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Mathematica

Unexpected result from a pattern constraint with the construct ?test

wojtek potocki

Posted 9 years ago

Hello Below is a line which works as expected: Cases[\!\(\* TagBox[ StyleBox[ RowBox[{"Times", "[", RowBox[{ RowBox[{"Power", "[", RowBox[{"_", ",", "n_"}], "]"}], ",", "x_"}], "]"}], ShowSpecialCharacters->False, ShowStringCharacters->True, NumberMarks->True], FullForm]\) /; EvenQ[n]] @@ {1 + y x^2 + y x^3 } I expect the line below to have an identical result (only change: use of the n?EvenQ construct instead of EvenQ[n]). Cases[{ 1 + y x^2 + y x^3}, \!\(\* TagBox[ StyleBox[ RowBox[{"Times", "[", RowBox[{ RowBox[{"Power", "[", RowBox[{"_", ",", "n_"}], "]"}], ",", "x_"}], "]"}], ShowSpecialCharacters->False, ShowStringCharacters->True, NumberMarks->True], FullForm]\) /; n?EvenQ, Infinity] It is accepted by MMA but since it returns nothing, It must be interpreted differently. But how? True I always saw in examples that the construct form ' _?test' is being used but I never read that the `_` cannot be replaced here by a pattern variable

Hello

Below is a line which works as expected:

  Cases[\!\(\*
  TagBox[
  StyleBox[
  RowBox[{"Times", "[", 
  RowBox[{
  RowBox[{"Power", "[", 
  RowBox[{"_", ",", "n_"}], "]"}], ",", "x_"}], "]"}],
  ShowSpecialCharacters->False,
  ShowStringCharacters->True,
  NumberMarks->True],
  FullForm]\) /; EvenQ[n]] @@ {1 + y x^2 + y x^3 }

I expect the line below to have an identical result (only change: use of the n?EvenQ construct instead of EvenQ[n]).

 Cases[{ 1 + y x^2 + y x^3}, \!\(\*
 TagBox[
 StyleBox[
 RowBox[{"Times", "[", 
 RowBox[{
 RowBox[{"Power", "[", 
 RowBox[{"_", ",", "n_"}], "]"}], ",", "x_"}], "]"}],
 ShowSpecialCharacters->False,
 ShowStringCharacters->True,
 NumberMarks->True],
 FullForm]\) /;  n?EvenQ, Infinity]

It is accepted by MMA but since it returns nothing, It must be interpreted differently. But how?

True I always saw in examples that the construct form ' _?test' is being used but I never read that the _ cannot be replaced here by a pattern variable

POSTED BY: wojtek potocki

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Frank Kampas

Frank Kampas, Physicist at Large Consulting

Posted 9 years ago

Your previous version had incorrect syntax. As described in the documentation for Condition: https://reference.wolfram.com/language/ref/Condition.html "patt /; test is a pattern which matches only if the evaluation of test yields True".