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Re: Scoping question

  • To: mathgroup at smc.vnet.net
  • Subject: [mg85713] Re: Scoping question
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Wed, 20 Feb 2008 06:50:33 -0500 (EST)
  • Organization: University of Bergen
  • References: <fpdv0k$ref$1@smc.vnet.net>

Yaroslav Bulatov wrote:
> The code below defines a function with parameter x. The body contains
> another function definition which uses this parameter. Why do "inner x
> is" and "outer x is" lines start reporting different values at one
> point?
> 
> f1[x_] := (
>   f2[y_] := (Print["inner x is ", x]; If[x + 1 < 3, f1[x + 1], 1]);
>   (Print["outer x is ", x]; f2[#]) & /@ Range[2];
>   )
> f1[1]
> 

Hi Yaroslav,

Scoping in Mathematica is slippery terrain, but this is not about 
scoping.  If you step through your functions manually, you will 
understand what happens.

Here is a rewritten, cleaner version of the same algorithm.  (This could 
be directly translated to a static language like C.) Perhaps this will 
make things clear:

(* 'global' is a global variable *)

fun2 :=
  (Print["fun2: ", global];
   If[global <= 1,
    fun1[global + 1]
    ])

fun1[arg_] :=
  (global = arg;
   Print["fun1: ", arg]; fun2;
   Print["fun1: ", arg]; fun2;)

When the last "inner x" is printed, the global variable (i.e. f2 in your 
version) is already redefined to have the value 2.  The outermost f1 
(i.e. outermost in the recursion) is not changing it back to 1, so it 
remains 2.  But f1 does not print 'global'.  It prints the argument that 
was passed to it.  Therefore the last "outer x" is 1, not 2.

I hope this helps,
Szabolcs


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