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# Ruby Arrays

Ruby arrays are ordered, integer-indexed collections of any object. Each element in an array is associated with and referred to by an index.

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.

Ruby arrays can hold objects such as String, Integer, Fixnum, Hash, Symbol, even other Array objects. Ruby arrays are not as rigid as arrays in other languages. Ruby arrays grow automatically while adding elements to them.

## Creating Arrays:

There are many ways to create or initialize an array. One way is with the new class method:

    names = Array.new

You can set the size of an array at the time of creating array:

    names = Array.new(20)

The array names now has a size or length of 20 elements. You can return the size of an array with either the size or length methods:

    months.size  # This returns 20
    months.length # This also returns 20

You can assign a value to each element in the array as follows:

    #!/usr/bin/ruby

    names = Array.new(4, "mac")

    puts "#{names}"

This will produce following result:

    macmacmacmac

You can also use a block with new, populating each element with what the block evaluates to:

    #!/usr/bin/ruby

    nums = Array.new(10) { |e| e = e * 2 }

    puts "#{nums}"

This will produce following result:

    024681012141618

There is another method of Array, []. It works like this:

    nums = Array.[](1, 2, 3, 4,5)

One more form of array creation is as follows :

    nums = Array[1, 2, 3, 4,5]

The Kernel module available in core Ruby, has an Array method, which only accepts a single argument. Here the method takes a range as an argument to create an array of digits:

    #!/usr/bin/ruby

    digits = Array(0..9)

    puts "#{digits}"

This will produce following result:

    0123456789

# Array Built-in Methods:

We need to have an instance of Array object to call a Array method. As we have seen, following is the way to create an instance of Array object:

    Array.[](...) [or] Array[...] [or] [...]

This will returns a new array populated with the given objects. Now using created object we can call any available instance methods. For example:

    #!/usr/bin/ruby

    digits = Array(0..9)

    num = digits.at(6)

    puts "#{num}"

This will produce following result:

    6

Following are the public array methods ( Assuming array is an array object ):

SN | Methods with Description
--- | ---
1 | array & other_array Returns a new array containing elements common to the two arrays, with no duplicates.
2 | array * int [or] array * str Returns a new array built by concatenating the int copies of self. With a String argument, equivalent to self.join(str).
3 | array + other_array Returns a new array built by concatenating the two arrays together to produce a third array.
4 | array . other_array Returns a new array that is a copy of the original array, removing any items that also appear in other_array.
5 | str <=> other_str Compares str with other_str, returning -1 (less than), 0 (equal), or 1 (greater than). The comparison is casesensitive.
6 | array | other_array Returns a new array by joining array with other_array, removing duplicates.
7 | array << obj Pushes the given object onto the end of array. This expression returns the array itself, so several appends may be chained together.
8 | array <=> other_array Returns an integer (-1, 0, or +1) if this array is less than, equal to, or greater than other_array.
9 | array == other_array 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.
10 | array[index] [or] array[start, length] [or] array[range] [or] array.slice(index) [or] array.slice(start, length) [or] array.slice(range) Returns the element at index, or returns a subarray starting at start and continuing for length elements, or returns a subarray specified by range. Negative indices count backward from the end of the array (-1 is the last element). Returns nil if the index (or starting index) is out of range.
11 | array[index] = obj [or] array[start, length] = obj or an_array or nil [or] array[range] = obj or an_array or nil Sets the element at index, or replaces a subarray starting at start and continuing for length elements, or replaces a subarray specified by range. If indices are greater than the current capacity of the array, the array grows automatically. Negative indices will count backward from the end of the array. Inserts elements if length is zero. If nil is used in the second and third form, deletes elements from self.
12 | array.abbrev(pattern = nil) Calculates the set of unambiguous abbreviations for the strings in self. If passed a pattern or a string, only the strings matching the pattern or starting with the string are considered.
13 | array.assoc(obj) Searches through an array whose elements are also arrays comparing obj with the first element of each contained array using obj.==. Returns the first contained array that matches , or nil if no match is found.
14 | array.at(index) Returns the element at index. A negative index counts from the end of self. Returns nil if the index is out of range.
15 | array.clear Removes all elements from array.
16 | array.collect { |item| block } [or] array.map { |item| block } Invokes block once for each element of self. Creates a new array containing the values returned by the block.
17 | array.collect! { |item| block } [or] array.map! { |item| block } Invokes block once for each element of self, replacing the element with the value returned by block.
18 | array.compact Returns a copy of self with all nil elements removed.
19 | array.compact! Removes nil elements from array. Returns nil if no changes were made.
20 | array.concat(other_array) Appends the elements in other_array to self.
21 | array.delete(obj) [or] array.delete(obj) { block } Deletes items from self that are equal to obj. 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.
22 | array.delete_at(index) Deletes the element at the specified index, returning that element, or nil if the index is out of range.
23 | array.delete_if { |item| block } Deletes every element of self for which block evaluates to true.
24 | array.each { |item| block } Calls block once for each element in self, passing that element as a parameter.
25 | array.each_index { |index| block } Same as Array#each, but passes the index of the element instead of the element itself.
26 | array.empty? Returns true if the self array contains no elements.
27 | array.eql?(other) Returns true if array and other are the same object, or are both arrays with the same content.
28 | array.fetch(index) [or] array.fetch(index, default) [or] array.fetch(index) { |index| block } Tries to return the element at position index. If index lies outside the array, the first form throws an IndexError exception, the second form returns default, and the third form returns the value of invoking block, passing in index. Negative values of index count from the end of the array.
29 | array.fill(obj) [or] array.fill(obj, start [, length]) [or] array.fill(obj, range) [or] array.fill { |index| block } [or] array.fill(start [, length] ) { |index| block } [or]
array.fill(range) { |index| block }
The first three forms set the selected elements of self to obj. A start of nil is equivalent to zero. A length of nil is equivalent to self.length. The last three forms fill the array with the value of the block. The block is passed the absolute index of each element to be filled.
30 | array.first [or] array.first(n) Returns the first element, or the first n elements, of the array. If the array is empty, the first form returns nil, and the second form returns an empty array.
31 | array.flatten Returns a new array that is a one-dimensional flattening of this array (recursively).
32 | array.flatten! Flattens array in place. Returns nil if no modifications were made. (array contains no subarrays.)
33 | array.frozen? Returns true if array is frozen (or temporarily frozen while being sorted).
34 | array.hash Compute a hash-code for array. Two arrays with the same content will have the same hash code
35 | array.include?(obj) Returns true if obj is present in self, false otherwise.
36 | array.index(obj) Returns the index of the first object in self that is == to obj. Returns nil if no match is found.
37 | array.indexes(i1, i2, ... iN) [or] array.indices(i1, i2, ... iN) This methods is deprecated in latest version of Ruby so please use Array#values_at.
38 | array.indices(i1, i2, ... iN) [or] array.indexes(i1, i2, ... iN) This methods is deprecated in latest version of Ruby so please use Array#values_at.
39 | array.insert(index, obj...) Inserts the given values before the element with the given index (which may be negative).
40 | array.inspect Creates a printable version of array.
41 | array.join(sep=$,) Returns a string created by converting each element of the array to a string, separated by sep.
42 | array.last [or] array.last(n) Returns the last element(s) of self. If array is empty, the first form returns nil.
43 | array.length Returns the number of elements in self. May be zero.
44 | array.map { |item| block } [or] array.collect { |item| block } Invokes block once for each element of self. Creates a new array containing the values returned by the block.
45 | array.map! { |item| block } [or] array.collect! { |item| block } Invokes block once for each element of array, replacing the element with the value returned by block.
46 | array.nitems Returns the number of non-nil elements in self. May be zero.
47 | array.pack(aTemplateString) Packs the contents of array into a binary sequence according to the directives in aTemplateString. 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 platformindependent size. Spaces are ignored in the template string. ( See templating Table below )
48 | array.pop Removes the last element from array and returns it, or nil if array is empty.
49 | array.push(obj, ...) Pushes (appends) the given obj onto the end of this array. This expression returns the array itself, so several appends may be chained together.
50 | array.rassoc(key) 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.
51 | array.reject { |item| block } Returns a new array containing the items array for which the block is not true.
52 | array.reject! { |item| block } Deletes elements from array for which the block evaluates to true, but returns nil if no changes were made. Equivalent to Array#delete_if.
53 | array.replace(other_array) Replaces the contents of array with the contents of other_array, truncating or expanding if necessary.
54 | array.reverse Returns a new array containing array's elements in reverse order.
55 | array.reverse! Reverses array in place.
56 | array.reverse_each {|item| block } Same as Array#each, but traverses array in reverse order.
57 | array.rindex(obj) Returns the index of the last object in array == to obj. Returns nil if no match is found.
58 | array.select {|item| block } Invokes the block passing in successive elements from array, returning an array containing those elements for which the block returns a true value.
59 | array.shift Returns the first element of self and removes it (shifting all other elements down by one). Returns nil if the array is empty.
60 | array.size Returns the length of array (number of elements). Alias for length.
61 | array.slice(index) [or] array.slice(start, length) [or] array.slice(range) [or] array[index] [or] array[start, length] [or] array[range] Returns the element at index, or returns a subarray starting at start and continuing for length elements, or returns a subarray specified by range. Negative indices count backward from the end of the array (-1 is the last element). Returns nil if the index (or starting index) are out of range.
62 | array.slice!(index) [or] array.slice!(start, length) [or] array.slice!(range) Deletes the element(s) given by an index (optionally with a length) or by a range. Returns the deleted object, subarray, or nil if index is out of range.
63 | array.sort [or] array.sort { | a,b | block } Returns a new array created by sorting self.
64 | array.sort! [or] array.sort! { | a,b | block } Sorts self.
65 | array.to_a Returns self. If called on a subclass of Array, converts the receiver to an Array object.
66 | array.to_ary Returns self.
67 | array.to_s Returns self.join.
68 | array.transpose Assumes that self is an array of arrays and transposes the rows and columns.
69 | array.uniq Returns a new array by removing duplicate values in array.
70 | array.uniq! Removes duplicate elements from self. Returns nil if no changes are made (that is, no duplicates are found).
71 | array.unshift(obj, ...) Prepends objects to the front of array, other elements up one.
72 | array.values_at(selector,...) Returns an array containing the elements in self corresponding to the given selector (one or more). The selectors may be either integer indices or ranges.
73 | array.zip(arg, ...) [or] array.zip(arg, ...){ | arr | block } Converts any arguments to arrays, then merges elements of array with corresponding elements from each argument.

## Array pack directives:

Following table lists pack directives for use with Array#pack.

Directive | Description
--- | ---
@ | 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, d | Double-precision float, native format.
E | Double-precision float, little-endian byte order.
e | Single-precision float, little-endian byte order.
F, 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 RFC 2045).
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.
Q, q | 64-bit number.
S | Unsigned short.
s | Short.
U | UTF-8.
u | UU-encoded string.
V | Long, little-endian byte order.
v | Short, little-endian byte order.
w | BER-compressed integer \fnm.
X | Back up a byte.
x | Null byte.
Z | Same as a, except that null is added with *.

## Example:

Try following example to pack various data.

    a = [ "a", "b", "c" ]
    n = [ 65, 66, 67 ]
    a.pack("A3A3A3")   #=> "a  b  c  "
    a.pack("a3a3a3")   #=> "a\000\000b\000\000c\000\000"
    n.pack("ccc")      #=> "ABC"

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