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Re: setps problem/how to set matrix elements
*To*: mathgroup at smc.vnet.net
*Subject*: [mg65923] Re: setps problem/how to set matrix elements
*From*: "Nasser Abbasi" <nma at 12000.org>
*Date*: Sun, 23 Apr 2006 06:17:43 -0400 (EDT)
*References*: <200604170628.CAA08745@smc.vnet.net> <e22hg8$e6o$1@smc.vnet.net>
*Reply-to*: "Nasser Abbasi" <nma at 12000.org>
*Sender*: owner-wri-mathgroup at wolfram.com
"Szabolcs Horvát" <szhorvat at gmail.com> wrote in message
news:e22hg8$e6o$1 at smc.vnet.net...
>
> and you get
>
> {{0.5,0.5,0.5,0.5},{0.5,0.5,0.5,0.5},{0.5,0.5,0.5,0.5},{0.5,0.5,0.5,0.5}}[[1,1]]
> = 5;
>
> which is illegal.
in other words:
A = {{1,2},{3,4}}
A[[1,1]] <----OK
{{1,2},{3,4}} [[1,1]] <---- OK
but when we try to assign to it, the second form fails
A[[1,1]] = 5 <--- OK
{{1,2},{3,4}} [[1,1]] = 5 <---- not OK
on a side question, does Mathematica always pass arguments to functions by
value? (i.e. default is to evaluate all arguments being passed before the
function being called gets hold of them?)
If a matrix A was one million by one million, then
F[a__]:= ... use matrix 'a' here...
and then I do F[myBigMatrixA] then the whole thing is passed by value? and
put of the stack?
So to force passing by reference, we prevent the evaluation of that specific
argument?
Is there a document that describes more how Mathematica does these things.
Calling convention, passing mechanism, and things sort of thing?
Nasser
This is explained in the documentation of the error
> message (in version 5.2, not all messages are documented in all
> versions).
>
> You must prevent the first argument of FSD from being evaluated, so r
> will be replaced by p instead of the value of p. See the documentation
> of attribute HoldFirst.
>
> In[4]:=
> SetAttributes[FSD, HoldFirst]
>
> In[5]:=
> FSD[p,256,256]
>
> In[6]:=
> p
>
> Out[6]=
> {{5,0.5,0.5,0.5},{0.5,0.5,0.5,0.5},{0.5,0.5,0.5,0.5},{0.5,0.5,0.5,0.5}}
>
> After applying the attribute, the function works correctly, and
> p[[1,1]] is set to 5.
>
> Szabolcs HorvÃ¡t
>
>
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