Creates a new hash populated with the given objects. Equivalent to the literal { key, value, … }. Keys and values occur in pairs, so there must be an even number of arguments.

   Hash["a", 100, "b", 200]       #=> {"a"=>100, "b"=>200}
   Hash["a" => 100, "b" => 200]   #=> {"a"=>100, "b"=>200}
   { "a" => 100, "b" => 200 }     #=> {"a"=>100, "b"=>200}

Returns a new, empty hash. If this hash is subsequently accessed by a key that doesn‘t correspond to a hash entry, the value returned depends on the style of new used to create the hash. In the first form, the access returns nil. If obj is specified, this single object will be used for all default values. If a block is specified, it will be called with the hash object and the key, and should return the default value. It is the block‘s responsibility to store the value in the hash if required.

   h = Hash.new("Go Fish")
   h["a"] = 100
   h["b"] = 200
   h["a"]           #=> 100
   h["c"]           #=> "Go Fish"
   # The following alters the single default object
   h["c"].upcase!   #=> "GO FISH"
   h["d"]           #=> "GO FISH"
   h.keys           #=> ["a", "b"]

   # While this creates a new default object each time
   h = Hash.new { |hash, key| hash[key] = "Go Fish: #{key}" }
   h["c"]           #=> "Go Fish: c"
   h["c"].upcase!   #=> "GO FISH: C"
   h["d"]           #=> "Go Fish: d"
   h.keys           #=> ["c", "d"]

Try to convert obj into a hash, using to_hash method. Returns converted hash or nil if obj cannot be converted for any reason.

   Hash.try_convert({1=>2})   # => {1=>2}
   Hash.try_convert("1=>2")   # => nil

Equality—Two hashes are equal if they each contain the same number of keys and if each key-value pair is equal to (according to Object#==) the corresponding elements in the other hash.

   h1 = { "a" => 1, "c" => 2 }
   h2 = { 7 => 35, "c" => 2, "a" => 1 }
   h3 = { "a" => 1, "c" => 2, 7 => 35 }
   h4 = { "a" => 1, "d" => 2, "f" => 35 }
   h1 == h2   #=> false
   h2 == h3   #=> true
   h3 == h4   #=> false

Element Reference—Retrieves the value object corresponding to the key object. If not found, returns the a default value (see Hash::new for details).

   h = { "a" => 100, "b" => 200 }
   h["a"]   #=> 100
   h["c"]   #=> nil

Element Assignment—Associates the value given by value with the key given by key. key should not have its value changed while it is in use as a key (a String passed as a key will be duplicated and frozen).

   h = { "a" => 100, "b" => 200 }
   h["a"] = 9
   h["c"] = 4
   h   #=> {"a"=>9, "b"=>200, "c"=>4}

Searches through the hash comparing obj with the key using ==. Returns the key-value pair (two elements array) or nil if no match is found. See Array#assoc.

   h = {"colors"  => ["red", "blue", "green"],
        "letters" => ["a", "b", "c" ]}
   h.assoc("letters")  #=> ["letters", ["a", "b", "c"]]
   h.assoc("foo")      #=> nil

Removes all key-value pairs from hsh.

   h = { "a" => 100, "b" => 200 }   #=> {"a"=>100, "b"=>200}
   h.clear                          #=> {}

Makes hsh to compare its keys by their identity, i.e. it will consider exact same objects as same keys.

   h1 = { "a" => 100, "b" => 200, :c => "c" }
   h1["a"]        #=> 100
   h1.compare_by_identity
   h1.compare_by_identity? #=> true
   h1["a"]        #=> nil  # different objects.
   h1[:c]         #=> "c"  # same symbols are all same.

Returns true if hsh will compare its keys by their identity. Also see Hash#compare_by_identity.

Returns the default value, the value that would be returned by hsh[key] if key did not exist in hsh. See also Hash::new and Hash#default=.

   h = Hash.new                            #=> {}
   h.default                               #=> nil
   h.default(2)                            #=> nil

   h = Hash.new("cat")                     #=> {}
   h.default                               #=> "cat"
   h.default(2)                            #=> "cat"

   h = Hash.new {|h,k| h[k] = k.to_i*10}   #=> {}
   h.default                               #=> nil
   h.default(2)                            #=> 20

Sets the default value, the value returned for a key that does not exist in the hash. It is not possible to set the a default to a Proc that will be executed on each key lookup.

   h = { "a" => 100, "b" => 200 }
   h.default = "Go fish"
   h["a"]     #=> 100
   h["z"]     #=> "Go fish"
   # This doesn't do what you might hope...
   h.default = proc do |hash, key|
     hash[key] = key + key
   end
   h[2]       #=> #<Proc:0x401b3948@-:6>
   h["cat"]   #=> #<Proc:0x401b3948@-:6>

If Hash::new was invoked with a block, return that block, otherwise return nil.

   h = Hash.new {|h,k| h[k] = k*k }   #=> {}
   p = h.default_proc                 #=> #<Proc:0x401b3d08@-:1>
   a = []                             #=> []
   p.call(a, 2)
   a                                  #=> [nil, nil, 4]

Sets the default proc to be executed on each key lookup.

   h.default_proc = proc do |hash, key|
     hash[key] = key + key
   end
   h[2]       #=> 4
   h["cat"]   #=> "catcat"

Deletes and returns a key-value pair from hsh whose key is equal to key. If the key is not found, returns the default value. If the optional code block is given and the key is not found, pass in the key and return the result of block.

   h = { "a" => 100, "b" => 200 }
   h.delete("a")                              #=> 100
   h.delete("z")                              #=> nil
   h.delete("z") { |el| "#{el} not found" }   #=> "z not found"

Deletes every key-value pair from hsh for which block evaluates to true.

   h = { "a" => 100, "b" => 200, "c" => 300 }
   h.delete_if {|key, value| key >= "b" }   #=> {"a"=>100}

Calls block once for each key in hsh, passing the key-value pair as parameters.

   h = { "a" => 100, "b" => 200 }
   h.each {|key, value| puts "#{key} is #{value}" }

produces:

   a is 100
   b is 200

Calls block once for each key in hsh, passing the key as a parameter.

   h = { "a" => 100, "b" => 200 }
   h.each_key {|key| puts key }

produces:

   a
   b

Calls block once for each key in hsh, passing the key-value pair as parameters.

   h = { "a" => 100, "b" => 200 }
   h.each {|key, value| puts "#{key} is #{value}" }

produces:

   a is 100
   b is 200

Calls block once for each key in hsh, passing the value as a parameter.

   h = { "a" => 100, "b" => 200 }
   h.each_value {|value| puts value }

produces:

   100
   200

Returns true if hsh contains no key-value pairs.

   {}.empty?   #=> true

Returns true if hash and other are both hashes with the same content.

Returns a value from the hash for the given key. If the key can‘t be found, there are several options: With no other arguments, it will raise an KeyError exception; if default is given, then that will be returned; if the optional code block is specified, then that will be run and its result returned.

   h = { "a" => 100, "b" => 200 }
   h.fetch("a")                            #=> 100
   h.fetch("z", "go fish")                 #=> "go fish"
   h.fetch("z") { |el| "go fish, #{el}"}   #=> "go fish, z"

The following example shows that an exception is raised if the key is not found and a default value is not supplied.

   h = { "a" => 100, "b" => 200 }
   h.fetch("z")

produces:

   prog.rb:2:in `fetch': key not found (KeyError)
    from prog.rb:2

Returns a new array that is a one-dimensional flattening of this hash. That is, for every key or value that is an array, extract its elements into the new array. Unlike Array#flatten, this method does not flatten recursively by default. If the optional level argument determines the level of recursion to flatten.

   a =  {1=> "one", 2 => [2,"two"], 3 => "three"}
   a.flatten    # => [1, "one", 2, [2, "two"], 3, "three"]
   a.flatten(2) # => [1, "one", 2, 2, "two", 3, "three"]

Returns true if the given key is present in hsh.

   h = { "a" => 100, "b" => 200 }
   h.has_key?("a")   #=> true
   h.has_key?("z")   #=> false

Returns true if the given value is present for some key in hsh.

   h = { "a" => 100, "b" => 200 }
   h.has_value?(100)   #=> true
   h.has_value?(999)   #=> false

Compute a hash-code for this array. Two arrays with the same content will have the same hash code (and will compare using eql?).

Returns true if the given key is present in hsh.

   h = { "a" => 100, "b" => 200 }
   h.has_key?("a")   #=> true
   h.has_key?("z")   #=> false

Replaces the contents of hsh with the contents of other_hash.

   h = { "a" => 100, "b" => 200 }
   h.replace({ "c" => 300, "d" => 400 })   #=> {"c"=>300, "d"=>400}

Return the contents of this hash as a string.

    h = { "c" => 300, "a" => 100, "d" => 400, "c" => 300  }
    h.to_s   #=> "{\"c\"=>300, \"a\"=>100, \"d\"=>400}"

Returns a new hash created by using hsh‘s values as keys, and the keys as values.

   h = { "n" => 100, "m" => 100, "y" => 300, "d" => 200, "a" => 0 }
   h.invert   #=> {0=>"a", 100=>"m", 200=>"d", 300=>"y"}

Returns the key for a given value. If not found, returns nil.

   h = { "a" => 100, "b" => 200 }
   h.key(200)   #=> "b"
   h.key(999)   #=> nil

Returns true if the given key is present in hsh.

   h = { "a" => 100, "b" => 200 }
   h.has_key?("a")   #=> true
   h.has_key?("z")   #=> false

Returns a new array populated with the keys from this hash. See also Hash#values.

   h = { "a" => 100, "b" => 200, "c" => 300, "d" => 400 }
   h.keys   #=> ["a", "b", "c", "d"]

Returns the number of key-value pairs in the hash.

   h = { "d" => 100, "a" => 200, "v" => 300, "e" => 400 }
   h.length        #=> 4
   h.delete("a")   #=> 200
   h.length        #=> 3

Returns true if the given key is present in hsh.

   h = { "a" => 100, "b" => 200 }
   h.has_key?("a")   #=> true
   h.has_key?("z")   #=> false

Returns a new hash containing the contents of other_hash and the contents of hsh, overwriting entries in hsh with duplicate keys with those from other_hash.

   h1 = { "a" => 100, "b" => 200 }
   h2 = { "b" => 254, "c" => 300 }
   h1.merge(h2)   #=> {"a"=>100, "b"=>254, "c"=>300}
   h1             #=> {"a"=>100, "b"=>200}

Adds the contents of other_hash to hsh. If no block is specified entries with duplicate keys are overwritten with the values from other_hash, otherwise the value of each duplicate key is determined by calling the block with the key, its value in hsh and its value in other_hash.

   h1 = { "a" => 100, "b" => 200 }
   h2 = { "b" => 254, "c" => 300 }
   h1.merge!(h2)   #=> {"a"=>100, "b"=>254, "c"=>300}

   h1 = { "a" => 100, "b" => 200 }
   h2 = { "b" => 254, "c" => 300 }
   h1.merge!(h2) { |key, v1, v2| v1 }
                   #=> {"a"=>100, "b"=>200, "c"=>300}

Searches through the hash comparing obj with the value using ==. Returns the first key-value pair (two elements array) that matches. See also Array#rassoc.

   a = {1=> "one", 2 => "two", 3 => "three", "ii" => "two"}
   a.rassoc("two")    #=> [2, "two"]
   a.rassoc("four")   #=> nil

Rebuilds the hash based on the current hash values for each key. If values of key objects have changed since they were inserted, this method will reindex hsh. If Hash#rehash is called while an iterator is traversing the hash, an RuntimeError will be raised in the iterator.

   a = [ "a", "b" ]
   c = [ "c", "d" ]
   h = { a => 100, c => 300 }
   h[a]       #=> 100
   a[0] = "z"
   h[a]       #=> nil
   h.rehash   #=> {["z", "b"]=>100, ["c", "d"]=>300}
   h[a]       #=> 100

Same as Hash#delete_if, but works on (and returns) a copy of the hsh. Equivalent to hsh.dup.delete_if.

Equivalent to Hash#delete_if, but returns nil if no changes were made.

Replaces the contents of hsh with the contents of other_hash.

   h = { "a" => 100, "b" => 200 }
   h.replace({ "c" => 300, "d" => 400 })   #=> {"c"=>300, "d"=>400}

Returns a new hash consisting of entries which the block returns true.

   h = { "a" => 100, "b" => 200, "c" => 300 }
   h.select {|k,v| k > "a"}  #=> {"b" => 200, "c" => 300}
   h.select {|k,v| v < 200}  #=> {"a" => 100}

Removes a key-value pair from hsh and returns it as the two-item array [ key, value ], or the hash‘s default value if the hash is empty.

   h = { 1 => "a", 2 => "b", 3 => "c" }
   h.shift   #=> [1, "a"]
   h         #=> {2=>"b", 3=>"c"}

Returns the number of key-value pairs in the hash.

   h = { "d" => 100, "a" => 200, "v" => 300, "e" => 400 }
   h.length        #=> 4
   h.delete("a")   #=> 200
   h.length        #=> 3

Element Assignment—Associates the value given by value with the key given by key. key should not have its value changed while it is in use as a key (a String passed as a key will be duplicated and frozen).

   h = { "a" => 100, "b" => 200 }
   h["a"] = 9
   h["c"] = 4
   h   #=> {"a"=>9, "b"=>200, "c"=>4}

Converts hsh to a nested array of [ key, value ] arrays.

   h = { "c" => 300, "a" => 100, "d" => 400, "c" => 300  }
   h.to_a   #=> [["c", 300], ["a", 100], ["d", 400]]

Returns self.

Alias for inspect

Adds the contents of other_hash to hsh. If no block is specified entries with duplicate keys are overwritten with the values from other_hash, otherwise the value of each duplicate key is determined by calling the block with the key, its value in hsh and its value in other_hash.

   h1 = { "a" => 100, "b" => 200 }
   h2 = { "b" => 254, "c" => 300 }
   h1.merge!(h2)   #=> {"a"=>100, "b"=>254, "c"=>300}

   h1 = { "a" => 100, "b" => 200 }
   h2 = { "b" => 254, "c" => 300 }
   h1.merge!(h2) { |key, v1, v2| v1 }
                   #=> {"a"=>100, "b"=>200, "c"=>300}

Returns true if the given value is present for some key in hsh.

   h = { "a" => 100, "b" => 200 }
   h.has_value?(100)   #=> true
   h.has_value?(999)   #=> false

Returns a new array populated with the values from hsh. See also Hash#keys.

   h = { "a" => 100, "b" => 200, "c" => 300 }
   h.values   #=> [100, 200, 300]

Return an array containing the values associated with the given keys. Also see Hash.select.

  h = { "cat" => "feline", "dog" => "canine", "cow" => "bovine" }
  h.values_at("cow", "cat")  #=> ["bovine", "feline"]