searching for "_ ?" returned none. When you say I could "search for both", what could I enter in the search box (one search) or how could I combined multiple searches? (process) to identify relevant entries? Also, following links I arrive at controlling order of evaluation "define the function f to only evaluate if it receives a numeric value by using NumericQ and pattern testing. Append the pattern ?NumericQ to the argument of the function f"... TBC – Julian Moore Oct 29 '16 at 09:21
  • I also tried a[x_?VectorQ] := Module[{}, Print["Executed"]; True]; b[x_List] := Module[{}, Print["Executed"]; True] and didn't find any difference with various inputs. Under what conditions might they be different? – Julian Moore Oct 29 '16 at 09:22
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    Searching for ? produces precisely three results. It has two uses in Mathematica syntax and one in regular expressions. Go to Help -> Wolfram Documentation, type ? and hit Enter. Use the builtin documentation, not Google or the Wolfram website. – Szabolcs Oct 29 '16 at 09:22
  • "Under what conditions might they be different" <- Please do read the comment I linked to above, where I gave two examples. The VectorQ documentation explains more. – Szabolcs Oct 29 '16 at 09:25
  • I am searching the Documentation Center as a whole (and in fact get 10547 hits). I must be searching in the wrong place. Ah, no... you meant search for "?" WITH QUOTES; I was searching for "?" without quotes. I think that's quite an important difference ;) – Julian Moore Oct 29 '16 at 09:28
  • Let us continue this discussion in chat. – Julian Moore Oct 29 '16 at 09:30
  • For completeness here: as @Szabolcs said, one can search... however different versions of MMA work differently: in V9(.0.1, tested on Windows 7 64-bit) to find the limited number of results expected by a user of a later version, one must put the question mark in double quotes thus "?" if one is to find a sensible number of results: using only ? in that version of MMA returned over 10,000 results. – Julian Moore Oct 29 '16 at 11:20

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