Re: Re: "dereference" variable

*To*: mathgroup at smc.vnet.net*Subject*: [mg82119] Re: [mg82077] Re: [mg82033] "dereference" variable*From*: DrMajorBob <drmajorbob at bigfoot.com>*Date*: Fri, 12 Oct 2007 03:03:17 -0400 (EDT)*References*: <22397821.1192019277612.JavaMail.root@m35> <op.tzzu5kmlqu6oor@monster.gateway.2wire.net> <28399250.1192087108813.JavaMail.root@m35>*Reply-to*: drmajorbob at bigfoot.com

> a local variable which has been assigned the as-yet-unevaluated > expression No such expression can be assigned in the first place, unless you've set, for instance s = Hold[Sum[x, {i, 1, 20}]] or similarly used HoldForm (or some such), which ALSO requires that you "have the luxury" of typing the original, unevaluated sum. It makes no difference what a function can or cannot do, if you can't create the argument you're talking about. Bobby On Wed, 10 Oct 2007 23:23:32 -0500, Todd Johnson <johnsong at ics.uci.edu> wrote: > That is true. But, as I understand it, this only works if you have the > luxury of typing in "type[Sum[x, {i, 1, 20}]]"; if all you have is a > local variable which has been assigned the as-yet-unevaluated expression > "Sum[x, {i, 1, 20}]", then that solution doesn't seem to work. > > todd. > > > DrMajorBob wrote: >> Possibly this will make it clearer: >> >> Clear[type, m, p, q, k] >> SetAttributes[type, HoldFirst]; >> >> type[arg : Sum[x_, it_]] = {"Sum", HoldForm[arg], arg}; >> type[arg : Times[x_, y_]] = {"Times", HoldForm[arg], arg}; >> type[x_Symbol] = {"Symbol", HoldForm[x], x}; >> >> m1[x_] := Module[{s = Sum[x, {i, 1, 20}]}, type[s]] >> m2[x_] := type[Evaluate@Sum[x, {i, 1, 20}]] >> m3[x_] := type[Sum[x, {i, 1, 20}]] >> >> m1[k] >> {Symbol,s$1748,20 k} >> >> m2[k] >> {Times,20 k,20 k} >> >> m3[k] >> {Sum,\!\( >> \*UnderoverscriptBox[\(\[Sum]\), \(i = 1\), \(20\)]k\),20 k} >> >> m1 passes s as a symbol (setting s beforehand has no effect), m2 >> evaluates the sum to a product, and m3 passes the Sum unevaluated. >> >> Bobby >> > > > -- DrMajorBob at bigfoot.com