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?
Charlie -Small, well written answers like this make me wish I could up vote your answer twice.
Frank Rosario
2009-05-02 23:01:13
So in short, filter is a map that uses the characteristic function.
Azder
2009-05-02 23:03:28
Um, yes and no, @Azder. If you did a map with the same odd(x) function, you'd get { 1, 0, 1, 0, 1, 0, ...} A filter is really a projection; you it's *only* those elements forwhich the predicate holds.
Charlie Martin
2009-05-02 23:07:03
@Frank, thanks very much. You could always get a second login ;-)
Charlie Martin
2009-05-02 23:07:27
Thanks about that comment Charlie, I forgot the ranges of map and filter are different.
Azder
2009-05-02 23:24:32
+1
A:
Filter takes a "list" and a function, applies the function to every member of the list and returns a new list containing only members where the application of the function returned true. For instance:
l = [1,2,3,4]
l = filter(lambda x: x < 3, l)
print l # [1,2]
Map does the same thing, but returns a list containing the results of the function application:
l = [1,2,3,4]
l = map(lambda x: x < 3, l)
print l # [True,True,False,False]
Patrick
2009-05-02 22:50:12
A:
A filter determines if an item should be kept or removed. A mapper just translates the value in to another. As a consequence: The output set of a map operation is always equal in size with the input set. The output on a filter operation is smaller than the input set.
Emil H
2009-05-02 22:50:22
A:
Generally, map functions take an input set and a function, and returns a set containing the function output for each input element. A filter takes an input set and a boolean function, and returns a set containing the input values for which the function returns true.
Alex
2009-05-02 23:21:16