Re: Telling Mathematica that a symbol is going to be a List?

*To*: mathgroup at smc.vnet.net*Subject*: [mg50838] Re: Telling Mathematica that a symbol is going to be a List?*From*: p-valko at tamu.edu (Peter Valko)*Date*: Thu, 23 Sep 2004 05:27:19 -0400 (EDT)*References*: <ciqvop$in9$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

The error is happening, because an element of the list is referred to before the list turns out to be a list. To change the order of happenings, I would suggest: m[d_]:= { {d[[1]], 0, 0}, {0, d[[2]], 0}, {0, 0, d[[3]]} } m[d/. d -> {d1, d2, d3} ] // MatrixForm Then the symbol d turns out to be a list of {d1,d2,d3} before it is used in the evaluation of m and there will be no error message. Peter AES/newspost <siegman at stanford.edu> wrote in message news:<ciqvop$in9$1 at smc.vnet.net>... > I want to define a function containing parameters that are going to be > elements in a list, e.g. something like > > m := { { d[[1]], 0, 0}, {0, d[[2]], 0}, {0, 0, d[[3]]} > > but not define the list of values of d until later (and not put the > parameters in as explicit arguments of m just to keep the appearance > less cluttered and typing easier). > > If I then give an input line > > m /. {d -> {d1, d2, d3} } // MatrixForm > > I get "Part::partd" errors -- but the matrix then displays correctly. > > I can of course just Off[] the errors -- but is there a better way.