Some uses of Arrays that were not apparent at first.

Enumeration

each is the basic enumerator method but there is also each_with_index (and for hashes each_pair)

['a', 'b', 'c', 'd'].each {|item| puts item }
['a', 'b', 'c', 'd'].each_with_index {|item,index| puts "#{index}:#{item}" }
{:a=>1, :b=>2}.each_pair {|key,value| puts "#{key}=>#{value}" }

Sorting

Sort is very easy with ruby if you want a basic sort [4,5,3,2,7].sort will do. However sorting by any key is possible using sort_by. Sort_by performs a Schwartzian Transform

['a','B','D','c'].sort
#=> ["B", "D", "a", "c"]
['a','B','D','c'].sort_by {|x| x.downcase }
#=> ["a", "B", "c", "D"]

Return Arrays

When you want an array, map modifies the contents of each element:

[1,2,3,4].map {|i| i*2 }
#=> [2,4,6,8]
# or with any enumerable (Range)
(1..4).map {|i| i*2 }

select and reject can be used to select and remove elements from the enumerable, and return a new enumerable, they are immutable methods.

[0,1,2,3,4,5].select { |x| x > 2 }
#=> [3,4,5]
[0,1,2,3,4,5].reject { |x| x < 3 }
#=> [3,4,5]

select! and reject! (bang operators) modify the existing enumerable. They have more natural names that I think should be used when appropriate, keep_if and delete_if.

[0,1,2,3,4,5].keep_if { |x| x > 2 }
#=> [3,4,5]
[0,1,2,3,4,5].delete_if { |x| x < 3 }
#=> [3,4,5]

Reduction to a Single Value

inject can be called on any enumerable object, it iterates over each item with an accumulator.

[1,2,3,4].inject {|memory, item| memory * (item+1) }

For basic operations there is the reduce method which can be used for summing and multiplying out the arrays.

Sum:

 [1,2,3,4].reduce :+

Multiply:

[1,2,3,4].reduce :*

Booleans

include? is used to check if the array contains the supplied object.

['a','b','c'].include?( 'b' )
#=> true

all? and any? can be used to check if the array matches against given criteria.

[1,2,3,4].any?{|x| x > 3 }
#=> true
[1,2,3,4].all?{|x| x > 3 }
#=> false
[1,2,3,4].any?{|x| x > 5 }
#=> false
[1,2,3,4].all?{|x| x > 0 }
#=> true

I often use any? with an array of regular expressions to match a string to any item.

text   = "one"
search = [/one/, /two/, /three/]
found  = search.any? { |regexp| text.match( regexp ) }
#=> true