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Re: need little help - no longer!

  • To: mathgroup at
  • Subject: [mg23619] Re: [mg23530] need little help - no longer!
  • From: Mark Fisher <mefisher at>
  • Date: Wed, 24 May 2000 02:16:20 -0400 (EDT)
  • References: <8g9nm8$>
  • Sender: owner-wri-mathgroup at

Following up on Andrzej's point, Ken's solution is indeed elegant, but
Ted Ersek's solution can be compiled. Here is Ted's solution (slightly

With[{x = Random[]}, 
  Which[x < .2, 1, x < .6, 2, x < .7, 3, x < 1., 4]

Here's a function that takes a list and returns a CompiledFuntion:

compileRandomIndex[list_List/; Plus @@ list == 1] := 
Block[{x, y}, 
  Compile @@ 
    (Hold[{}, With[{x = Random[]}, y], 
      {{Which[__], _Integer}}] /. y -> 
        Which @@ (Flatten @
          Transpose[{Thread[x < Rest @ FoldList[Plus, 0, list]], 

The Hold is used to make an inert container that facilitates the
construction of the arguments to Compile. The idea is to insert the
evaluated Which expression into the Hold container without evaluating
the call to Random, and only then slap on the Compile head. (The third
argument to Compile alerts Compile to the fact that the Which expression
returns an Integer.)

Here is the usage:

m = {0.2, 0.4, 0.1, 0.3}
ran = compileRandomIndex[m]

It runs faster than Ken's solution, with the advantage growing with the
length of the list. 


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