Re: Converting set of equations to matrix

*To*: mathgroup at smc.vnet.net*Subject*: [mg26736] Re: Converting set of equations to matrix*From*: Brian Higgins <bghiggins at ucdavis.edu>*Date*: Fri, 19 Jan 2001 02:14:13 -0500 (EST)*References*: <9462b7$gc8@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Helge, Here is one way to do it if each equation consists of more than 1 term. In the set you gave your last equation reduces to 3a[0,2]=0. I have modified it for this illustration. I am sure the Mathematica gurus have a slicker way that even works for your case. myeqns = {3 a[0, 0] + 2 a[0, 2] + 4 a[2, 0] == 0, -1 a[0, 0] + 0.3 a[2, 0] == 0, 4 a[0, 2] - 1 a[0, 1] == 0} In[10]:= Map[Drop[#, -1] &, Map[Cases[#, x_?NumberQ, 3] &, eqns]] Out[10]= {{3, 2, 4}, {-1, 0.3}, {-1, 4}} Cheers, Brian In article <9462b7$gc8 at smc.vnet.net>, Helge Kreutzmann <helgek at studserv.stud.uni-hannover.de> wrote: > Hello ! > I have a set of equations I create following some rules, like > > 3 a[0,0] + 2 a[0,2] + 4 a[2,0] = 0 > -1 a[0,0] + 0.3 a[2,0] = 0 > 4 a[0,2] - 1 a[0,2] = 0 > > The numbers of coefficients (the a[i,j]) is different in each run > but fixed. For further calculations I would like to transform the > above equations into the matrix > {{3,2,4 },{-1,0,0.3},{0,4,-1}} > > Currently I have a list (called coeffli) where I can get the equations > from. The existing coefficients are stored in a second list (klist). > I also have a list where the values i,j are stored (cplist). > > The above list I would create by > For[i = 1, i <= Length[kflist], i++, > Print[Collect[coeffli[[cplist[[i]][[1]]]][[cplist[[i]][[2]]]], klist]]] > > I can create a list of the equations but I do not know how to tear > apart the sum into matrix elements. > > Thanks > > Helge > > Sent via Deja.com http://www.deja.com/