Inside Enumerable

Mike Bowler

Gargoyle Software Inc.

Agenda

What is Enumerable?

What can you do with it?

How does it do that?

What is it?

It’s a group of related methods all having to do with collections

It’s the interesting part behind Array, Set, Hash and Range

It’s a Module (often called a mixin)

hash = { :a => 1, :b => 2} hash.collect do |key, value| value * 4 end [4,8] [4,8,12] [4,8,12] array = [1,2,3] array.collect do |value| value * 4 end range = 1..3 range.collect do |value| value * 4 end

hash = { :a => 1, :b => 2} hash.collect do |key, value| value * 4 end array = [1,2,3] array.collect do |value| value * 4 end range = 1..3 range.collect do |value| value * 4 end module Enumerable def collect ... end end

multiple inheritance

mixins

Mixins Cannot be instantiated Can be mixed in

Mixins usually don’t stand alone don’t stand alone

Typically...

mixins aren’t completely self-contained

they rely on certain methods being present on the class they are mixed into

This isn’t mandatory but is very common

Let’s look at an example...

Enumerable requires each() [1,2,3].collect {|i| i*2} [2,4,6]

Enumerable.collect class Foo include Enumerable def each yield 1 yield 2 yield 3 end end Foo.new.collect do |num| num * 2 end module Enumerable def collect array = [] each do |a| array << yield (a) end array end end

Enumerable.collect module Enumerable def collect array = [] each do |a| array << yield (a) end array end end class Foo include Enumerable def each yield 1 yield 2 yield 3 end end Foo.new.collect do |num| num * 2 end

Enumerable.collect class Foo include Enumerable def each yield 1 yield 2 yield 3 end end Foo.new.collect do |num| num * 2 end module Enumerable def collect array = [] each do |a| array << yield (a) end array end end

Enumerable.collect module Enumerable def collect array = [] each do |a| array << yield (a) end array end end class Foo include Enumerable def each yield 1 yield 2 yield 3 end end Foo.new.collect do |num| num * 2 end

Enumerable.collect class Foo include Enumerable def each yield 1 yield 2 yield 3 end end Foo.new.collect do |num| num * 2 end module Enumerable def collect array = [] each do |a| array << yield (a) end array end end Result [2,4,6]

Required methods

collect() would not have worked if Foo hadn’t declared each()

All methods in Enumerable require each()

A couple also require <=>

min(), max(), sort()

Comparison operator

Enumerable methods all?, any?, collect, detect, each_cons, each_slice, each_with_index, entries, enum_cons, enum_slice, enum_with_index, find, find_all, grep, include?, inject, map, max, member?, min, partition, reject, select, sort, sort_by, to_a, to_set, zip

any? [1,2,3,4].any? {|a| a > 2} Returns true if this block... ...returns true for any of these values

all? [1,2,3,4].all? {|a| a > 2} Returns true if this block... ...returns true for ALL of these values

collect, map

“Returns a new array with the results of running block once for every element in enum”

map is an alias for collect

[1,2,3].collect {|i| i*2} [2,4,6]

find, detect

find(ifnone = nil) {| obj | block } => obj or nil

Passes each entry in enum to block. Returns the first for which block is not false. If no object matches, calls ifnone and returns its result when it is specified, or returns nil

detect is an alias for find

find_all, select

find_all {| obj | block } => array

Returns an array containing all elements of enum for which block is not false

select is an alias for find_all

grep

grep(pattern) => array

Returns an array of every element in enum for which Pattern === element. If the optional block is supplied, each matching element is passed to it, and the block‘s result is stored in the output array

Case equality

For most objects, === is the same as ==

Case equality Regular equality

Case equality case myvar when 1..50 # do something when 51 # do something when /^d+$/ # do something end Within the range This exact number Matches this regular expression Different kind of equality as a convenience in case statements

Case equality in grep [1,2,3].grep(1..50) [1,2,3].grep(51) [1,2,3].grep(/^d+$/) Within the range This exact number Matches this regular expression

Wrap up