Arrays

June 20, 2021

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An array is a collection of items surrounded by square brackets [ ]. Arrays in Julia are 1-based therefore the first item in an array is at the 1-index.

# Array of strings representing fruits
fruits = ["orange", "melon", "apple", "lemon"]

# Get the first item
julia> fruits[1]
"orange"

# Get items two through three
julia> fruits[2:3]
2-element Vector{String}:
 "melon"
 "apple"

An item in an array can be replaced by assigning the index of that item to a new value.

# Array of numbers
numbers = [4.2, 8, 10, 3, 4]

# Replace the value of the first item
numbers[1] = 20

julia> numbers
5-element Vector{Float64}:
 20.0
  8.0
 10.0
  3.0
  4.0

Use the copy() function to copy an array to a new variable.

# Array of numbers
numbers2 = [4.2, 8, 10, 3, 4]

# Copy the array to a new variable
numbers3 = copy(numbers2)

# Replace value of the first item in original array
numbers2[1] = 100

julia> numbers2
5-element Vector{Float64}:
 100.0
   8.0
  10.0
   3.0
   4.0

# New array contains values from original array
julia> numbers3
5-element Vector{Float64}:
  4.2
  8.0
 10.0
  3.0
  4.0

Use an array comprehension to create an array from a range of numbers.

julia> z = [i * 2 for i in 1:5]
5-element Vector{Int64}:
  2
  4
  6
  8
 10

Julia's arrays are stored in column-major order. Therefore, iterating over the columns first in a 2D array will execute quicker than iterating over the rows first. This is demonstrated in the example below where calcA() finishes in about 0.7 seconds while calcB() completes after 2.5 seconds for 10,000 iterations on a MacBook Pro.

# iterate over columns first then rows (faster)
function calcA(n)
    x = rand(n, n)
    z = zeros(n, n)

    for j in 1:n
        for i in 1:n
            xij = x[i, j]
            z[i, j] = xij^2
        end
    end

    return z
end

# iterate over rows first then columns (slower)
function calcB(n)
    x = rand(n, n)
    z = zeros(n, n)

    for i in 1:n
        for j in 1:n
            xij = x[i, j]
            z[i, j] = xij^2
        end
    end

    return z
end

julia> @time calcA(10_000);
  0.733511 seconds (4 allocations: 1.490 GiB, 6.50% gc time)

julia> @time calcB(10_000);
  2.527453 seconds (4 allocations: 1.490 GiB, 4.04% gc time)