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Re: Disappearing variables

• To: mathgroup at smc.vnet.net
• Subject: [mg13799] Re: [mg13759] Disappearing variables
• From: Jon Prudhomme <prudhomj at elwha.evergreen.edu>
• Date: Fri, 28 Aug 1998 04:18:22 -0400
• Sender: owner-wri-mathgroup at wolfram.com

> I want to to define a variable inside a function, as follows: f:=x=123;
> Whenever I run f, x should be given the value of 123, and if x doesn't
> exist, it should be created. However:
> 
> In[1]:= f:=x=123;
> In[2]:=f
> Out[2]:=123
> In[3]:=x
> Out[3]:=123        ** x exists
> In[4]:=Remove[x];
> In[5]:=f
> Out[5]:=123  ** f seems to work 
> In[6]:=x
> Out[6]:=x     ** x doesn't exist anymore, even though f worked
> 
> It seems like x is created when I initially define f, whereas since f is
> SetDelayed,  x should be created when I run f.  Does anyone know why x
> is not created when I run f?

Your problem is that when you Remove a symbol, Mathematica goes through 
all of the previous definitions to note that that symbol has been
removed.

In[7]:=?f
Out[7]:=Global`f
	f:=Removed["x"]=123


'f:=ToExpression["x=123"]' is probably closer to what you want.