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

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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