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MathGroup Archive 1997

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Re: rules->function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg7404] Re: [mg7338] rules->function
  • From: seanross at worldnet.att.net
  • Date: Fri, 30 May 1997 01:20:25 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Lord of Darkness wrote:
> 
> Hello All,
> 
> If I solve a set of simultaneous equations and get a simple answer like
> zrule = z-> x^2 + y^2
> 
> but I then want to evaluate
> fn[x_,y_] := z + some more stuff.....
> 
> I tried saying
> 
> fn[x_,y_] := Replace[z,zrule] + ....
> 
> but it doesn't seem to handle it the way I wanted...
> 
> Is there a correct way of doing this ?
> 
> thanks
> 
> John B


No, there is no single correct way to do it.  My particular way is like
this:

f[a_,b_]=Evaluate[z/.zrule]/.{x->a,y->b}

The reason your approach did not work is that mathematica has real
trouble when function arguments appear on the left hand side of
replacement rules.  You also used := which waits to execute the
instructions until called upon.  By that time, x and y are numbers, not
generic symbols and that gets you into trouble as well.

P.S.  Whats with the "Lord of Darkness" handle?  Were you Darth Vader in
a previous life?


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