Question

I know about map/reduce alghoritm and its use. It’s using functions that are called Mappers and Reducers, but I also find people use the word Filters.

Are Filters same as Mappers or is there some significant difference?

Answer 1

A filter is like a map for which the passed function is always a “characteristic function”, that is a function that returns either “yes” or “no” to the question “does this belong here?”

In other words, think of a set defined as {x | x ∈ X and P(x) }. Filter takes the base set, tests to see if P(x) is true, and returns only those members for which it is true.

So { x | x is a natural number and odd(x) } is {1,3,5,7…}.

A map applies an arbitrary function, so you can think of that as a set like { y | x ∈ X and y = f(x) }.

So { y | x is a natural number and y = x² } is {1,4,9,16,…}.