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Re: Use of delayed assignment, :=, with a list
*To*: mathgroup at smc.vnet.net
*Subject*: [mg91785] Re: Use of delayed assignment, :=, with a list
*From*: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
*Date*: Sun, 7 Sep 2008 22:54:30 -0400 (EDT)
*References*: <ga077v$oto$1@smc.vnet.net>
phillman5 wrote:
> 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 := . . .
FWIW,
The delayed assignment you provided works as expected on my system.
Something else must be going on on your system.
In[1]:= {{a, b}, {c, d}} = {{1, 2}, {3, 4}}
Out[1]= {{1, 2}, {3, 4}}
In[2]:= {{a, b}, {c, d}}
Out[2]= {{1, 2}, {3, 4}}
In[3]:= {{a, b}, {c, d}} := {{1, zomeasure}, {-(1/f), 1 - zomeasure/f}}
Out[3]= {{Null, Null}, {Null, Null}}
In[4]:= {{a, b}, {c, d}}
Out[4]=
1 zomeasure
{{1, zomeasure}, {-(-), 1 - ---------}}
f f
In[5]:= $Version
Out[5]= 6.0 for Mac OS X x86 (64-bit) (May 21, 2008)
Regards,
-- Jean-Marc
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