Re: Re: Re: Can't assign value to symbols

*To*: mathgroup at smc.vnet.net*Subject*: [mg58513] Re: [mg58458] Re: [mg58444] Re: [mg58409] Can't assign value to symbols*From*: Andrzej Kozlowski <akozlowski at gmail.com>*Date*: Tue, 5 Jul 2005 01:57:52 -0400 (EDT)*References*: <30126504.1120490281439.JavaMail.root@vms064.mailsrvcs.net>*Sender*: owner-wri-mathgroup at wolfram.com

To tell the truth, I don't understand what you are trying to do. Why do you want to reevaluate the line X=... ? If you do not want at the end of your Do loop the variables a,b,c to have assigned values the simplest way is to use Block: Block[{a, b, c}, Do[ Print@MapThread[Set, {ToExpression[X, StandardForm, Unevaluated], T[[k]]}], {k, 2, Length@T}]]; All the assignments will now take place locally inside Block and the variables {a,b,c} are automatically cleared on exit-ting Block. Andrzej Kozlowski On 5 Jul 2005, at 00:18, Bruce Colletti wrote: > Andrzej > > Using your reply, I offer this code for Lee's consideration. > > Although it works when first run, if re-run without quitting the > kernal, the "X= line" causes an error because a,b,c are instantiated. > > At the start of the program, how can I clear these variables > WITHOUT explicitly declaring them? I've failed to find the right > argument for Clear (e.g., Clear at Subscript...can't find the right > counterpart for Subscript). > > Thankx. > > Bruce > > > T={{a,b,c},{3,4,2},{12,2,1},{6,5,-3}}; > > X=Map[SymbolName,T[[1]]]; > > Do[Apply[Clear,ToExpression[X,StandardForm,Unevaluated]]; > Print@MapThread[Set,{ToExpression[X,StandardForm,Unevaluated],T > [[k]]}], > {k,2,Length@T}]; > > ===================== > From: Andrzej Kozlowski <akozlowski at gmail.com> To: mathgroup at smc.vnet.net > Date: Sun Jul 03 02:57:11 CDT 2005 > Subject: [mg58513] [mg58458] Re: [mg58444] Re: [mg58409] Can't assign value > to symbols > > Instead of ToExpression use > > ToExpression[#, StandardForm, Unevaluated] & > > For example: > > > In[1]:= > parameters = {{"a", "b", "c"}, {1, 2, 3}, {4, 5, 6}, {7, 8, 9}}; > > In[2]:= > MapThread[(#1 = #2) & , {ToExpression[#,StandardForm,Unevaluated]& /@ > parameters[[1]], > parameters[[3]]}] ; > > In[3]:= > a > > Out[3]= > 4 > > In[4]:= > MapThread[(#1 = #2) & , {ToExpression[#,StandardForm,Unevaluated]& /@ > parameters[[1]], > parameters[[2]]}] ; > > In[5]:= > a > > Out[5]= > 1 > > > Of course instead of Standardform you can use TraditionalForm or even > InputForm. > > Andrzej Kozlowski > > Chiba, Japan > > > On 2 Jul 2005, at 17:07, Lee Newman wrote: > > >> Thanks for the solutions. I'm running with David Park's solution, >> but >> now realize there is one additional problem. Once I've executed the >> statement MapThread[(#1 = #2) & , {parameters[[1]], parameters >> [[3]]}] , >> because I've specified the first row of "parameters" as symbols, this >> row now loses the symbol names and takes on the assigned values, such >> that my attempt to execute the MapThread Statement again for a >> different >> set of values, e.g. parameters[[2]], fails. >> >> in: MapThread[(#1 = #2) & , {parameters[[1]], parameters[[3]]}] ; >> in: {a,b,c} >> Out: {4,5,6} >> in: MapThread[(#1 = #2) & , {parameters[[1]], parameters[[2]]}] ; >> out: Set:: setraw : Cannot assign to raw object 4 ... etc >> >> I want to preserve the first row of "parameters" as the symbol >> names, so >> one solution is to use strings, and modify David Park's solution to >> turn >> the strings into symbol names when assigned, This works the first >> time >> it is run, and parameters[[1]] still contains the strings, but ut >> still >> fails the second time. but why? >> in: parameters = {{"a", "b", "c"}, {1, 2, 3}, {4, 5, 6}, {7, 8, 9}}; >> in: MapThread[(#1 = #2) & , {ToExpression /@ parameters[[1]], >> parameters[[3]]}] ; >> in: {a,b,c} >> out {4,5,6} >> in: MapThread[(#1 = #2) & , {ToExpression /@ parameters[[1]], >> parameters[[3]]}] ; >> out: Set:: setraw : Cannot assign to raw object 4 ... etc >> >> why does the MapThread statement fail the second time? It seems >> ToExpression/@parameters[[1]] converts the strings "a", "b", "c" to >> symbols but then evaluates them prior to returning them, so the >> attempt >> to assign to them in MapThread fails. >> >> How can I modify the MapThread statement so I can call it multiple >> times, each time assigning values to the symbols identified by the >> strings in the first row?? >> >> Lee >> >> >> >> David Park wrote: >> >> >>> Lee, >>> >>> Set (=) has a higher precedence Than Function (&). So all you have >>> to do is >>> add parentheses. >>> >>> parameters = {{a, b, c}, {1, 2, 3}, {4, 5, 6}, {7, 8, 9}}; >>> >>> MapThread[(#1 = #2) & , {parameters[[1]], parameters[[3]]}] >>> {4, 5, 6} >>> >>> David Park >>> djmp at earthlink.net >>> http://home.earthlink.net/~djmp/ >>> >>> From: Lee Newman [mailto:leenewm at umich.edu] To: mathgroup at smc.vnet.net >>> >>> >>> >>> Hello, >>> >>> Situation: I have a table that contains parameters and sets of >>> values >>> that I want to assign to the parameters for the purposes of >>> running a >>> simulation. >>> - parameters = {{a,b,c},{1,2,3},{4,5,6},{7,8,9}} >>> - want to assign values in a given row to the symbols listed in the >>> first row. >>> - tried using: MapThread[ #1 = #2 &, {parameters[[1]], parameters >>> [[3]]} ] >>> - fails with error "Tag Slot in #1 is Protected" >>> - tried adding Unprotect[#1] and a variety of other attemps, but >>> can't >>> get it to work. >>> >>> Anyone know how might accomplish this? >>> >>> Thanks, >>> Lee >>> >>> >>> >>> >> >> >> >> >> > >