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Re: copying a variable

  • To: mathgroup at smc.vnet.net
  • Subject: [mg92336] Re: copying a variable
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Sat, 27 Sep 2008 22:23:00 -0400 (EDT)
  • Organization: University of Bergen
  • References: <gbl303$m03$1@smc.vnet.net>

ramiro wrote:
> Hello,
> This must be very simple.  Consider the following code:
> 
> k[1] = 2;
> sub[x_] := With[{z = x}, z[1] = 4; z]
> r = sub[k];
> r[1]
> k[1]
>  
> The output is
> 4
> 4
> 
> I would like to have the assignment inside the function (on z) don't 
> affect the parameter (x).  I would like to copy x so that whatever 
> assignment I do on z doesn't affect it, in other words, I would like to 
> return _a copy_ of x. So that the output should be:
> 
> 4
> 2
> 

I don't understand your question fully.  What is the purpose of having 
two symbols with the same definition?  Why couldn't you just do r[1] = 4 
instead of r = sub[k]?

Are you trying to transfer the other definitions associated with k to r, 
so that if k[2]=Pi was defined then after evaluating r = sub[k], r[2]=Pi 
would be automatically defined too?

I suspect that there might be better solutions to your *actual* problem 
than copying definitions ...   But if you really want to do this, you 
could try something like DownValues[r] = DownValues[k] /. k :> r for 
copying ... Be warned that this is very bug prone and is very likely to 
cause trouble with non-trivial definitions.


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