Ruby Array

Topics

• Array Literal
• How to create new array
• 2-dimensional array
• Add element to array
• Refer value
• Delete element
• Each

Array Literal

Array class

a = []
b = [10, true, "30"] # Add any object
b[0]  #=> 10

How to create new array

[], Array.new

a = [1,2,3]
a.class     #=> Array
array[1,2,3]
Array.new(3, "str") # Init
Array.new([1,2,3])
Array.new(3){ |i| * 3}

%

v1 = %w(homuhomu madoka mami)   # v1 = ["homuhomu", "madoka", "mami"]
v1.join()           # homuhomumadokamami
v1.join("_")            # homuhomu_madoka_mami

Instantiate

a = Array.new(5)    # [nil, nil, nil, nil, nil]
a.length
Array.new(2){|index| index + 10 }   # [10, 11]

Initialize

a = Array.new(2, "a")   #=> ["a", "a"] a[0], a[1]が同一のオブジェクト
a[0].replace("b")   #=> a[0]も, a[1]も変わる
p a         #=> ["b", "b"]

Block

a = Array.new(2){"a"}   #=> ["a", "a"]
a[0].replace("b")
p a     # ["b", "a"]

2-dimensional array

a = [[1,2], [3,4]]
a[0][1] # 1
a[1][2] # 4

Add element to array

<<, push, concat

a = [1,2,3]
a << 4    #=> [1,2,3,4]
a.concat([5,6])     #=> [1,2,3,4,5,6]
a.insert(3,9)       #=> [1,2,3,9,4,5,6]
a.unshift(10)       #=> [10,1,2,3]

If there are no argument in unshift, do nothing.

Change element

a = [1,2,3]
a[1] = 10
a[1..2] = [11,12]

Over index

v1 = [10]
v1.length   #=> 1
v1[3] = "aa"    # Can add over index
v1.length   #=> 4
v1[2]       #=> nil   element between elements(inserted value) is nil
v1[100]     #=> nil

Refer value

[], slice, values_at, at, fetch, first, last, assoc, rassoc

a = [1,2,3]
a[1]        # 2
a.at(1)     # 2
a[1..2]     #=> [2,3]
a.value_at(1)
 
a.fetch(4)  # IndexError
a.fetch(4){|n| "ERROR" #{n}"}       # "ERROR 4"
 
a.first     #=> 1
a.last      #=> 3
a = [1,2,3,4,5]
a.first(3)  #=> [1,2,3]  3 elements from the beginning

assoc

Search array of array, if the argument match the first element of array, return this array.
This is example

a = [[1,2],[3,4], [5,6],[7,8]]
a.assoc(3)  #=> [3,4]

rassoc

This is similar with assoc, the difference is to match with index 1 element not first.

a = [[1,2], [3,4], [5,6], [7,8]]
a.rassoc(4) #=> [3,4]

Look for element?

include?, index, rindex

a = [1,2,3,4,5]
a.include?(3)   #=> true
a.include?(10)  #=> false
a.index(4)  #=> 3
a.rindex(4) #=> 3   Search from the last

Return nil if there are no match value when using index,rindex.

Delete element

each, each_index, cycle, reverse_each, clear, slice!, shift, pop

List

delete_at

a = [1,2,3,4,5]
a.delete_at(2)  #=> [1,2,4,5]

delete_if

a = [1,2,3,4,5]
a.delete_if{|n| n % 2 == 0} #=> [1,3,5]

delete

a = [1,2,3,4,5]
a.delete(3) #=> [1,2,4,5]

slice!

a = [1,2,3,4,5]
a.slice!(2,2)   #=> [3,4] 2 elements from index 2

shift

a = [1,2,3,4,5]
a.shift(2)  #=> [1,2]

pop

a = [1,2,3,4,5]
a.pop(2)    #=> [4,5]
a.pop       #=> 3
a       #=> [1,2]

–

a = [1,2,3,4,5]
a - [1,2]   #=> [3,4,5]

Each

each, each_index, cycle, reverse_each

for

a = [2,3,4]
for i in a do
  p i
end # 2 3 4

for i in a do
end

for i in [2,3,4]
  bar = 1
end
p bar   #=> 1 Oh! really?

for doesn’t create scope.
So, bar still alive.

each

[2,3,4].each do
   bar = 1
end
p bar   #=> NameError

cycle

[1,2,3].cycle{|n| puts n}

Connect(join)

[1,2,3].join(", ") #=> "1,2,3"