Re: Multiplying a vector over multiple vectors

*To*: mathgroup at smc.vnet.net*Subject*: [mg91199] Re: Multiplying a vector over multiple vectors*From*: Ray Koopman <koopman at sfu.ca>*Date*: Sat, 9 Aug 2008 07:45:34 -0400 (EDT)*References*: <g7ed6u$2pm$1@smc.vnet.net>

In[1]:= m = Table[Random[Integer,{-100,100}],{10^5},{2}]; First@Timing[p = Transpose[{a,b} Transpose@m];] First@Timing[q = Transpose[{a m[[All,1]], b m[[All,2]]}];] First@Timing[r = Inner[Times,m,{a,b},List];] SameQ[p,q,r] Out[2]= 0.7 Second Out[3]= 0.71 Second Out[4]= 0.55 Second Out[5]= True On Aug 7, 1:53 am, Donald DuBois <don... at comcast.net> wrote: > How does one get a vector to distribute over > a list of vectors under multiplication? > > For example: > > {{1,2}, {3,4}, {5,6}} times {a,b} should > equal (under this operation) > > {{a, 2b}, {3a, 4b}, {5a, 6b}} > > If {a,b} is duplicated three times one can > use MapThread as in: > > MapThread[Times, {{{1,2}, {3,4}, {5,6}}, {{a,b}, {a,b}, {a,b}}}] > > but duplicating {a,b} using Table is very slow > when the vectors are large (consisting of thousands > of elemets). > > Thanks in advance for any help you can give me. > > Don