Arrays in Julia

Arrays in Julia are a collection of elements just like other collections like Sets, Dictionaries, etc. Arrays are different from Sets because arrays are ordered collection of elements, and can hold duplicate values, unlike sets which require all the elements to be unique. Arrays are N-Dimensional containers divided into rows and columns. Arrays are mutable data types which mean their content can be modified, deleted, overwritten, etc.
Arrays are the heterogeneous type of containers and hence, they can hold elements of any data type. It is not mandatory to define the data type of an array before assigning the elements to the array. Julia automatically decides the data type of the array by analyzing the values assigned to it. Because of the ordered nature of an array, it makes it easier to perform operations on its values based on their index. 

Types of Arrays

Based on their dimensions, arrays can be of three types: 

• 1D Array
• 2D Array
• 3D Array

1D Array:

A 1D array or 1 dimensional array is a linear representation of elements. A 1D array can only have either a row or a column. It represents a type of list that can be accessed by subsequent memory locations. A 1D array in Julia is known as a Vector. Unlike other types of arrays, Vectors allow to add elements from front or back resulting in resizing the array. 
Example:

A = [1, 2, 3, 4]
4-element Array{Int64, 1}:
 1
 2
 3
 4

2D Array:

A 2D array or 2 dimensional array is a representation of items in the form of rows and columns. Unlike Vectors, 2D arrays are a tabular representation of data. A 2D array in Julia is known as a Matrix. Elements in Matrix are accessed with the use of both row and column index. A 2D array can be created by removing the commas between the elements from a Vector at the time of array creation.

A = [1 2; 3 4]

2×2 Array{Int64, 2}:
 1  2
 3  4

3D Array:

A 3D array or 3 dimensional array is an array of arrays. A 3D array is also known as a multi-dimensional array. A 3D array is of the type NxNxN where combining arrays can be of any dimensions. A 3D array can be created with the use of cat or reshape command.
Example: 

A = cat([1 2; 3 4], [5 6; 7 8], dims=3)

2×2×2 Array{Int64, 3}:
[:, :, 1] =
 1  2
 3  4

[:, :, 2] =
 5  6
 7  8

Here, you can give any value for ‘dims‘ and an array of that dimension will be created. 

Creating an Array

An Array in Julia can be created with the use of a pre-defined keyword Array() or by simply writing array elements within square brackets([]). There are different ways of creating different types of arrays. 

• A 1D array can be created by simply writing array elements within square brackets separated by commas(, ) or semicolon(;). 
  
• A 2D array can be created by writing a set of elements without commas and then separating that set with another set of values by a semicolon. 
  
• Similarly, a 3D array can be created by using ‘cat’ command. This command takes arrays and number of dimensions required as input and then concatenates the arrays in given dimensions. 
  

Syntax:

Array1 = [Value1, Value2, Value3, ...]
or
Array1 = [Value1 Value2 ; Value3 Value4]
or
Array1 = cat([Value Value; Value Value], [Value Value; Value Value], dims=3)

Python3

# Julia program to illustrate
# the use of Arrays

# Creating a 1D array
Array1 = [1, 2, 3, 4]
println(Array1)

# Creating a 2D array
Array2 = [1 2 3; 4 5 6]
println(Array2)

# Creating a 3D array
# using 'cat' command
Array3 = cat([1 2; 3 4], [5 6; 7 8], [2 2; 3 4], dims = 3)
println(Array3)

Output:

Accessing Array elements

Elements of an array can be easily extracted with the use of square brackets([]). Loops can also be used to extract more than one element from an array. Also, a range of elements can also be extracted from an array by passing a range within square brackets just like Tuples.
Example: Accessing elements in a 1D array

Python3

# Julia program to illustrate
# the use of Arrays

# Creating a 1D Array
Array1 = [1, 2, 3, 4, "Hello", "Geeks"]

# Passing index value
println(Array1[3])

# Accessing last value
println(Array1[end])

# Passing a bunch of index values
println(Array1[[3, 5, 6]])

# Passing a range of indices
println(Array1[3:end])

# Using true-false to print values
println(Array1[[true, true, false, false, false, true]])

Example: Accessing elements in a 2D Array

Python3

# Julia program to illustrate
# the use of Arrays

# Creating a 2D Array
Array1 = [1 2 "Hello"; 3 4 "Geeks"]

# Passing index value
# row-column wise
println(Array1[1, 3])

# Accessing last value
println(Array1[2, end])

# Using getindex() function
println(getindex(Array1, 2, 3))

# Using colon to indicate every row
println(Array1[:, 3])

# Using colon to indicate every column
println(Array1[1, :])

Example: Accessing elements in a 3D Array

Python3

# Julia program to illustrate
# the use of Arrays

# Creating a 3D array
# using 'cat' command
Array3 = cat([1 2; 3 4],
              ["hello" "Geeks"; "Welcome" "GFG"],
              [5 6; 7 8], dims = 3)

# Accessing element using index value
println(Array3[1, 2, 2])

# Accessing Range of elements
println(Array3[1:2, 1:2, 2])

# Using colon to access every row or column
println(Array3[:, :, 3])

# Using getindex function
println(getindex(Array3, 1, 2, 2))

# Using getindex with colon operator
println(getindex(Array3, 1, :, 2))

Adding elements to an Array

Arrays are mutable type collections in Julia, hence, their values can be modified with the use of certain pre-defined keywords. Julia allows adding new elements in an array with the use of push! command. Elements in an array can also be added at a specific index by passing the range of index values in the splice! function.
Example: Adding element in 1D Array

Python3

# Julia program to illustrate
# the use of Arrays

# Creating a 1D Array
Array1 = [1, 2, 3, 4, "Hello", "Geeks"]

# Adding element at the end
push !(Array1, "Welcome")
println(Array1)

# Adding element at the beginning
pushfirst !(Array1, 0)
println(Array1)

# Adding element at specific position
splice !(Array1, 3:4, "GFG")
println(Array1)

# Adding a range of elements
splice !(Array1, 6:7, [1, 2, 3])
println(Array1)

Example: Adding elements in a 2D Array

Python3

# Julia program to illustrate
# the use of Arrays

# Creating a 2D Array
Array1 = [1 2 "Hello"; 3 4 "Geeks"]

# Array of elements which are to be added
Array2 = [5 6 7; 8 9 10]

# Appending arrays
Array3 = [Array1; Array2]
println(Array3)

Example: Adding elements in a 3D Array

Python3

# Julia program to illustrate
# the use of Arrays

# Creating a 2D Array
Array1 = cat([1 2; 3 4],
             ["hello" "Geeks"; "Welcome" "GFG"],
             dims = 3)

# Array of elements which is to be added
Array2 = cat([0 2; 1 4],
             ["GFG" "Geeks"; "abc" "def" ],
             dims = 3)

# Appending arrays
Array3 = [Array1; Array2]
println(Array3)

Updating existing elements

Arrays in Julia are mutable and hence it allows modification of its content. Elements in arrays can be removed or updated as per requirement. To update an existing element of an array, pass the index value of that element within a square bracket. 
Example:

Python3

# Julia program to illustrate
# the use of Arrays

# Creating a 1D array
Array1 = [1, 2, 3, 4, 5, 6, "a", "b"]
println("Existing 1D Array:\n", Array1)

# Updating value at specific index
Array1[4] = "Hello"
println("\nUpdated 1D Array:\n", Array1)

# Creating a 2D array
Array2 = [1 2 3; 4 5 6]
println("\nExisting 2D Array:\n", Array2)

# Updating value at specific index
Array2[2, 2] = 10
println("\nUpdated 2D Array:\n", Array2)

# Creating a 3D array
Array3 = cat([1 2; 3 4],
             ["hello" "Geeks"; "Welcome" "GFG"],
             dims = 3)
println("\nExisting 3D Array:\n", Array3)

# Updating value at specific index
Array3[2, 2, 2] = "World"
println("\nUpdated 3D Array:\n", Array3)

Removing elements from an Array

Julia allows to delete an element from a 1D array by using predefined function pop!. This function will simply remove the last element of the array and reduce the array size by 1. To delete first element of the array, one can use popfirst! function. If there’s a need to delete an element from a specific index in the array, splice! and deleteat! functions can be used. 
These methods work only on 1D array because 2D or other multidimensional arrays doesn’t allow to delete a specific element from the matrix to maintain their (row, column) order. However, one can either assign zero or undef at the place of the element to be deleted. This will not affect the order of matrix. Another way to do the same is by deleting an entire row or column.
Example: Deleting elements from 1D array

Python3

# Julia program to illustrate
# the use of Arrays

# Creating a 1D array
Array1 = [1, 2, 3, 4, 5, 6, "Hello", "Geeks"]
println("Initial Array:\n", Array1)

# Delete last element
# using pop !()
pop !(Array1)

# Printing the array
println("\nArray after deleting last element:\n", Array1)

# Delete first element
# using popfirst !()
popfirst !(Array1)
println("\nArray after deleting first element:\n", Array1)

# Deleting a specific element
# using splice !()
splice !(Array1, 4)

println("\nArray after deleting a specific(e.g. 4th) element:\n", Array1)

# Deleting a range of elements
# using deleteat !()
deleteat !(Array1, 2:4)

println("\nArray after deleting a range(e.g. 2:4) of elements:\n", Array1)

# Deleting all the elements
# using empty !()
empty !(Array1)

println("\nArray after deleting all the elements:\n", Array1)

Example: Deleting elements from 2D Array

Python3

# Julia program to illustrate
# the use of arrays

Array2 = [1 2 3; 4 5 6; "Hello" "Geeks" "World"]
println("Initial Array:\n", Array2)

# Assigning zero in place of
# element to be deleted(e.g. element at 1, 3)
Array2[1, 3] = 0

println("\nZero in place of element to be deleted:\n", Array2)

# Assigning undefined value in place of
# element to be deleted(e.g. element at 2, 3)
Array2[2, 3] = undef

println("\nundef in place of element to be deleted:\n", Array2)

# Deleting entire row of elements(e.g. 2nd row)
Array2 = Array2[1:size(Array2, 1) .!= 2, :]

println("\nArray after deleting 2nd row:\n", Array2)

# Deleting entire column of elements(e.g. 2nd column)
Array2 = Array2[:, 1:size(Array2, 2) .!= 2]

println("\nArray after deleting 2nd column:\n", Array2)

In the above code, while deleting an entire row or column, we create another array of the same size and assign the elements to the new array excluding the row or column to be deleted. This is done because the deletion of a row or column from a matrix is not allowed in Julia. 

Example: Deleting elements from 3D Array

Python3

# Julia program to illustrate
# the use of arrays

# Creating a 3D array
Array3 = cat([1 2 3; 3 4 5; 5 6 7],
             ["a" "b" "c"; "c" "d" "e"; "e" "f" "g"],
             dims = 3)
println("Initial 3D Array:\n", Array3)

# Assigning zero in place of
# element to be deleted(e.g. element at 1, 3, 1)
Array3[1, 3, 1] = 0

println("\nZero in place of element to be deleted:\n", Array3)

# Assigning undefined value in place of
# element to be deleted(e.g. element at 1, 3, 2)
Array3[1, 3, 2] = undef

println("\nundef in place of element to be deleted:\n", Array3)

# Deleting entire row of elements(e.g. 2nd row)
Array3 = Array3[1:size(Array3, 2) .!= 2, :, :]

println("\nArray after deleting 2nd row:\n", Array2)

# Deleting entire column of elements(e.g. 2nd column)
Array3 = Array3[:, 1:size(Array3, 2) .!= 2, :]

println("\nArray after deleting 2nd column:\n", Array2)

Array Methods

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Methods	Description
axes()	Returns the tuple or valid range of valid indices for specified array
broadcast()	Used to broadcast the function f over the specified arrays, tuples or collections.
broadcast!()	This function is same as broadcast() function but stores the result of broadcast(f, As…) in the specified dest array
cat()	Used to concatenate the given input arrays along the specified dimensions in the iterable dims.
colon()	Used to indicate and access every row and column
conj()	Used to compute the complex conjugate of a specified complex number z.
conj!()	Transforms the specified array to its complex conjugate in-place
copyto!()	Used to copy all elements from the specified collection src to array dest
count()	Counts the number of elements in the specified array
Dims()	Used to represent the dimensions of the specified AbstractArray
eachindex()	Creates an iterable object for visiting each index of the specified array
fill()	Returns an array of specified dimensions filled with a specific value passed to it as parameter
fill!()	Takes an existing array as argument and fills it with a new specified value
findall()	Returns a vector of indices or keys of the all true values from the specified array
findfirst()	Return the index or key of the first true value in the specified array
findlast()	Returns the index or key of the last true value in the specified array
findnext()	Returns the next coming index after or including i of a true element of the specified array
findprev()	Returns the previous index before or including i of a true element of the specified array
first()	Returns the first element of the specified array
getindex()	Returns the constructed array of the specified type and also the element of the array at a specific index.
hcat()	Used to concatenate the given arrays along second dimension
hvcat()	Used to concatenate the given arrays horizontally and vertically in one call
isassigned()	Tests whether the given index contains a value in the specified array or not
last()	Returns the last element of the specified array
length()	Returns the number of elements present in the specified array
ndims()	Returns the number of dimension of the specified array A
parent()	Returns the parent array of a specified array view type (i.e, SubArray) or the array itself if it is not a view
repeat()	Constructs an array by repeating the specified array elements with the specified number of times
reshape()	Returns an array with the same data as the specified array, but with different specified dimension sizes
reverse()	Returns a reversed copy of the vector.
reverse!()	Returns an in-place reversed copy of the vector.
reverseind()	Returns the corresponding index in v so that v[reverseind(v, i)] == reverse(v)[i].
setindex()	Used to store values from the given array X within some subset of A as specified by inds.
similar()	Used to create an uninitialized mutable array with the given element type and size, based upon the given source array.
size()	Returns a tuple containing the dimensions of the specified array
stride()	Returns the distance in memory between the specified adjacent elements in specified dimension
strides()	Returns a tuple of the memory strides in each dimension
to_indices()	Used to convert the given tuple I to a tuple of indices for use in indexing into array A
vcat()	Used to concatenate the given arrays along first dimension
vec()	Reshapes the specified array as a one-dimensional column vector i.e, 1D array
vect()	Used to create a vector with the passed elements as parameter
view()	Used to return a view into the given parent array A with the given indices instead of making a copy.
@view()	Used to create a sub array from the given indexing expression.
@views()	Used to convert every given array-slicing operation in the given expression.