# Programming Ruby ### The Pragmatic Programmer's Guide
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class Array |
Parent: | Object
Version: | 1.6

### Index:

[ ] new & * + -- << <=> == === [ ] [ ]= | assoc at clear collect collect! compact compact! concat delete delete_at delete_if each each_index empty? eql? fill first flatten flatten! include? index indexes indices join last length map! nitems pack pop push rassoc reject! replace reverse reverse! reverse_each rindex shift size slice slice! sort sort! to_a to_ary to_s uniq uniq! unshift

Arrays are ordered, integer-indexed collections of any object. Array indexing starts at 0, as in C or Java. A negative index is assumed relative to the end of the array---that is, an index of -1 indicates the last element of the array, -2 is the next to last element in the array, and so on.

mixins
Enumerable: | collect, detect, each_with_index, entries, find, find_all, grep, include?, map, max, member?, min, reject, select, sort, to_a

class methods
[ ] | Array[ [anObject]* ] -> anArray
|  | Returns a new array populated with the given objects. Equivalent to the operator form `Array.[](` ... `)`.
`Array.[]( 1, 'a', /^A/ )` | » | `[1, "a", /^A/]`
`Array[ 1, 'a', /^A/ ]` | » | `[1, "a", /^A/]`
`[ 1, 'a', /^A/ ]` | » | `[1, "a", /^A/]`
new | Array.new( anInteger=0, anObject=nil ) -> anArray
|  | Returns a new array, optionally with a size and initial value (that is, anInteger references to the same anObject).
`Array.new` | » | `[]`
`Array.new(2)` | » | `[nil, nil]`
`Array.new(5, "A")` | » | `["A", "A", "A", "A", "A"]`
`Array.new(2, Hash.new)` | » | `[{}, {}]`

instance methods
& | arr & anOtherArray -> anArray
|  | Set Intersection---Returns a new array containing elements common to the two arrays, with no duplicates.
`[ 1, 1, 3, 5 ] & [ 1, 2, 3 ]` | » | `[1, 3]`
* | arr * anInteger -> anArray arr * aString -> anOtherString
|  | Repetition---With a `String` argument, equivalent to `arr.join(aString)`. Otherwise, returns a new array built by concatenating the anInteger copies of arr.
`[ 1, 2, 3 ] * 3` | » | `[1, 2, 3, 1, 2, 3, 1, 2, 3]`
+ | arr + anOtherArray -> anArray
|  | Concatenation---Returns a new array built by concatenating the two arrays together to produce a third array.
`[ 1, 2, 3 ] + [ 4, 5 ]` | » | `[1, 2, 3, 4, 5]`
-- | arr - anOtherArray -> anArray
|  | Set Difference---Returns a new array that is a copy of the original array, removing any items that also appear in anOtherArray and duplicated items.
`[ 1, 1, 2, 2, 3, 3, 3, 4, 5 ] - [ 1, 2, 4 ]` | » | `[3, 5]`
<< | arr << anObject -> arr
|  | Append---Pushes the given object on to the end of this array. This expression returns the array itself, so several appends may be chained together. See also `Array#push` .
`[ 1, 2 ] << "c" << "d" << [ 3, 4 ]` | » | `[1, 2, "c", "d", [3, 4]]`
<=> | arr <=> anOtherArray -> -1, 0, +1
|  | Comparison---Returns an integer -1, 0, or +1 if this array is less than, equal to, or greater than anOtherArray. Each object in each array is compared (using `<=>`). If any value isn't equal, then that inequality is the return value. If all the values found are equal, then the return is based on a comparison of the array lengths. Thus, two arrays are ``equal'' according to `Array#<=>` if and only if they have the same length and the value of each element is equal to the value of the corresponding element in the other array.
`[ "a", "a", "c" ] <=> [ "a", "b", "c" ]` | » | `-1`
`[ 1, 2, 3, 4, 5, 6 ] <=> [ 1, 2 ]` | » | `1`
== | arr == anOtherArray -> `true` or `false`
|  | Equality---Two arrays are equal if they contain the same number of elements and if each element is equal to (according to `Object#==` ) the corresponding element in the other array.
`[ "a", "c" ] == [ "a", "c", 7 ]` | » | `false`
`[ "a", "c", 7 ] == [ "a", "c", 7 ]` | » | `true`
`[ "a", "c", 7 ] == [ "a", "d", "f" ]` | » | `false`
=== | arr === anOtherArray -> `true` or `false`
|  | Case Equality---Equality as evaluated by `case` expressions. For arrays, this is the same as `Array#==` .
[ ] | arr[anInteger] -> anObject or `nil` arr[start, length] -> aSubArray or `nil` arr[aRange] -> aSubArray or `nil`
|  | Element Reference---Returns the element at index anInteger, or returns a subarray starting at index start and continuing for length elements, or returns a subarray specified by aRange. Negative indices count backward from the end of the array (-1 is the last element). Returns `nil` if any indices are out of range.
`a = [ "a", "b", "c", "d", "e" ]`
`a[2] + a[0] + a[1]` | » | `"cab"`
`a[6]` | » | `nil`
`a[1, 2]` | » | `["b", "c"]`
`a[1..3]` | » | `["b", "c", "d"]`
`a[4..7]` | » | `["e"]`
`a[6..10]` | » | `nil`
`a[-3, 3]` | » | `["c", "d", "e"]`
[ ]= | arr[anInteger] = anObject -> anObject arr[start, length] = aSubArray -> aSubArray arr[aRange] = aSubArray -> aSubArray
|  | Element Assignment---Sets the element at index anInteger, or replaces a subarray starting at index start and continuing for length elements, or replaces a subarray specified by aRange. If anInteger is greater than the current capacity of the array, the array grows automatically. A negative anInteger will count backward from the end of the array. Inserts elements if length is zero. If subArray is `nil`, deletes elements from arr. An `IndexError` is raised if a negative index points past the beginning of the array. See also `Array#push` , `Array#unshift` .
`a = Array.new` | » | `[]`
`a[4] = "4"; a` | » | `[nil, nil, nil, nil, "4"]`
`a[0, 3] = [ 'a', 'b', 'c' ]; a` | » | `["a", "b", "c", nil, "4"]`
`a[1..2] = [ 1, 2 ]; a` | » | `["a", 1, 2, nil, "4"]`
`a[0, 2] = "?"; a` | » | `["?", 2, nil, "4"]`
`a[0..2] = "A"; a` | » | `["A", "4"]`
`a[-1] = "Z"; a` | » | `["A", "Z"]`
`a[1..-1] = nil; a` | » | `["A"]`
| | arr | anOtherArray -> anArray
|  | Set Union---Returns a new array by joining this array with anOtherArray, removing duplicates.
`[ "a", "b", "c" ] | [ "c", "d", "a" ]` | » | `["a", "b", "c", "d"]`
assoc | arr.assoc( anObject ) -> anArray or `nil`
|  | Searches through an array whose elements are also arrays comparing anObject with the first element of each contained array using anObject `.==` . Returns the first contained array that matches (that is, the first associated array), or `nil` if no match is found. See also `Array#rassoc` .
`s1 = [ "colors", "red", "blue", "green" ]`
`s2 = [ "letters", "a", "b", "c" ]`
`s3 = "foo"`
`a = [ s1, s2, s3 ]`
`a.assoc("letters")` | » | `["letters", "a", "b", "c"]`
`a.assoc("foo")` | » | `nil`
at | arr.at( anInteger ) -> anObject or `nil`
|  | Returns the element at index anInteger. A negative index counts from the end of arr. Returns `nil` if the index is out of range. See also `Array#[]` . ( `Array#at` is slightly faster than `Array#[]` , as it does not accept ranges and so on.)
`a = [ "a", "b", "c", "d", "e" ]`
`a.at(0)` | » | `"a"`
`a.at(-1)` | » | `"e"`
clear | arr.clear -> arr
|  | Removes all elements from arr.
`a = [ "a", "b", "c", "d", "e" ]`
`a.clear` | » | `[]`
collect | arr.collect {| obj | block } -> anArray
|  | Returns a new array by invoking block once for every element, passing each element as a parameter to block. The result of block is used as the given element in the new array. See also `Array#collect!` .
`a = [ "a", "b", "c", "d" ]`
`a.collect {|x| x + "!" }` | » | `["a!", "b!", "c!", "d!"]`
`a` | » | `["a", "b", "c", "d"]`
collect! | arr.collect! {| obj | block } -> arr
|  | Invokes block once for each element of arr, replacing the element with the value returned by block. See also `Array#collect` .
`a = [ "a", "b", "c", "d" ]`
`a.collect! {|x| x + "!" }` | » | `["a!", "b!", "c!", "d!"]`
`a` | » | `["a!", "b!", "c!", "d!"]`
compact | arr.compact -> anArray
|  | Returns a new array based on the arr with all `nil` elements removed.
`[ "a", nil, "b", nil, "c", nil ].compact` | » | `["a", "b", "c"]`
compact! | arr.compact! -> arr or `nil`
|  | Same as `Array#compact` , but modifies the receiver in place. Returns `nil` if no changes were made.
`[ "a", nil, "b", nil, "c" ].compact!` | » | `["a", "b", "c"]`
`[ "a", "b", "c" ].compact!` | » | `nil`
concat | arr.concat( anOtherArray ) -> arr
|  | Appends the elements in anOtherArray to arr.
`[ "a", "b" ].concat( ["c", "d"] )` | » | `["a", "b", "c", "d"]`
delete | arr.delete( anObject ) -> anObject or `nil` arr.delete( anObject ) {| | block } -> anObject or `nil`
|  | Deletes items from the self that are equal to anObject. If the item is not found, returns `nil`. If the optional code block is given, returns the result of block if the item is not found.
`a = [ "a", "b", "b", "b", "c" ]`
`a.delete("b")` | » | `"b"`
`a` | » | `["a", "c"]`
`a.delete("z")` | » | `nil`
`a.delete("z") { "not found" }` | » | `"not found"`
delete_at | arr.delete_at( anIndex ) -> anObject or `nil`
|  | Deletes the element at the specified index, returning that element, or `nil` if the index is out of range. See also `Array#slice!` .
`a = %w( ant bat cat dog )`
`a.delete_at(2)` | » | `"cat"`
`a` | » | `["ant", "bat", "dog"]`
`a.delete_at(99)` | » | `nil`
delete_if | arr.delete_if {| | block } -> arr
|  | Deletes every element of arr for which block evaluates to `true`.
`a = [ "a", "b", "c" ]`
`a.delete_if {|x| x >= "b" }` | » | `["a"]`
each | arr.each {| item | block } -> arr
|  | Calls block once for each element in arr, passing that element as a parameter.
    a = [ "a", "b", "c" ]
    a.each {|x| print x, " -- " }
produces:
    a -- b -- c --
each_index | arr.each_index {| anIndex | block } -> arr
|  | Same as `Array#each` , but passes the index of the element instead of the element itself.
    a = [ "a", "b", "c" ]
    a.each_index {|x| print x, " -- " }
produces:
    0 -- 1 -- 2 --
empty? | arr.empty? -> `true` or `false`
|  | Returns `true` if arr array contains no elements.
`[].empty?` | » | `true`
eql? | arr.eql?( anOtherArray ) -> `true` or `false`
|  | An array is equal to another array if the lengths are equal and each corresponding element is equal (according to `Object#eql?` ). See also `Array#<=>` . `eql?` is used for `Hash` comparison.
`[ "a", "b", "c" ].eql?(["a", "b", "c"])` | » | `true`
`[ "a", "b", "c" ].eql?(["a", "b"])` | » | `false`
`[ "a", "b", "c" ].eql?(["b", "c", "d"])` | » | `false`
fill | arr.fill( anObject ) -> arr arr.fill( anObject, start [, length ] ) -> arr arr.fill( anObject, aRange ) -> arr
|  | Sets the selected elements of arr (which may be the entire array) to anObject. A start of `nil` is equivalent to zero. A length of `nil` is equivalent to arr.length.
`a = [ "a", "b", "c", "d" ]`
`a.fill("x")` | » | `["x", "x", "x", "x"]`
`a.fill("z", 2, 2)` | » | `["x", "x", "z", "z"]`
`a.fill("y", 0..1)` | » | `["y", "y", "z", "z"]`
first | arr.first -> anObject or `nil`
|  | Returns the first element of the array. If the array is empty, returns `nil`.
`a = [ "q", "r", "s", "t" ]`
`a.first` | » | `"q"`
flatten | arr.flatten -> anArray
|  | Returns a new array that is a one-dimensional flattening of this array (recursively). That is, for every element that is an array, extract its elements into the new array.
`s = [ 1, 2, 3 ]` | » | `[1, 2, 3]`
`t = [ 4, 5, 6, [7, 8] ]` | » | `[4, 5, 6, [7, 8]]`
`a = [ s, t, 9, 10 ]` | » | `[[1, 2, 3], [4, 5, 6, [7, 8]], 9, 10]`
`a.flatten` | » | `[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]`
flatten! | arr.flatten! -> arr or `nil`
|  | Same as `Array#flatten` , but modifies the receiver in place. Returns `nil` if no modifications were made (i.e., arr contains no subarrays.)
`a = [ 1, 2, [3, [4, 5] ] ]`
`a.flatten!` | » | `[1, 2, 3, 4, 5]`
`a.flatten!` | » | `nil`
`a` | » | `[1, 2, 3, 4, 5]`
include? | arr.include?( anObject ) -> `true` or `false`
|  | Returns `true` if the given object is present in arr (that is, if any object `==` anObject), `false` otherwise.
`a = [ "a", "b", "c" ]`
`a.include?("b")` | » | `true`
`a.include?("z")` | » | `false`
Template characters for `Array#pack`
Directive | Meaning
@ | Moves to absolute position
A | ASCII string (space padded, count is width)
a | ASCII string (null padded, count is width)
B | Bit string (descending bit order)
b | Bit string (ascending bit order)
C | Unsigned char
c | Char
d | Double-precision float, native format
E | Double-precision float, little-endian byte order
e | Single-precision float, little-endian byte order
f | Single-precision float, native format
G | Double-precision float, network (big-endian) byte order
g | Single-precision float, network (big-endian) byte order
H | Hex string (high nibble first)
h | Hex string (low nibble first)
I | Unsigned integer
i | Integer
L | Unsigned long
l | Long
M | Quoted printable, MIME encoding (see RFC2045)
m | Base64 encoded string
N | Long, network (big-endian) byte order
n | Short, network (big-endian) byte-order
P | Pointer to a structure (fixed-length string)
p | Pointer to a null-terminated string
S | Unsigned short
s | Short
U | UTF-8
u | UU-encoded string
V | Long, little-endian byte order
v | Short, little-endian byte order
X | Back up a byte
x | Null byte
Z | Same as ``A''
index | arr.index( anObject ) -> anInteger or `nil`
|  | Returns the index of the first object in arr such that the object `==` anObject. Returns `nil` if no match is found.
`a = [ "a", "b", "c" ]`
`a.index("b")` | » | `1`
`a.index("z")` | » | `nil`
indexes | arr.indexes( i1, i2, ... iN ) -> anArray
|  | Returns a new array consisting of elements at the given indices. May insert `nil` for indices out of range.
`a = [ "a", "b", "c", "d", "e", "f", "g" ]`
`a.indexes(0, 2, 4)` | » | `["a", "c", "e"]`
`a.indexes(0, 2, 4, 12)` | » | `["a", "c", "e", nil]`
indices | arr.indices( i1, i2, ... iN ) -> anArray
|  | Synonym for `Array#indexes` .
join | arr.join( aSepString=`$,` ) -> aString
|  | Returns a string created by converting each element of the array to a string, separated by aSepString.
`[ "a", "b", "c" ].join` | » | `"abc"`
`[ "a", "b", "c" ].join("-")` | » | `"a-b-c"`
last | arr.last -> anObject or `nil`
|  | Returns the last element of arr. If the array is empty, returns `nil`.
`[ "w", "x", "y", "z" ].last` | » | `"z"`
length | arr.length -> anInteger
|  | Returns the number of elements in arr. May be zero.
`[ 1, 2, 3, 4, 5 ].length` | » | `5`
map! | arr.map! {| obj | block } -> arr
|  | Synonym for `Array#collect!` .
nitems | arr.nitems -> anInteger
|  | Returns the number of non-`nil` elements in arr. May be zero.
`[ 1, nil, 3, nil, 5 ].nitems` | » | `3`
pack | arr.pack ( aTemplateString ) -> aBinaryString
|  | Packs the contents of arr into a binary sequence according to the directives in aTemplateString (see Table 22.1 on page 285). Directives ``A,'' ``a,'' and ``Z'' may be followed by a count, which gives the width of the resulting field. The remaining directives also may take a count, indicating the number of array elements to convert. If the count is an asterisk (```*`''), all remaining array elements will be converted. Any of the directives ```sSiIlL`'' may be followed by an underscore (```_`'') to use the underlying platform's native size for the specified type; otherwise, they use a platform-independent size. Spaces are ignored in the template string. See also `String#unpack` on page 378.
`a = [ "a", "b", "c" ]`
`n = [ 65, 66, 67 ]`
`a.pack("A3A3A3")` | » | `"abc"`
`a.pack("a3a3a3")` | » | `"a\000\000b\000\000c\000\000"`
`n.pack("ccc")` | » | `"ABC"`
pop | arr.pop -> anObject or `nil`
|  | Removes the last element from arr and returns it, or `nil` if the array is empty (as with a stack).
`a = [ "a", "m", "z" ]`
`a.pop` | » | `"z"`
`a` | » | `["a", "m"]`
push | arr.push( [ anObject ]+ ) -> arr
|  | Appends the given argument(s) to the end of arr (as with a stack).
`a = [ "a", "b", "c" ]`
`a.push("d", "e", "f")` | » | `["a", "b", "c", "d", "e", "f"]`
rassoc | arr.rassoc( key ) -> anArray or `nil`
|  | Searches through the array whose elements are also arrays. Compares key with the second element of each contained array using `==`. Returns the first contained array that matches. See also `assoc`.
`a = [ [ 1, "one"], [2, "two"], [3, "three"], ["ii", "two"] ]`
`a.rassoc("two")` | » | `[2, "two"]`
`a.rassoc("four")` | » | `nil`
reject! | arr.reject! {| | block } -> arr or `nil`
|  | Equivalent to `Array#delete_if` , but returns `nil` if no changes were made.
replace | arr.replace( anOtherArray ) -> arr
|  | Replaces the contents of arr with the contents of anOtherArray, truncating or expanding if necessary.
`a = [ "a", "b", "c", "d", "e" ]`
`a.replace( [ "x", "y", "z" ] )` | » | `["x", "y", "z"]`
`a` | » | `["x", "y", "z"]`
reverse | arr.reverse -> anArray
|  | Returns a new array using arr's elements in reverse order.
`[ "a", "b", "c" ].reverse` | » | `["c", "b", "a"]`
`[ 1 ].reverse` | » | `[1]`
reverse! | arr.reverse! -> arr or `nil`
|  | Same as `reverse`, but returns `nil` if arr is unchanged (arr `.length` is zero or one).
`a = [ "a", "b", "c" ]`
`a.reverse!` | » | `["c", "b", "a"]`
`a` | » | `["c", "b", "a"]`
`[ 1 ].reverse!` | » | `nil`
reverse_each | arr.reverse_each {| | block }
|  | Same as `Array#each` , but traverses arr in reverse order.
    a = [ "a", "b", "c" ]
    a.reverse_each {|x| print x, " " }
produces:
    c b a
rindex | arr.rindex( anObject ) -> anInteger or `nil`
|  | Returns the index of the last object in arr such that the object `==` anObject. Returns `nil` if no match is found.
`a = [ "a", "b", "b", "b", "c" ]`
`a.rindex("b")` | » | `3`
`a.rindex("z")` | » | `nil`
shift | arr.shift -> anObject or `nil`
|  | Returns the first element of arr and removes it (shifting all other elements down by one). Returns `nil` if the array is empty.
`args = [ "-m", "-q", "filename" ]`
`args.shift` | » | `"-m"`
`args` | » | `["-q", "filename"]`
size | arr.size -> anInteger
|  | Synonym for `Array#length` .
slice | arr.slice( anInteger ) -> anObject arr.slice( start, length ) -> aSubArray arr.slice( aRange ) -> aSubArray
|  | Synonym for `Array#[ ]` .
`a = [ "a", "b", "c", "d", "e" ]`
`a.slice(2) + a.slice(0) + a.slice(1)` | » | `"cab"`
`a.slice(6)` | » | `nil`
`a.slice(1, 2)` | » | `["b", "c"]`
`a.slice(1..3)` | » | `["b", "c", "d"]`
`a.slice(4..7)` | » | `["e"]`
`a.slice(6..10)` | » | `nil`
`a.slice(-3, 3)` | » | `["c", "d", "e"]`
slice! | arr.slice!( anInteger ) -> anObject or `nil` arr.slice!( start, length ) -> aSubArray or `nil` arr.slice!( aRange ) -> aSubArray or `nil`
|  | Deletes the element(s) given by an index (optionally with a length) or by a range. Returns the deleted object, subarray, or `nil` if the index is out of range. Equivalent to:
    def slice!(*args)
      result = self[*args]
      self[*args] = nil
      result
    end
`a = [ "a", "b", "c" ]`
`a.slice!(1)` | » | `"b"`
`a` | » | `["a", "c"]`
`a.slice!(-1)` | » | `"c"`
`a` | » | `["a"]`
`a.slice!(100)` | » | `nil`
`a` | » | `["a"]`
sort | arr.sort -> anArray arr.sort {| a,b | block } -> anArray
|  | Returns a new array created by sorting arr. Comparisons for the sort will be done using the `<=>` operator or using an optional code block. The block implements a comparison between a and b, returning -1, 0, or +1.
`a = [ "d", "a", "e", "c", "b" ]`
`a.sort` | » | `["a", "b", "c", "d", "e"]`
`a.sort {|x,y| y <=> x }` | » | `["e", "d", "c", "b", "a"]`
sort! | arr.sort! -> arr arr.sort! {| a,b | block } -> arr
|  | Same as `Array#sort` , but modifies the receiver in place. arr is effectively frozen while a sort is in progress.
`a = [ "d", "a", "e", "c", "b" ]`
`a.sort!` | » | `["a", "b", "c", "d", "e"]`
`a` | » | `["a", "b", "c", "d", "e"]`
to_a | arr.to_a -> arr
|  | Returns arr.
to_ary | arr.to_ary -> arr
|  | Synonym for `Array#to_a` .
to_s | arr.to_s -> aString
|  | Returns arr `.join`.
`[ "a", "e", "i", "o" ].to_s` | » | `"aeio"`
uniq | arr.uniq -> anArray
|  | Returns a new array by removing duplicate values in arr.
`a = [ "a", "a", "b", "b", "c" ]`
`a.uniq` | » | `["a", "b", "c"]`
uniq! | arr.uniq! -> arr or `nil`
|  | Same as `Array#uniq` , but modifies the receiver in place. Returns `nil` if no changes are made (that is, no duplicates are found).
`a = [ "a", "a", "b", "b", "c" ]`
`a.uniq!` | » | `["a", "b", "c"]`
`b = [ "a", "b", "c" ]`
`b.uniq!` | » | `nil`
unshift | arr.unshift( anObject ) -> arr
|  | Prepends anObject to the front of arr, and shifts all other elements up one.
`a = [ "b", "c", "d" ]`
`a.unshift("a")` | » | `["a", "b", "c", "d"]`

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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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