hash

hash is declared inside { }

ex:

animal= { 

    :one=> “dog”,

    :two=>”rabbit”

}

說明:

animal[:one] = “dog”

animal[:two]=”rabbit”

ex:

animal= {

    :one=>[‘v’, ‘version’ ]

}

CONSTANT is often defined as hash

ex:

OPTIONS={

    :command=>[‘r’, ‘run’]

}

keys:

return the keys of hash in the array

ex:

{ :a=>”name”, :b=>”age”}.keys

-->  [ :a,  :b ]

values:

return the values of hash in the array

ex:

{ :name=>"peter"}.values

--->  ["peter"]

values_at

get multiple element

ex:

{ :a=>"a", :b=>"b", :c=>"c"}.values_at(:a,:b)

---> ["a", "b"]

has_key? :

ex:

{:a=>"a"}.has_key?(:a)

--->  true

sort_by:

由小排到大

ex:

a={ :c=>"C", :b=>"B" }

a.keys.sort_by{ |i| a[i] }

---> [:b, :c]

ex:

sort_by{ |a, b| }

sort according to a first, according to b in case of a tie

merge:

ex:

{ :a=>"a", :b=>"b"}.merge( {:a=>"c", :d=>"d" } )

-->  { :a=>"c", :d=>"d",  :b=>"b" }

each_key:

ex:

c= { :a=>"aa", :b=>"bb"}

{ :a=>"aa", :b=>"bb"}.each_key { |i| puts c[i] }

-->  aa

      bb

each_pair:

{ "name"=>"peter"}.each_pair do |i,j|

     puts   i+ " "+ j

end

---> name peter

sort:

return nested array

default(without pass block parameter) :

ascending order  & compare hash's value

ex:

{ "b"=>1,  "a"=>2}.sort

--> [ ["b", 1],  ["a", 2] ]

Hash.new(0)

set default value of hash for nonexistent keys, the key is still nonexistent

ex:

a =Hash.new(0)

a[2]

--->  0

set default value of hash for nonexistent keys, the key is added

ex:

a=Hash.new { |hash, key| hash[key]=0 }