Keyword Array Hash String 2D Case Class Console Convert DateTime Duplicates Exception File Format If Iterator Math Method Nil Number Recursion Regexp Replace Set Sort Split Substring While

Array. An array stores elements, one after another. In Ruby, the Array class is used to construct these linear collections. Arrays are a fundamental facet of this language.
To create an array, we use the Array class. We can omit "Array" and just use the initializer. Or we can assign each element as we go along.
New Array. We first construct a new array with four integer elements. Then we modify the first element (at index 0). We display the array values. They are stored in the specified order.

First element:The first element in an array is located at index 0, not 1. This is standard for most programming languages.

Based on: Ruby 2

Ruby program that uses array

# An array of integers.
values = Array[10, 20, 30, 40]

# Modify the first element.
values[0] = 100

# Display the array.
puts values


First, last. In life there is always a beginning and an end. This is true too in an array where at least one element exists. First and last are helpful methods.

Warning:In a zero-element array, first and last return nil. So we must check the result if our array may be empty.

Info:The first array element is located at the index 0. And the last is located at the position equal to length minus one.

Ruby program that uses first, last

elements = ["ruby", "diamond", "gold"]

# First is the 0 index.
puts elements.first
puts elements[0]

# Last index is length minus one.
puts elements.last
puts elements[elements.length - 1]


Mutable. Arrays are mutable. When we modify an element, no array copy is made. The element is simply replaced. In algorithms that change elements, these mutable arrays help improve speed.

Sizing:Arrays are automatically resized. We have no need to specify how many elements will eventually be stored in them.

Terms:It is a topic of debate whether Array is technically a List. Often in languages, an Array is not resizable, but a list is.

Push. This method adds the specified element value to the end of the array. This is possible with a mutable collection. Here we start with an empty array.

And:We add the string "tree" and then "grass" to the array. The first element is "tree." The last is "grass."

Ruby program that uses push

# Create an empty array.
values = Array[]

# Push two elements.

# Print elements.
print "First: ", values.first, "\n"
print "Last:  ", values.last, "\n"


First: tree
Last:  grass
Push, operator. This is an alternative to the push method. It does the same thing but requires less source code. The << operator can be chained. Many elements can be added in one statement.
Ruby program that uses append operator

items = Array[]
# Append two strings, one after another.
items << "cat" << "dog"
# Append another element.
items << "bird"

p items


["cat", "dog", "bird"]
Delete. An array element can be deleted. When this occurs, the slot in the array is eliminated. Elements that come after are shifted forward.

Delete_at:With delete_at(), we remove an element by specifying its index. No search is required.

Delete:With delete(), we remove the first element of the specified value. We remove the "lime" element without knowing its index.

Ruby program that deletes elements

# Citrus fruits.
arr = Array["orange", "lemon", "lime"]

# Delete element at index 1.
puts "Fruit:", arr

# Delete element with value of "lime".
puts "Fruit:", arr


Delete_if. This method receives a block that evaluates to true or false. The block is tested against each element in the array. If it evaluates to true, the element is deleted.

Block:The block must be surrounded by curly brackets. In the block, we supply a name for our element variable.

Here:I use "e." Another name like "x" could instead be used. The expression deletes all elements with values >= 40.

Ruby program that uses delete_if

# An array of integers.
numbers = [10, 20, 30, 40, 50]

# Delete numbers greater than or equal to 40.
numbers.delete_if {|e| e >= 40}
puts numbers


Pop, empty. The pop method deletes the final element in an array. It simplifies the syntax for this operation. Empty(), also shown, returns true if an array has zero elements.

Here:We create an array of three elements. We then pop it three times, yielding an empty array.

Tip:The final array element is removed each time pop() is called. The actual array is modified—no copy is created.

Ruby that pops an array

# Create and display an array.
names = Array["Conrad", "James", "Joyce"]
puts names, ""

# Pop the last element.
puts names, ""

# Pop again.
puts names, ""

# Pop again and see if the array is empty.
puts names.empty?


Conrad   3 elements

Conrad   2 elements

Conrad   1 element

true     0 elements
Each. Any loop can iterate over an array. But some methods, called iterators, make looping over an array more elegant. They reduce complexity. They reduce possible errors.

Syntax:We declare an iteration variable between two vertical bars. It is important to memorize the syntax here.


Tip:Each() enumerates over each element in the array. Here we use the identifier "number" to access elements.

Ruby that uses each

# Some Wagstaff primes.
primes = [3, 11, 43, 683, 2731]

# Loop over primes and display them.
primes.each do |number|
    puts number


Each, short syntax. Ruby is known for its expressive, glittering syntax. Here we use a short syntax form to write an iterator on one line.

Tip:We omit the "do" keyword and use curly brackets. This style makes it harder to add more statements.

Ruby that uses each, short syntax form

# Some even numbers.
# ... Let's get even.
evens = [2, 4, 6, 8]

# Use short syntax form for the iterator block.
evens.each {|ev| puts ev}


Each_index. Sometimes we want to access all indexes in an array. This can be done with a loop, but the each_index iterator is also available. It yields all the indexes of the array.For Loop

Here:We use each_index over a three-element array. It has indexes 0, 1 and 2. We print them.

Ruby that uses each_index

# Array has indexes 0, 1 and 2.
values = ["cat", "dog", "sheep"]

# Loop over all indexes in the array.
values.each_index do |index|
    puts index


Index. This searches an array for the specified element. It starts its search at the beginning and proceeds to the following elements. If it finds the element, it returns its index value.

Important:If index() does not find the specified element, it returns nil. This is a special object.

Ruby that uses index

items = ["Boots", "Cloak", "Dagger"]

# Get index of this element.
result = items.index("Cloak")
puts result

# Call index again.
result = items.index("Dagger")
puts result

# Call index with nonexistent element.
result = items.index("Helmet")
if result == nil
    puts "Not found"


Not found
For-loop. In the range of the for-loop, we can specify an array's lowest and highest indexes. To loop over the entire array, we specify the highest valid index as the maximum of the range.

So:We start at zero. We progress to the inclusive maximum of the array's length minus one. We then access each element.

Advantages:With a for-loop, we can use the index in the loop iteration for other operations. We can access adjacent elements in the array.

Usually:The simplest syntax possible for an operation is best. This leads to simpler (and thus higher-quality) programs.

Ruby that uses for-loop, array

# An array with four strings.
values = ["cat", "dog", "rabbit", "giraffe"]

# Loop over all indexes in the array.
for i in 0..values.length-1
    # Access the element.
    puts values[i]


Uniq. This removes all duplicate elements. We also can use the "uniq!" method to modify the array in-place. In this example, we call uniq to eliminate two duplicate element (a 1 and 4).Remove Duplicates
Ruby that uses uniq

# Contains some duplicate elements.
values = [1, 1, 2, 3, 4, 4]

# Remove non-unique elements in-place.
puts values


Collect. This applies (maps) each element in an array to a new value based on a function. For example we can multiply each element by two. With "collect!" the array is modified in-place.

Map:In other languages like Python, the map() built-in has similar functionality as collect.

Block:The syntax for collect requires a block. In this code we use the identifier "e," but other identifiers work just as well.

Ruby that uses collect

# Contains three elements.
elements = [1, 0, 100]

# Use collect to multiply all elements by 2.
result = elements.collect{|e| e * 2}

# Display the result array.
p result


[2, 0, 200]
Ranges. With range syntax we can change parts of arrays in single operations. So we can replace a range of elements with a series of new elements. We can insert ranges this way.

Tip:Single elements too can be assigned. And often assigning one element at the time is easiest.

Ruby that assigns range of elements

elements = [10, 20, 30, 40, 50]

# Assign range of elements at indexes 1, 2, and 3 to a new array.
elements[1..3] = [100, 200]
puts elements


Sort, reverse. Arrays have sort() and reverse() methods. With sort, we can specify a block to sort elements by a method. In this way we implement ascending and descending sort orders.Sort, reverse
Copy. How can we copy an array? The slice syntax is helpful here. We take a total-array slice. And this returns (with little hassle) a copied array.Copy Array
Two-dimensional. We can create arrays of arrays—2D arrays. These are jagged arrays. Each subarray can have a different length. We can use 2D array indexing syntax here.2D Array

Flatten:The flatten method converts a multidimensional array into one dimension. It copies all nested arrays.

Arrays are powerful. They are resizable. They can be searched and tested. As a result, they are used in many programs. They often form of the basis of other classes and collections.
With array methods, we avoid repetitive code. This leads to clearer syntax forms and better programs. With arrays in Ruby, we build on solid foundations.