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Compile[{{x, _Real, 1}}, MemberQ[x, 2]][{2}]

outputs False.

In fact, it seems to get compiled as False for every input if you look at a CompilePrint. It doesn't call the main evaluator, and MemberQ is included in the Compile`CompilerFunctions[] list

Any ideas?

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    $\begingroup$ Compile[{{x, _Integer, 1}}, MemberQ[x, 2]] works; Compile[{{x, _Real, 1}}, MemberQ[x, 2.]] also works. MemberQ[{2.}, 2] might help you figure out why. $\endgroup$ – J. M.'s ennui♦ May 23 '12 at 5:15
  • $\begingroup$ @JM, I'm not sure of the protocol, but I'll post an answer and if that's not what I'm supposed to do, I'll delete it. I'll sure delete it if you post yours $\endgroup$ – Rojo May 23 '12 at 5:42
  • $\begingroup$ No worries, I upvoted your answer. I didn't feel like elaborating today, and wanted you to piece things together yourself, so I just left a comment. $\endgroup$ – J. M.'s ennui♦ May 23 '12 at 6:11
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Some more insight can be obtained using CompilePrint:

Needs["CompiledFunctionTools`"];
CompilePrint[
 Compile[{{x, _Real, 1}},
  MemberQ[x, 2]]
 ]

Mathematica graphics

(ie, it works out that it's always false due to the type), versus

Needs["CompiledFunctionTools`"];
CompilePrint[
 Compile[{{x, _Integer, 1}},
  MemberQ[x, 2]]
 ]

Mathematica graphics

which does the usual thing.

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  • $\begingroup$ huh, didn't know about CompilePrint, nice! $\endgroup$ – tkott May 23 '12 at 10:38
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OK, as J. M. pointed out, it was an issue of types. x was defined to be of Real type, so it was converted and treated as such in the compiled code. Since a 2 can never be a member of a vector of Reals, the compiler hard-coded the result False, which is correct.

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    $\begingroup$ In general: if you're going to be dealing with reals entirely within your compiled code, make all hard-coded constants reals, and similarly with integers. With[] might be useful here for injecting things like N[Sqrt[2]] or N[Pi] into your code to be compiled. $\endgroup$ – J. M.'s ennui♦ May 23 '12 at 6:13

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