Re: "dereference" variable

*To*: mathgroup at smc.vnet.net*Subject*: [mg82071] Re: [mg82033] "dereference" variable*From*: Todd Johnson <johnsong at ics.uci.edu>*Date*: Thu, 11 Oct 2007 00:20:27 -0400 (EDT)*References*: <200710100822.EAA26488@smc.vnet.net> <30346C40-5665-4B7C-B0FA-96E651F05229@jeol.com>

>> so that what L actually receives is a value like "s$123456". Q, on >> the other hand, evaluates "s" entirely, and arrives at "20*k". You >> may wonder, "but wait, that is what s evaluates to, why can't your >> code deal with that?" The simplest answer is that sometimes >> Mathematica seems to decide erroneously that the summation can be >> turned into multiplication, > > It's not erroneous you need to follow the evaluation sequence, > Mathematica does not operate the way languages like C or Java do, it > uses a strict read eval loop, it reads an expression checks if there's > a rule that can be used to evaluate it and continues doing so until > there is nothing left to evaluate. For that matter the idea of > references and passing by value or reference does not map well. > >> which leaves me with dummy variables from summations lying around >> unbound. > > What variables are unbound? I don't see any. I'm sorry, I wasn't very clear. This was a very much stripped down/faked-up version of the code which has these problems. This code doesn't have any unbound variables because it's so simple. But sometimes I'll write something like Sum[Sum[j,{j, 1, someweird[nestedexpression[with,i,as,an,argument,somewhere]]}],{i,1,imax}] and get imax*Sum[j,{j, 1, someweird[nestedexpression[with,i,as,an,argument,somewhere]]}] and now the "i" in the iterator for the inner loop doesn't have any meaning. Although, to be honest, I haven't seen an example of that behavior in a while, so I can't draw up an example that displays it. >> Also, this is a problem at other times, such as when there is an >> appreciable difference between "x[[i]]-x[[i]]" and "0", because I >> really mean "x[[i]]" to stand in for some vector of as-yet-unknown >> length, so that it minus itself is a zero vector, which is different >> from plain 0. > > Again you are fighting the evaluation sequence, if you don't want to > evaluate x[[i]]-x[[i]] because you are going to define an evaluation > rule for x later, you shouldn't evaluate that expression until the > right time. The goal of the bit of code I'm writing is to examine/modify/optimize some code to be executed later, so it's not really an option to wait until I have a value for x. > s=mySum[x,{k,1,20}] > > L[mySum[x__]]:=Print["here"] I tried that, and it works OK. But I lose a lot of Mathematica's knowledge of the properties of Sum, which I would rather not do. > > Regards, > Thanks for your help. It seems pretty clear that what I want to do can't be done, and I will just have to work around that. > Ssezi todd.

**References**:**"dereference" variable***From:*Todd Johnson <johnsong@ics.uci.edu>