Re: small question

*To*: mathgroup at smc.vnet.net*Subject*: [mg74880] Re: small question*From*: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>*Date*: Tue, 10 Apr 2007 05:11:11 -0400 (EDT)*Organization*: The Open University, Milton Keynes, UK*References*: <evd3or$5fo$1@smc.vnet.net>

KFUPM wrote: > Dear Group > > I have two variables multiplied by each other > > N1 x (N1 times x) which is a part of an equation. > > What i want to do is to replace this multiplication by Nx1 (one > variable) and i want to do this automatically using Mathematica, > because i have a huge number of such terms, for example, N2 y, N3 z, > N4 x, N5 y.... etc. > > Thereofore, i need to get this transformation: > > N1 x ------> Nx1 > N2 y ------> Ny2 > N3 z ------> Nz3 > and so on. > > Please notice that the letter N doesn't change only numbers(1,2,...) > and (x,y,z....) do. > > > Any help in this regard is highly appreciated. You could use a transformation rule. In the following expression, you just have to provide the list of the variables not in the format N1, N2, etc. expr = N1*x + N2*x + N3*x + N1*y + N2*y + N3*y + N1*z + N2*z + N3*z; vars = {x, y, z}; expr /. (n_)*(x_ /; MemberQ[vars, x]) :> ToExpression[StringInsert[ToString[n], ToString[x], 2]] Nx1 + Nx2 + Nx3 + Ny1 + Ny2 + Ny3 + Nz1 + Nz2 + Nz3 Regards, Jean-Marc