Re: string-variable-Set-snarl
- To: mathgroup at smc.vnet.net
- Subject: [mg22526] Re: string-variable-Set-snarl
- From: "Allan Hayes" <hay at haystack.demon.co.uk>
- Date: Thu, 9 Mar 2000 03:24:16 -0500 (EST)
- References: <89qhdg$amm@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
James and Tania,
After reading the replies of Bojan Bistrovic and Hartmut Wolf I came up
with the following
Attributes[SetByNameOrSymbol] = HoldAll;
SetByNameOrSymbol[s_String, val_] :=
(# = val) &[Unevaluated @@ MakeExpression[ s]]
SetByNameOrSymbol[s_, val_] := s = val
Tests:
SetByNameOrSymbol["cow", 2]; cow
2
SetByNameOrSymbol[cow, 3]; cow
3
SetByNameOrSymbol["cow", cow + 2]; cow
5
If we have allow
animal = "cow";
Then the following will work
SetByNameOrSymbol[Evaluate[animal], 1]; cow
1
Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565
"James Fuite and Tania Nordli" <jfuite at ualberta.ca> wrote in message
news:89qhdg$amm at smc.vnet.net...
> Dear Advanced Mathematica Users,
>
> The following is a short and perhaps trivial problem that has caused me
> some frustration. I cannot write a program that allows me to change the
> value of any given variable if given the name of the variable as a
> string. What follows is an equivalent situation. Let
>
> In[3]= {cow = Random[], emu = Random[], ant = Random[]}
> Out[3]= {0.2, 0.3, 0.4}
>
> . . . . some selection process . . . .
>
> In[25]= animalname
> Out[25]= cow
>
> In[25]= Head[animalname]
> Out[25]= String
>
> Given a string, here "cow", corresponding to a variable, here cow, I
> cannot clear or set a value for the variable itself. Of course I cannot
> use Clear[cow] or cow = 2 because I do not know which variable will be
> chosen, as represented by the variable animalname. My lame attempts
> such as Clear[animalname] or ToExpression[animalname] = 2 only fail to
> affect cow - it seems that I am caught trying to work on the wrong side
> of the equals sign (Set)! My goal would be a line such as:
>
> In[28]= ToExpression[animalname]
> Out[28]= 2
>
> which I think would imply that cow = 2 and has thus been affected. Who
> can guide me from line 25 to line 28? What are the general
> considerations?
>
> Sincerely,
> James J. Fuite.
>
>