reduce

Reduce is a standard function in Lisp and other functional programming languages. The basic reduce function has two arguments reduce(f,list), where the function f takes two arguments; f is initially applied to the first two elements in the list, then the function is applied again to the result and the third item, and so on. For example:
      reduce( '+', (1,2,3,4) ) = (((1+2)+3)+4).
Reduce functions typically allow an optional third argument that is used as an initial value.

The reduce function is one of the inspirations for the MapReduce infrastructure for massive scale processing of big data.