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MathGroup Archive 2002

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RE: ToExpression

  • To: mathgroup at smc.vnet.net
  • Subject: [mg35714] RE: [mg35697] ToExpression
  • From: "DrBob" <majort at cox-internet.com>
  • Date: Sat, 27 Jul 2002 06:43:15 -0400 (EDT)
  • Reply-to: <drbob at bigfoot.com>
  • Sender: owner-wri-mathgroup at wolfram.com

Just a small change:

insertx[str_] :=
     Module[{foo = str, vars, n, pos}, foo = StringReplace[foo, {")(" ->
"),(
      ", ".(" -> ".,(", ")." -> "),.", ".." -> ".,."}];
    foo = StringReplace[foo, {")" -> "]", "(" -> "Cross["}];
    n = Length[StringPosition[foo, "."]];
    Do[pos = StringPosition[foo, "."][[1]];
      foo = StringReplacePart[foo, ToString[Slot[i]], pos]
      (*Print[foo]*), {i, n}];
    ToExpression[foo <> "&"]] (* put & here *)
insertx[str1] 
insertx[str1][j, k, i, k, k, i] (* not here *)

Bobby Treat

-----Original Message-----
From: Heubach, Silvia [mailto:sheubac at exchange.calstatela.edu] 
To: mathgroup at smc.vnet.net
Subject: [mg35714] [mg35697] ToExpression

Hi all, 

I have a problem with using the function ToExpression to make a string 
into an expression that can then be evaluated. Something goes wrong with

the evaluation - when I type the same expression in "by hand", then it 
evaluates properly. Either I am totally off in my understanding of how 
ToExpression works, or there is a potential bug (I am using version 
4.0). Please reply to this email 
(sheubac at calstatela.edu) rather than to the mathgroup, as I am 
travelling and cannot subscribe to the mathgroup right now - it would 
make my mailbox overflow....:) 

Thanks for any tips on this! 
Silvia 

******************************** 

i = {1, 0, 0}; j = {0, 1, 0}; k = {0, 0, 1}; zero = {0, 0, 0}; 
a = {i, j, k} 

str1 = "(((..)(..))(..))" 



The function insertx is to convert this string of parentheses and dots 
into a cross product, where each dot represents a different variable, 
and the parentheses indicate the groupings of the cross product. 



insertx[str_] := Module[{foo = str, vars, n, pos}, 
    foo = 
      StringReplace[ 
        foo, {")(" -> "),(", ".(" -> ".,(", ")." -> "),.", ".." -> 
".,."}]; 
    foo = StringReplace[foo, {")" -> "]", "(" -> "Cross["}]; 
         n = Length[StringPosition[foo, "."]]; 
    Do[pos = StringPosition[foo, "."][[1]]; 
          foo = StringReplacePart[foo, ToString[Slot[i]], pos] 
         (*Print[foo]*) , {i, n}]; 
    ToExpression[foo]] 



The function properly converts the string into the associated cross 
product, but does not evaluate correctly. Checking whether what is 
returned by the function is still a string gives a false, and it should,

as the function should return an expression. (the output is indented) 

insertx[str1] 

   ((#1\[Cross]#2)\[Cross](#3\[Cross]#4))\[Cross](#5\[Cross]#6) 

insertx[str1] &[j, k, i, k, k, i] 

   ((#1\[Cross]#2)\[Cross](#3\[Cross]#4))\[Cross](#5\[Cross]#6) 

StringQ[insertx[str1]] 

   False 



Typing in the corresponding cross product "by hand" and evaluating it 
returns the correct result. 



Cross[Cross[Cross[#1, #2], Cross[#3, #4]], Cross[#5, #6]] &[j, k, i, k, 
k, i] 

   {1, 0, 0} 

What is wrong??? I have spent many hours on trying to understand this, 
so either I am completely off or there is a bug. 






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