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MathGroup Archive 2004

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Re: Vector-valued functions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg47360] Re: Vector-valued functions
  • From: drbob at bigfoot.com (Bobby R. Treat)
  • Date: Tue, 6 Apr 2004 06:37:05 -0400 (EDT)
  • References: <c4rdik$mcf$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

g[i_,j_,k_]:=f[i,j,k][[2]]

Bobby

me at my.co (me) wrote in message news:<c4rdik$mcf$1 at smc.vnet.net>...
> Hi, Experts !
> I often encounter the following situation:
> I have a function that creates a list (=vector) as output.
> For example:
> f[i_,j_,k_]:={a[i,j,k],b[i,j,k],c[i,j,k]}
> I have to use, for example, the second part of the vector
> in a subsequent expression. When I try that as, for example,
> g[i_,j_,k_]=f[i,j,k][[2]]
> Mathematica gives the error message 'Part n does not exist' 
> because f[i,j,k] is indeed not yet evaluated.
> What is the best way to I make [[...]] behave like [...] so that it stays 
> unevaluated unless it can be evaluated ?
> I can think of three ways:
> - Use functional arguments [...] instead of lists [[...]] alltogether.
> - Rename Part[...] into something else so that Mathematica does not complain.
> - Make frequent use of Hold[...].
> All three ways are a bit ugly, I feel. Which (other) method is preferred ?
> I could imagine that this is a rather common question but I found no
> discussion about it.
> 
> Thank you !
> Michael


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