Julia dictionary
last modified October 25, 2023
In this article we work with dictionaries in Julia.
A dictionary is a collection of key/value pairs. Dictionaries are created with
Dict type.
Dictionaries are also called maps or associative arrays.
Simple example
In the first example, we construct a simple dictionary in Julia.
d = Dict("sk" => "Slovakia", "ru" => "Russia",
"de" => "Germany", "no" => "Norway")
println(d)
println(d["sk"])
println(d["no"])
A dictionary of a few countries is created. The values are the country names and their keys are the countries' abbreviations.
d = Dict("sk" => "Slovakia", "ru" => "Russia",
"de" => "Germany", "no" => "Norway")
A dictionary is created with the Dict type. Each item has a key and
a value; these are separated with the => characters. The pairs
are separated with commas.
println(d)
We print the whole dictionary.
println(d["sk"]) println(d["no"])
We print the names of two countries. The countries are accesse by their keys. The keys are specified inside a pair of square brackets.
$ julia main.jl
Dict("ru" => "Russia", "sk" => "Slovakia", "no" => "Norway", "de" => "Germany")
Slovakia
Norway
Looping over a dictionary
In the next example, we go over a dictionary with for loop.
d = Dict(:a => "book", :b => "storm", :c => "pencil", :d => "green")
for (k, v) in d
println("$(k): $(v)")
end
We define a dictionary of words. The keys are symbols and the values are words. We go over the dictionary with a for loop.
for (k, v) in d
println("$(k): $(v)")
end
In each cycle, the key and the value of the current pair is stored in the
k and v variables. They are then printed to the console.
$ julia main.jl a: book b: storm d: green c: pencil
The keys and values functions
We can use the keys and values functions to get all
the keys and values of a dictionary.
d = Dict(:a => "book", :b => "storm",
:c => "pencil", :d => "green")
println(keys(d))
println(values(d))
for e in keys(d)
println(e)
end
for e in values(d)
println(e)
end
The program uses the keys and values functions
d = Dict(:a => "book", :b => "storm",
:c => "pencil", :d => "green")
We define a dictionary of words.
println(keys(d))
We get all the keys of the dictionary. The function returns an array.
$ julia main.jl [:a, :b, :d, :c] ["e-books", "storm", "green", "pencil"] a b d c book storm green pencil
Types
The next example works with types.
d = Dict{Symbol, String}(:a => "book", :b => "storm",
:c => "pencil", :d => "green")
println(keytype(d))
println(valtype(d))
We explicitly define the dictionary key/value types and use the
keytype and valtype functions to determine the types
of keys and values at runtime.
d = Dict{Symbol, String}(:a => "book", :b => "storm",
:c => "pencil", :d => "green")
We can explicitly specify the types for the keys and values within the
{} brackets.
$ julia main.jl Symbol String
Merging dictionaries
The merge function can be used to merge dictionaries.
d1 = Dict(:a => "book", :b => "storm", :c => "pencil") d2 = Dict(:d => "water", :e => "form", :f => "lamp") d = merge(d1, d2) println(d)
In the example, we merge two dictionaries of words with merge.
$ julia main.jl
Dict(:a => "book", :b => "storm", :f => "lamp", :d => "water",
:e => "form", :c => "pencil")
The haskey function
With the haskey function, we can determine if the dictionary
contains the given key.
d = Dict(:a => "book", :b => "storm",
:c => "pencil", :d => "green")
if haskey(d, :c)
println("the dictionary contains the :c key")
else
println("the dictionary does not contain the :c key")
end
The program checks if the dictionary contains the :c key.
The in operator
We can check if a pair is in the dictionary with the in operator.
d = Dict(:a => "book", :b => "storm",
:c => "pencil", :d => "green")
p = Pair(:a, "book")
if p in d
println("the $(p) is in the dictionary")
else
println("the item is not in the dictionary")
end
The program determines whether the defined dictionary contains the
Pair(:a, "book").
We can also use the ∈ and ∉ operators. (Written with
\in and \notin).
d = Dict(:a => "book", :b => "storm",
:c => "pencil", :d => "green")
p1 = Pair(:a, "book")
if p1 ∈ d
println("the $(p1) is in the dictionary")
else
println("the item is not in the dictionary")
end
p2 = Pair(:e, "war")
if p2 ∉ d
println("the $(p2) is not in the dictionary")
else
println("the item is in the dictionary")
end
The program uses the ∈ and ∉ operators.
The get and get! functions
The get function returns the value stored for the given key, or the
given default value if no mapping for the key is present. The get!
function returns the value stored for the given key, or if no mapping for
the key is present, it stores the given default pair and returns it.
d = Dict(:a => "book", :b => "storm", :c => "pencil",
:d => "car", :e => "book")
println(get(d, :a, "not present"))
println(d)
println(get(d, :f, "not present"))
println(d)
println(get!(d, :f, "cloud"))
println(d)
In the program we use th get and get! functions.
$ julia getfuns.jl book Dict(:a => "book", :b => "storm", :d => "car", :e => "book", :c => "pencil") not present Dict(:a => "book", :b => "storm", :d => "car", :e => "book", :c => "pencil") cloud Dict(:a => "book", :b => "storm", :f => "cloud", :d => "car", :e => "book", :c => "pencil")
Adding elements
In the next example, we add new pairs to a dictionary.
d = Dict(:a => "book", :b => "storm", :c => "pencil", :d => "green") println(d) push!(d, :e => "cloud") d[:x] = "sea" println(d)
In the program, we add two pairs.
push!(d, :e => "cloud")
In the first case, we use the push! function.
d[:x] = "sea"
Another way is to use the assignment operator.
$ julia main.jl Dict(:a => "book", :b => "storm", :d => "green", :c => "pencil") Dict(:a => "book", :b => "storm", :d => "green", :e => "cloud", :c => "pencil", :x => "sea")
Deleting elements
We can delete elements with delete! and pop!.
d = Dict(:a => "book", :b => "storm", :c => "pencil",
:d => "car", :e => "book")
delete!(d, :a)
println(d)
r = pop!(d)
println(r)
println(d)
In the program, we remove two elements.
delete!(d, :a)
We delete the pair that has key :a.
r = pop!(d)
The pop! removes an item in the dictionary and returns it. If
collection is an ordered container, the last item is returned; for unordered
containers, an arbitrary element is returned.
$ julia main.jl Dict(:b => "storm", :d => "car", :e => "book", :c => "pencil") :b => "storm" Dict(:d => "car", :e => "book", :c => "pencil")
In this article we have worked with dictionaries in Julia.
