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From: Daniel <dosadchy@its.jnj.com>
Newsgroups: comp.soft-sys.math.mathematica
Subject: Re: Compiler output type
Date: Tue, 13 May 2014 02:26:32 +0000 (UTC)
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Xref: csiph.com comp.soft-sys.math.mathematica:16942

This will do the trick:
collatz = Compile[{{n, _Integer}}, If[EvenQ[n], n/2, 3 n + 1]//Round]

I think that the problem is with n/2. Although the result is guaranteed to be integer, the compiler cannot analyze the code and know it, so the output is set to Real.

> Some compiled functions yield reals even if the
> e input and necessary output are integers.  How can I
> force the output to be an integer rather than a real
> in such cases?  To be clear, not all compiled program
> output produces reals; some compiled programs, given
> integer inputs, produce integer outputs.  But some do
> not.  An example is:
> 
> collatz = Compile[{{n, _Integer}}, If[EvenQ[n], n/2,
> 3 n + 1]]
> 
> It produces real rather than integer output, and I'd
> like to force integer output if possible.
> 
> 	Thanks
> 
> 	Best,
> 
> 	Harvey
>