Efficient way to sum a product of numbers...

[vsoler]

Hi,

After simplifying my problem, I can say that I want to get the sum of
the product of two culumns:

Say
         m= [[ 'a', 1], [ 'b', 2],[ 'a', 3]]
         r={'a':4, 'b':5, 'c':6}

What I need is the calculation

         1*4 + 2*5 + 3*4 = 4 + 10 + 12 = 26

That is, for each row list in variable 'm' look for its first element
in variable 'r' and multiply the value found by the second element in
row 'm'. After that, sum all the products.

What's an efficient way to do it? I have thousands of these
calculations to make on a big data file.

Thank you.