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 > >

**References**:**setps problem/how to set matrix elements***From:*Robert James Williamson <sparkles@stanford.edu>