Re: Can't assign value to symbols
- To: mathgroup at smc.vnet.net
- Subject: [mg58469] Re: Can't assign value to symbols
- From: Peter Pein <petsie at dordos.net>
- Date: Sun, 3 Jul 2005 03:57:26 -0400 (EDT)
- References: <da5jcd$243$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Lee Newman schrieb: > 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 >> >> > Hi Lee, use Clear to clear the assigned values: In[1]:= parameters = {{a, b, c}, {1, 2, 3}, {4, 5, 6}, {7, 8, 9}}; MapThread[(#1 = #2) & , {parameters[[1]], parameters[[3]]}]; {a, b, c} Out[3]= {4, 5, 6} In[4]:= Clear[a, b, c]; MapThread[(#1 = #2) & , {parameters[[1]], parameters[[2]]}]; {a, b, c} Out[6]= {1, 2, 3} -- Peter Pein Berlin
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