Re: Set::setps problems

*To*: mathgroup at smc.vnet.net*Subject*: [mg90270] Re: Set::setps problems*From*: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>*Date*: Fri, 4 Jul 2008 03:56:39 -0400 (EDT)*Organization*: The Open University, Milton Keynes, UK*References*: <g4i94f$3d2$1@smc.vnet.net>

ï¿½ wrote: > I found troubles with such a loop: > > T = Table[{s, 0}, {s, -Pi, Pi, step}]; > > For[i = 1, i <= Length[UU], i += 1, > UU[[i]][[2]] += 5]; > > (it's just the example of some general idea of changing values of > table's elements inside a loop) > > And Mathematica returned: > > Set::setps: UU[[i]] in the part assignment is not a symbol. >> > Set::setps: UU[[i]] in the part assignment is not a symbol. >> > Set::setps: UU[[i]] in the part assignment is not a symbol. >> > General::stop: Further output of Set::setps will be suppressed during \ > this calculation. >> You cannot use the shortcut += in this case because Mathematica mismatches the value of the symbol for the symbol itself when expanding the LHS (say, UU[[1,2]] == 5, so UU[[1,2]] += 5 is rewritten as 5 = 5 + 5 rather than the intended UU[[1,2]] = 5 + 5). So just write the expanded version UU[[i, 2]] = UU[[i, 2]] + 5. In[1]:= With[{step = .5}, UU = Table[{s, 0}, {s, -Pi, Pi, step}]] For[i = 1, i <= Length[UU], i++, UU[[i, 2]] = UU[[i, 2]] + 5] UU Out[1]= {{-3.14159, 0}, {-2.64159, 0}, {-2.14159, 0}, {-1.64159, 0}, {-1.14159, 0}, {-0.641593, 0}, {-0.141593, 0}, {0.358407, 0}, {0.858407, 0}, {1.35841, 0}, {1.85841, 0}, {2.35841, 0}, {2.85841, 0}} Out[3]= {{-3.14159, 5}, {-2.64159, 5}, {-2.14159, 5}, {-1.64159, 5}, {-1.14159, 5}, {-0.641593, 5}, {-0.141593, 5}, {0.358407, 5}, {0.858407, 5}, {1.35841, 5}, {1.85841, 5}, {2.35841, 5}, {2.85841, 5}} Note that a *much* more efficient way of doing the above is to manipulate a list as a whole: In[4]:= UU[[All, 2]] = UU[[All, 2]] + 5 UU Out[4]= {10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10, 10} Out[5]= {{-3.14159, 10}, {-2.64159, 10}, {-2.14159, 10}, {-1.64159, 10}, {-1.14159, 10}, {-0.641593, 10}, {-0.141593, 10}, {0.358407, 10}, {0.858407, 10}, {1.35841, 10}, {1.85841, 10}, {2.35841, 10}, {2.85841, 10}} You should spend some time familiarizing yourself with "List Manipulation" and "Functional Programming", as starters, to get an overall idea on how to program efficiently with Mathematica. guide/ListManipulation guide/FunctionalProgramming Regards, -- Jean-Marc