Trying to speed up a function

*To*: mathgroup at smc.vnet.net*Subject*: [mg91704] Trying to speed up a function*From*: gophergoon at gmail.com*Date*: Sat, 6 Sep 2008 02:05:52 -0400 (EDT)

Say I have a previously defined matrix 'vecs' and I want to calculate certain sums of columns. I find it easier to write in terms of the Transpose but it seems that when I put Transpose[vecs] in the definition of a table, mathematica evaluates the transpose over and over to get at every element of the matrix. Which takes much longer than doing it in 2 steps (2nd method). I only found this out by trial and error so I am curious about the explanation for this behaviour and how to avoid it in general. BTW I am aware that the expression can be written in 1 line without using Transpose also, but I'd still like to know how to tell that if a sub-expression in a function is going to be evaluated repeatedly. Thanks, Abhishek In[15]:= Dimensions[vecs] Out[15]= {1620, 1620} In[23]:= Table[Total[Transpose[vecs][[n]][[811 ;; 1620]]], {n, 1, 1620}]; // Timing Out[23]= {27.844, Null} In[24]:= vecst = Transpose[vecs]; Table[Total[vecst[[n]][[811 ;; 1620]]], {n, 1, 1620}]; // Timing Out[25]= {0.016, Null}