Programming Ruby

The Pragmatic Programmer's Guide
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class Hash
Parent: Object
Version: 1.6

Index:

[ ] new == [ ] [ ]= clear default default= delete delete_if each each_key each_pair each_value empty? fetch has_key? has_value? include? index indexes indices invert key? keys length member? rehash reject reject! replace shift size sort store to_a to_s update value? values

A Hash is a collection of key-value pairs. It is similar to an Array, except that indexing is done via arbitrary keys of any object type, not an integer index. The order in which you traverse a hash by either key or value may seem arbitrary, and will generally not be in the insertion order.

Hashes have a default value that is returned when accessing keys that do not exist in the hash. By default, that value is nil.
mixins
Enumerable: collect, detect, each_with_index, entries, find, find_all, grep, include?, map, max, member?, min, reject, select, sort, to_a

class methods
[ ] Hash[ [ key => value ]* ] -> aHash

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}

new Hash.new( anObject=nil ) -> aHash

Returns a new, empty hash. If anObject is specified, it will be used as the default value.

h = Hash.new("Go Fish")
h["a"] = 100
h["b"] = 200
h["a"] » 100
h["c"] » "Go Fish"

instance methods
== hsh == anOtherHash -> true or false

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

[ ] hsh[ aKeyObject ] -> aValueObject

Element Reference---Retrieves the aValueObject stored for aKeyObject. If not found, returns the default value.

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

[ ]= hsh[ aKeyObject ] = aValueObject -> aValueObject

Element Assignment---Associates the value given by aValueObject with the key given by aKeyObject. aKeyObject 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}

clear hsh.clear -> hsh

Removes all key-value pairs from hsh.

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

default hsh.default -> anObject

Returns the ``default value''---that is, the value returned for a key that does not exist in the hash. Defaults to nil. See also Hash#default= .

default= hsh.default = anObject -> hsh

Sets the ``default value''---that is, the value returned for a key that does not exist in the hash. Defaults to nil.

h = { "a" => 100, "b" => 200 }
h.default = "Go fish"
h["a"] » 100
h["z"] » "Go fish"

delete hsh.delete( aKeyObject ) -> aValueObject
hsh.delete( aKeyObject ) {| aKeyObject | block }

-> aValueObject

Deletes and returns a key-value pair from hsh whose key is equal to aKeyObject. 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"

delete_if hsh.delete_if {| key, value | block }

-> hsh

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}

each hsh.each {| key, value | block } -> hsh

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

h = { "a" => 100, "b" => 200 }
h.each {|key, value| print key, " is ", value, "\n" }
produces: 
a is 100
b is 200

each_key hsh.each_key {| key | block } -> hsh

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

each_pair hsh.each_pair {| key, value | block } -> hsh

Synonym for Hash#each .

each_value hsh.each_value {| value | block } -> hsh

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

empty? hsh.empty? -> true or false

Returns true if hsh contains no key-value pairs.

{}.empty? » true

fetch hsh.fetch( aKeyObject [, aDefObject ] ) -> anObject
hsh.fetch( aKeyObject ) {| aKeyObject | block }

-> anObject

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 IndexError exception; if aDefObject 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 (IndexError)
	from prog.rb:2

has_key? hsh.has_key?( aKeyObject ) -> true or false

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

has_value? hsh.has_value?( aValueObject ) -> true or 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

include? hsh.include?( aKeyObject ) -> true or false

Synonym for Hash#has_key? .

index hsh.index( aValueObject ) -> aKeyObject

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

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

indexes hsh.indexes( [ key ]+ ) -> anArray

Returns a new array consisting of values for the given key(s). Will insert the default value for keys that are not found.

h = { "a" => 100, "b" => 200, "c" => 300 }
h.indexes("a", "c") » [100, 300]
h.indexes("a", "c", "z") » [100, 300, nil]

indices hsh.indices( [ key ]+ ) -> anArray

Synonym for Hash#indexes .

invert hsh.invert -> aHash

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=>"n", 200=>"d", 300=>"y"}

key? hsh.key?( aKeyObject ) -> true or false

Synonym for Hash#has_key? .

keys hsh.keys -> anArray

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"]

length hsh.length -> aFixnum

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

member? hsh.member?( aKeyObject ) -> true or false

Synonym for Hash#has_key? .

rehash hsh.rehash -> hsh

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 IndexError 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

reject hsh.reject {| key, value | block }

-> aHash

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

reject! hsh.reject! {| key, value | block }

-> hsh or nil

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

replace hsh.replace( anOtherHash ) -> hsh

Replaces the contents of hsh with the contents of anOtherHash.

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

shift hsh.shift -> anArray or nil

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

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

size hsh.size -> aFixnum

Synonym for Hash#length .

sort hsh.sort -> anArray
hsh.sort {| a, b | block }

-> anArray

Converts hsh to a nested array of [ key, value ] arrays and sorts it, using Array#sort .

h = { "a" => 20, "b" => 30, "c" => 10  }
h.sort » [["a", 20], ["b", 30], ["c", 10]]
h.sort {|a,b| a[1]<=>b[1]} » [["c", 10], ["a", 20], ["b", 30]]

store hsh.store( aKeyObject, aValueObject ) -> aValueObject

Synonym for Element Assignment ( Hash#[]= ).

to_a hsh.to_a -> anArray

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

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

to_s hsh.to_s -> aString

Converts hsh to a string by converting the hash to an array of [ key, value ] pairs and then converting that array to a string using Array#join with the default separator.

h = { "c" => 300, "a" => 100, "d" => 400, "c" => 300  }
h.to_s » "a100c300d400"

update hsh.update( anOtherHash ) -> hsh

Adds the contents of anOtherHash to hsh, overwriting entries with duplicate keys with those from anOtherHash.

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

value? hsh.value?( aValueObject ) -> true or false

Synonym for Hash#has_value? .

values hsh.values -> anArray

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]

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Extracted from the book "Programming Ruby - The Pragmatic Programmer's Guide"
Copyright © 2001 by Addison Wesley Longman, Inc. This material may be distributed only subject to the terms and conditions set forth in the Open Publication License, v1.0 or later (the latest version is presently available at http://www.opencontent.org/openpub/)).

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