Re: Multiplying Elements of arrays: Inner product

*To*: mathgroup at smc.vnet.net*Subject*: [mg68518] Re: [mg68497] Multiplying Elements of arrays: Inner product*From*: Sseziwa Mukasa <mukasa at jeol.com>*Date*: Wed, 9 Aug 2006 04:18:05 -0400 (EDT)*References*: <200608081028.GAA29602@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

On Aug 8, 2006, at 6:28 AM, Gopinath Venkatesan wrote: > I have to dot multiply cd (3x3 matrix) by ab (3x1 column vector). > Also I defined values for the first row and the diagonal elements > of cd matrix, as well as defined the column vector ab with values. > Now ab = {1, 2, 3}, and cd = {{1, 2, 3}, {b[2,1], 1, b[2,3]}, {b > [3,1], b[3,2], 1}}. > > I expect the ef to be {14, 2+b[2,1]+3b[2,3], 3+b[3,1]+2b[3,2]} > column vector. But it returns the first element in unevaluated form > as 1.1+2.2+3.3. Why is this so? How to make it evaluate them? > Thanks for the suggestions. Please see below for my example. > > ab = Array[a, 3]; > cd = Array[b, {3, 3}]; > ef = Array[c, 3]; > Do[cd[[i, i]] = 1, {i, 1, 3}]; > cd[[1, 2]] = 2; > cd[[1, 3]] = 3; > Do[ab[[i]] = i, {i, 1, 3}]; > Do[ef[[i]] = 0, {i, 1, 3}]; Your problem is in the following line > Do[ef[[i]] = ef[[i]] + cd[[i, k]].ab[[k]], {i, 1, 3}, {k, 1, 3}]; cd[[i,k]] and ab[[k]] are scalars but you are trying to take their inner product with . The following would work Do[ef[[i]] = ef[[i]] + cd[[i, k]] ab[[k]], {i, 1, 3}, {k, 1, 3}] but it is a lot easier just to use Mathematica's inner product In[26]:= ab={1,2,3}; cd={{1,2,3},{b[2,1],1,b[2,3]},{b[3,1],b[3,2],1}}; ef=cd.ab Out[28]= {14,2+b[2,1]+3 b[2,3],3+b[3,1]+2 b[3,2]} Regards, Ssezi

**References**:**Multiplying Elements of arrays: Inner product***From:*Gopinath Venkatesan <gopinathv@ou.edu>