Re: Re: Variables names

*To*: mathgroup at smc.vnet.net*Subject*: [mg15092] Re: [mg15048] Re: [mg15002] Variables names*From*: Andrzej Kozlowski <andrzej at tuins.ac.jp>*Date*: Sat, 12 Dec 1998 03:58:57 -0500*Sender*: owner-wri-mathgroup at wolfram.com

On Wed, Dec 9, 1998, Jens-Peer Kuska <kuska at informatik.uni-leipzig.de> wrote: >Hi Kim, > >the best way is to use x[i] instead of x1, x2, ... > >You can still generate the names with > >ToExpression["x"<>ToString[i]] > >Hope that helps > Jens > >Kim Allemand wrote: > >> Hi, >> I have a square matrix Q of dimension N and would like to generate the >> quadratic function $\sum_{i,j}q_{ij}x_i x_j$. The problem is how to say >> to Mathamatica that the variables names are x1, x2, etc. Is it possible >> to generate variables names in an automatic way? A possibility would be >> to generate manually a vector NN={x1, x2, x3, ....} of dimension N and >> then to pick NN[[i]] for the ith variable name, but imagine when >> N=1000... !! >> Thanks, Kim > I think it may be interesting to point out the two approaches are not really equivalent. The difference between them, i.e. between x1, x2, x3 etc. and x[1],x[2], x[3] is that in the first case we have genuine variable names, in the second values of a function x. This means that in the first case pattern matching on the indices will have no effect on the "variables" , while in the second case it will. Compare the following two examples: In[1]:= x1^2+x2^2/.(2->3) Out[1]= 3 3 x1 + x2 In[2]:= x[1]^2+x[2]^2/.(2->3) Out[2]= 3 3 x[1] + x[3] I can imagine cases where one or the other behaviour is more desirable, so I do not think one can definitely say that one of these aproaches is superior to the other. Andrzej Kozlowski Toyama International University JAPAN http://sigma.tuins.ac.jp/ http://eri2.tuins.ac.jp/