Sum of products of two arrays (dotproduct)

Question

First off, I know my title can be formulated better, but my math classes are so far gone I can\'t remember the correct words anymore..

I need to do something like this (

Answer 1

With LINQ:

int dotProduct = digits1.Zip(digits2, (d1, d2) => d1 * d2)
                        .Sum();

Zipwill produce a streaming sequence containing the products of corresponding elements from both arrays, which is then summed into an integer with Sum.

Note that this will not fail like it should when the arrays of unequal length, so you probably need to validate the input:

//null checks here

if(digits1.Length != digits2.Length)
   throw new ArgumentException("...");

EDIT: As Jeff M points out,Enumerable.Zipwas only added to the framework in .NET 4.0. In .NET 3.5, you can do this (the idea is only efficient for collections that expose fast indexers):

int dotProduct = Enumerable.Range(0, digits1.Length)
                           .Sum(i => digits1[i] * digits2[i]);

//from Jeff M's comment:
int dotProduct = digits1.Select((n, i) => n * digits2[i])
                        .Sum();