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

*To*: mathgroup at smc.vnet.net*Subject*: [mg58534] Re: Re: [mg58513] Re: [mg58458] Re: [mg58444] Re: [mg58409] Can't assign value to symbols*From*: Bruce Colletti <vze269bv at verizon.net>*Date*: Wed, 6 Jul 2005 03:11:18 -0400 (EDT)*Sender*: owner-wri-mathgroup at wolfram.com

Andrzej Yes, your idea does work! Thanks. I've moved your Scan to the inside-end of the loop, since it kills two birds with one stone. The code below works as intended unless a,b,c have values (and then error messages abound). The only way I know to avoid this problem is to make Scan[Clear,Names["Global`*"]] the new first line...except that this (unfortunately) zaps ALL variables. There must be a better way (than my above idea) to clear the T[[1]]-variables (that may have values coming into the code below). How would you tell Mathematica to hold unevaluated the T[[1]]-elements so that they can be used to clear the associated variables? Bruce T={{a,b,c},{3,4,2},{12,2,1},{6,5,-3}}; X=Map[SymbolName,T[[1]]]; Do[Print@MapThread[Set,{ToExpression[X,StandardForm,Unevaluated],T[[k]]}]; Scan[Clear,X], {k,2,Length@T}]; ===================== From: Andrzej Kozlowski <akozlowski at gmail.com> To: mathgroup at smc.vnet.net n Subject: [mg58534] Re: [mg58513] Re: [mg58458] Re: [mg58444] Re: [mg58409] Can't assign value to symbols I see. In this case, does not Scan[Clear,X] after the Do loop do what you want? Andrzej On 5 Jul 2005, at 17:31, Bruce Colletti wrote: > Andrzej > > The aim is to explictly avoid (throughout the code) referring to > the variables. > > For instance, suppose matrix T (in my code) springs from an Excel > spreadsheet imported into Mathematica, one whose top row declares > the variable names, and whose later rows give the variables' values > for each iteration in a simulation. Suppose further that the > number of columns may change. > > Bruce > > > Bruce > > ===================== > From: Andrzej Kozlowski <akozlowski at gmail.com> To: mathgroup at smc.vnet.net > Date: Tue Jul 05 00:57:52 CDT 2005 > Subject: [mg58534] [mg58513] Re: [mg58458] Re: [mg58444] Re: [mg58409] Can't > assign value to symbols > > 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: [mg58534] [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 >>>> > 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 >>>> >>>> >>>> >>>> >>>> >>> >>> >>> >>> >>> >>> >> >> >> > >

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