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Re: Assigning Values to Many Symbols at Once

• To: mathgroup at smc.vnet.net
• Subject: [mg101789] Re: Assigning Values to Many Symbols at Once
• From: pfalloon <pfalloon at gmail.com>
• Date: Fri, 17 Jul 2009 05:03:19 -0400 (EDT)
• References: <200907141200.IAA05959@smc.vnet.net> <h3n5v7$2hj$1@smc.vnet.net>

On Jul 16, 10:20 pm, Gregory Lypny <gregory.ly... at videotron.ca> wrote:
> Thank you, Ssezi, Zach, and Leonid, for your thoughtful responses.
>
> Ssezi's solution works both in assigning string and numerical values  
> subject to the qualifier, noted by everyone, that you can only do it  
> once.
>
>         MapThread[(#=#2)&,{Table[Symbol["x" <> ToString[i]], {i=
,  
> 5}],Table["value" <> ToString[i],{i,5}]}]
>
> Leonid and Zach, both of you suggested direct list-to-list  
> assignments, which is what I thought should work but does not with you =
 
> create the list of new symbols using Table or mapping.  It does work =
 
> if you create the symbol list manually,  For example, typing
>
>         {x1, x2} = {9, 11}
>
> works as expected, creating two new symbols, x1 with a value of 9 and  
> x2 with a value of 10.  However,
>
>         Table[Symbol["x" <> ToString[i]], {i, 5}] = {1,2,3,4,5}
>
> and
>
>         Table[Symbol["x"<>ToString[i]],{i,5}]=Table[i,{i,5}]
>
> result in the error message
>
>         Set::write: Tag Table in Table[Symbol[xabc<>ToString[i]],=
{i,5}] is  
> Protected.
>
> I still haven't worked through Leonid's workaround.
>
>         Thanks again,
>
>                         Gregory
>
>
>
> Ssezi wrote:
> > One option is something like:
>
> > MapThread[(#=#2)&,{Table[Symbol["x" <> ToString[i]], {i,  
> > 5}],Table["value" <> ToString[i],{i,5}]}]
>
> > but you can't do this twice since the once the symbols are  
> > associated with values the evaluator with evaluate the symbol then  
> > attempt to assign the value resulting in assignment to a raw object.
>
> > Regards,
>
> > Ssezi
> Zach wrote:
> > Hi Gregory,
>
> > You can simply do list-to-list assignments:
> > {x1,x2,x3,x4,x5}={value1,value2,value3,value4,value5}
>
> > in whatever way you want, e.g.
> > Table[Symbol["x"<>ToString[i]],{i,5}]=Table[i,{i,5}]
>
> > Note that this is especially useful for doing parallel calculations  
> > to obtain values not derived from loops. E.g.
> > {x1,x2}={ParallelEvaluate[(*some long calculation*),{Kernels[]
> > [[1]]}],ParallelEvaluate[(*some different long calculation*),
> > {Kernels[][[2]]}]}
>
> > Cheers,
> > Zach
> Leonid Shifrin wrote:
> > Hi Gregory,
>
> > If your symbols don't have values, you can simply use Set:
>
> >  Table[Symbol["x" <> ToString[i]], {i, 5}] = {1,2,3,4,5}
>
> > for instance. If they do have values, this is a bit more work. The  
> > following
> > will do what you presumably want, in both cases:
>
> > In[1] =
>
> > ClearAll[assignToSymbols];
> > SetAttributes[assignToSymbols, HoldFirst];
> > assignToSymbols[base_String, {startIndex_Integer, endIndex_Integer},
> >     values_List] /; endIndex - startIndex + 1 == Length[values]=
 :=
> >   assignToSymbols @@
> >    Append[Unevaluated /@ Thread[
> >       Table[
> >        ToExpression[base <> ToString[i], InputForm, Hold],
> >        {i, startIndex, endIndex}], Hold], values];
>
> > assignToSymbols[symbs : {___Symbol}, values_List] /;
> >    Length[Hold @@ symbs] == Length[values] :=
> >   Unevaluated[symbs] = values;
>
> > In[2]=
> > Clear[x1, x2, x3, x4, x5];
> > assignToSymbols["x", {1, 5}, Range[5]];
> > {x1, x2, x3, x4, x5}
>
> > Out[2] = {1, 2, 3, 4, 5}
>
> > In[3] = assignToSymbols["x", {1, 5}, Range[5, 9]]
>
> > Out[3] = {5, 6, 7, 8, 9}
>
> > Regards,
> > Leonid
>
> > On Tue, Jul 14, 2009 at 5:00 AM, Gregory Lypny <gregory.ly...@videotron=
.ca
> > > wrote:
> > Hello everyone,
>
> > I can create a bunch of symbols on the fly using the Symbol command,
> > as in
>
> >        Table[Symbol["x" <> ToString[i]], {i, 5}]
>
> > to get
>
> >        {x1,x2,x3,x4,x5}
>
> > But how can I assign values to these all at once?
>
> > Regards,
>
> >        Gregory

Gregory,

(As I and several others suggested in earlier responses) you can make
this work by wrapping the LHS in an Evaluate:

In[81]:= Evaluate[Table[Symbol["x"<>ToString[i]],{i,5}]]=Table[i,{i,
5}]

Out[81]= {1, 2, 3, 4, 5}

In[83]:= {x1,x2,x3,x4,x5}

Out[83]= {1, 2, 3, 4, 5}

Cheers,
Peter.

• References:
  • Assigning Values to Many Symbols at Once
    • From: Gregory Lypny <gregory.lypny@videotron.ca>