Functional programming and looping(railspikes.com)
railspikes.com
Functional programming and looping
http://railspikes.com/2008/7/29/functional-loops-in-ruby-each-map-inject-select-and-for
2 comments
Of course, one of the common knocks on Ruby is that naively referring to a function executes it, and to pass functions around avoiding
[1] http://github.com/rails/rails/tree/master/activesupport/lib/...
[2] http://www.ruby-forum.com/topic/161089
list.map { |i| process i }
list.map &method(:process)
list.map &:process
[1] http://github.com/rails/rails/tree/master/activesupport/lib/...
[2] http://www.ruby-forum.com/topic/161089
map -- Perform a function on every value in a list individually, as a transformation. As an easy example, map square [1, 2, 3, 4, 5] -> [1, 4, 9, 16, 25]. This is usually just called "map" (it's short), but is different thing than a key->value map. The sequence of execution here is deliberately unspecified, and for potentially expensive/time-consuming operations this can be an easy operation to parallelize.
list.each -- Iterate through a list, doing something for each value. AKA foreach or List.iter. This is different from map in that it discards the result; it is just used for side-effects.
select -- AKA filter, called remove-if-not in Common Lisp. This returns a list of all values in a list meeting some criteria (a predicate, a function that returns a bool, is passed in): filter even [1, 2, 3, 4, 5] -> [2, 4].
reduce -- AKA fold - Starting with a default value (often the identity for a function, e.g. 0 for adding and 1 for multiplication), successively apply a two-argument operation to the current total and the values of the list, reducing it down to one overall result: reduce * 1 [1, 2, 3, 4, 5] -> (((((1 * 1) * 2) * 3) * 4) * 5). NB: Some languages make a distinction between a left fold and a right fold, which refers to whether it starts at the beginning or the end of the (potentially infinite!) list.