# Why is there a difference when formulating patterns for Select and FirstPosition?

Select[{1, 2, 4, 7, 6, 2}, # > 2 &]


Gives {4, 7, 6}. OK!

So one should think that the following would work:

myList = {-2, -1, 0, 1, 2};
FirstPosition[myList, # > 0 &]


But this fails. The correct command is

myList = {-2, -1, 0, 1, 2};
FirstPosition[myList, _?(# > 0 &)]


But why on earth do we have to use different syntax for those two cases??? Why is the _? necessary?

As it was commented by MarcoB:

Select takes a criterion function, while FirstPosition takes a pattern instead.

Can someone give a for dummies explanation what is the difference between both? Literally I would say a criterion is a pattern and a pattern is a criterion...

• Select takes a criterion function as an argument, not a pattern; FirstPosition takes a pattern instead. You are converting the criterion function from Select into a conditional pattern. On the other hand, Cases is similar to Select in functionality, but it takes a pattern argument instead. That's just how those functions were designed! – MarcoB Jul 14 '15 at 16:58
• But don't you aggree that is is very confusing? Why is it necessary to have criterion function and patterns. I don't even understand the difference! – newandlost Jul 14 '15 at 17:06
• Select[] came waay before FirstPosition[]. Now, why they chose to pattern FirstPosition[] after Cases[] rather than Select[] is a question for the language designers. – J. M. is slightly pensive Jul 14 '15 at 17:20
• @Guesswhoitis. makes sense, given that Position also takes patterns. Also SelectFirst and FirstCase are things too. – LLlAMnYP Jul 14 '15 at 17:23
• @Guesswhoitis. actually I'm begin to understand OPs confusion from a different angle. We have Select with criterions and Cases with patterns to get the elements of a list. We have Position with patterns to get the indices. Where's my CriterionPosition with criterions??? Also, what's a "gedanken version"? – LLlAMnYP Jul 14 '15 at 17:29

The high-level Mathematica language is arguably built on pattern matching. It it therefore natural that many functions also use, operate on, or work with patterns. For example Cases, Count, Position, Replace, Switch, Collect, Reap and standard definitions all natively use patterns. Patterns allow a generally way to express a specification.
Since one can make any Boolean test function into a pattern using PatternTest or Condition these may be considered a subset of pattern matching however they have their own benefits and conventions:
Since Mathematica uses a multi-paradigm language it is entirely reasonable for it to have functions dedicated to the use of Boolean test functions as well as its own pattern matching. Select is one such function. Note that it lacks the levelspec of the more general Cases which I believe is in keeping with the style of programming it is primarily intended for. It should be thought of as a specialized function. Your question could perhaps be recast as "Why isn't there a "BooleanPosition" function?" With the explosion of functions in recent versions I would not be surprised if one appears, but really it is not necessary as it could be simply constructed from the existing Position function.