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MathGroup Archive 2006

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  • To: mathgroup at
  • Subject: [mg66007] Compile&Which
  • From: gregorc <gregor.cernivec at>
  • Date: Wed, 26 Apr 2006 04:38:23 -0400 (EDT)
  • Sender: owner-wri-mathgroup at


I have trouble to understand how does the Compile function handle the 
Which statement. Following  there are two examples (just as 
ilustration), both are compiled OK, but the second one does not evaluate 
as a compiled expression. Can someone explain the evaluation error? It 
is probably because the Which statement, but I do not know how to do 
this correctly. Much thanks!

1st example:

f = Compile[{{x, _Real, 1}}, {x[[1]]}];
g = Compile[{{x, _Real, 1}}, {x[[2]]}];
r = Compile[{{x, _Real, 1}}, f[x] + g[x], {{f[x], _Real, 1}, {g[x], 
_Real, 1}}]
CompiledFunction[{x}, f[x] + g[x], -CompiledCode-]
r[{3, 4}]

2nd example:

f = Compile[{{x, _Real, 1}}, {x[[1]]}];
g = Compile[{{x, _Real, 1}}, {x[[2]]}];
r = Compile[{{x, _Real, 1}}, Which[x[[1]] ? x[[2]], f[x] +g[x], x[[1]] < 
x[[2]], f[x] -g[x]], {{f[x], _Real, 1}, {g[x], _Real, 1}}]
CompiledFunction[{x}, Which[x?1? ? x?2?, f[x] + g[x], x?1? < x?2?, f[x] 
- g[x]], -CompiledCode-]
r[{3, 4}]
CompiledFunction::cfse: Compiled expression {-1.} should be a 
machine-size \real number.
CompiledFunction::cfex: External evaluation error at instruction 2; 
\proceeding with uncompiled evaluation.


Cernivec Gregor

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