Pairs And Dicts in

About Pairs And Dicts

Pairs

A Pair is just two items joined together. The items are then imaginatively called first and second.

Create them either with the => operator or the Pair() constructor.

julia> p1 = "k" => 2
"k" => 2

julia> p2 = Pair("k", 2)
"k" => 2

# Both forms of syntax give the same result
julia> p1 == p2
true

# Each component has its own separate type
julia> dump(p1)
Pair{String, Int64}
  first: String "k"
  second: Int64 2

# Get a component using dot syntax
julia> p1.first
"k"

julia> p1.second
2

Dicts

A Vector of Pairs is like any other array: ordered, homogeneous in type, and stored consecutively in memory.

julia> pv = ['a' => 1, 'b' => 2, 'c' => 3]
3-element Vector{Pair{Char, Int64}}:
 'a' => 1
 'b' => 2
 'c' => 3

# Each pair is a single entry
julia> length(pv)
3

A Dict is superficially similar, but storage is now implemented in a way that allows fast retrieval by key, known as a "hash table", even when the number of entries grows large.

julia> pd = Dict('a' => 1, 'b' => 2, 'c' => 3)  # or Dict(pv) gives same result
Dict{Char, Int64} with 3 entries:
  'a' => 1
  'c' => 3
  'b' => 2

julia> pd['b']
2

# Key must exist
julia> pd['d']
ERROR: KeyError: key 'd' not found

# Generators are accepted in the constructor (and note the unordered output)
julia> Dict(x => x^2 for x in 1:5)
julia> Dict(x => 1 / x for x in 1:5)
Dict{Int64, Float64} with 5 entries:
  5 => 0.2
  4 => 0.25
  2 => 0.5
  3 => 0.333333
  1 => 1.0

In other languages, something very similar to a Dict might be called a dictionary (Python), a Hash (Ruby) or a HashMap (Java).

For Pairs, whether in isolation or in a Vector, there are few constraints on the type of each component.

To be valid in a Dict, the Pair must be a key => value pair, where the key is "hashable". Most importantly, this means the key must be immutable, so Char, Int, String, Symbol, and Tuple are all fine, but Vector is not allowed.

If mutable keys are important to you, there is a separate but much less common IdDict type that can allow this. See the manual for several other variants on the Dict type.

Modifying a Dict

Entries can be added, with a new key, or overwritten, with an existing key.

julia> pd
Dict{Char, Int64} with 3 entries:
  'a' => 1
  'c' => 3
  'b' => 2

# Add
julia> pd['d'] = 4
4

# Overwrite
julia> pd['a'] = 42
42

julia> pd
Dict{Char, Int64} with 4 entries:
  'a' => 42
  'c' => 3
  'd' => 4
  'b' => 2

To remove an entry, use the delete!() function, which will change the Dict if the key exists and silently do nothing otherwise.

julia> delete!(pd, 'd')
Dict{Char, Int64} with 3 entries:
  'a' => 42
  'c' => 3
  'b' => 2

Checking if a key or value exists

There are different approaches. To check a key, there is a haskey() function:

julia> haskey(pd, 'b')
true

Alternatively, search either the keys or the values:

julia> 'b' in keys(pd)
true

julia> 43 in values(pd)
false

julia> 42 ∈ values(pd)
true

This remains efficient at scale, as the keys() and values() functions each return an iterator with a fast search algorithm.