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Re: Use of delayed assignment, :=, with a list

  • To: mathgroup at
  • Subject: [mg91788] Re: Use of delayed assignment, :=, with a list
  • From: "David Park" <djmpark at>
  • Date: Sun, 7 Sep 2008 22:55:03 -0400 (EDT)
  • References: <ga077v$oto$>

You don't give us the precise code that causes the problem. But, in any 
case, instead of doing direct assignment of the elements, why not define an 
opticalMatrix with parameters. I suspect those are the things you would 
actually be working with.

opticalMatrix[f_,  zomeasure_] := {{1, zomeasure}, {-(1/f), 1 - 

Then you can directly multiply two matrices as follows:

opticalMatrix[f1, z1].opticalMatrix[f2, z2] // Simplify
{{1 - z1/f2, z1 + z2 - (z1 z2)/f2}, {-((f1 + f2 - z1)/(f1 f2)),
(f1 (f2 - z2) + z1 z2 - f2 (z1 + z2))/(f1 f2)}}

It is always better to localize parameters or variables to a definition by 
using arguments than having them floating around free in your notebook.

David Park
djmpark at

"phillman5" <PHILLMAN5 at> wrote in message 
news:ga077v$oto$1 at
>I use simple 2x2 matrixes for each item in a optical train to get the
> over all 2x2 matrix. The four elements are a, b, c, d, and I can
> assign them easily in one step.
> {{a, b},{c, d}} = any  2x2 matrix
> however some times the rhs has variables, so I'd like to use an
> delayed assignment
> {{a, b},{c, d}} := any  2x2 matrix
> this does not work, Mathematica says the two lists are not the same
> shape.  In particular:
> {{a, b},{c, d}} := {{1, zomeasure}, {-(1/f), 1 - zomeasure/f}}
> where f and zomeasure are variables (maybe not even defined yet).
> Mathematica says the lists are not the same shape, but if I use an
> assignment operator there is no problem.  I am not completely
> convinced it is a mere Mathematica notation misunderstanding on my
> part because a simpler 2x2 assignment:
> {{q, g}, {h, k}} := {{f, w}, {r, t}}
> works.  (Mathematica does respond {{Null, Null}, {Null, Null}}, but g
> does have a delayed assignment of w.
> Is there an easier way to do a delayed assignment than each member, a,
> b, c, d individually
> tempmatrix := {{1, zomeasure}, {-(1/f), 1 - zomeasure/f}}
> a := tempmatrix[[1]][[1]]
> b :=  . . .

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