# Dictionaries and pairs

In this module, we will explore two very useful data structures: dictionaries, which serve as “key-value” stores, and pairs, which serve as (essentially) the same thing but smaller.

What is a dictionary? Essentially, it is a data structure that is mutable (we can modify it), and associates a value to a key. Let’s start with a simple illustration, to give a taxonomic description of the common house mouse, Mus musculus:

In practice, and this is a feature we will explore later, representation of highly structured information is a good opportunity to define one’s own types.
mus_musculus = Dict([
"vernacular" => "House mouse",
"kingdom" => "Animalia",
"phylum" => "Chordata",
"class" => "Mammalia",
"order" => "Rodentia",
"family" => "Muridae",
"genus" => "Mus",
"subgenus" => "Mus",
"species" => "musculus",
"authority" => "L. 1758"
])

Dict{String, String} with 10 entries:
"phylum" => "Chordata"
"vernacular" => "House mouse"
"family" => "Muridae"
"genus" => "Mus"
"subgenus" => "Mus"
"class" => "Mammalia"
"order" => "Rodentia"
"authority" => "L. 1758"
"kingdom" => "Animalia"
"species" => "musculus"


The construction of the dictionary is a call to Dict, and the argument is an array (we know because it starts with [ and ends with ]) of pairs of elements. A pair is noted as key => value, which is something we will explore a little bit more in just a minute.

Note how the keys in the input and the keys in the output are in different orders. By default, the order of elements in dictionaries is not fixed, but this is not particulary concerning because we can access the different entries in a lot of ways.

Additionally, note that the dictionary we have created is parametric: its keys are of the String type, and its values are of the String type as well. In a few modules, we will explore a concept called “element dispatch”, where we can use this information. For now, just use this as a reminder of what is stored in your dictionary.

In some cases, we would prefer an ordered dictionary. This is a feature offered by a few different packages. As we tend to not rely on ordered dictionaries, we will not discuss these packages here.

In order to extract a value from a dictionary, we access is using its key:

mus_musculus["order"]

"Rodentia"


Of course, this is going to fail if there is no "order" key to be found. For example, not all species will have a "subgenus", or a "subspecies" key. It is therefore safer to handle data access with get:

get(mus_musculus, "subgenus", nothing)

"Mus"


The last argument of get is the value to be returned in case the key does not exist. We can check that this is indeed protecting us against exceptions by trying to access a key that is not in our object:

get(mus_musculus, "subspecies", nothing)


In order to get a list of the keys for a given dictionary, we can use the keys function:

keys(mus_musculus)

KeySet for a Dict{String, String} with 10 entries. Keys:
"phylum"
"vernacular"
"family"
"genus"
"subgenus"
"class"
"order"
"authority"
"kingdom"
"species"


This is a strange little object (a KeySet), which we can iterate over (and we will see what iteration is in a future module); to get it to a format we know, we can pass it through collect:

mus_keys = collect(keys(mus_musculus))

10-element Vector{String}:
"phylum"
"vernacular"
"family"
"genus"
"subgenus"
"class"
"order"
"authority"
"kingdom"
"species"


We can do the same thing with the values of a dictionary:

mus_values = collect(values(mus_musculus))

10-element Vector{String}:
"Chordata"
"House mouse"
"Muridae"
"Mus"
"Mus"
"Mammalia"
"Rodentia"
"L. 1758"
"Animalia"
"musculus"


An interesting little command is zip: we can use it to create a dictionary from two arrays, one with the keys and one with the values:

mus_zip = Dict(zip(mus_keys, mus_values))

Dict{String, String} with 10 entries:
"phylum" => "Chordata"
"vernacular" => "House mouse"
"family" => "Muridae"
"genus" => "Mus"
"subgenus" => "Mus"
"class" => "Mammalia"
"order" => "Rodentia"
"authority" => "L. 1758"
"kingdom" => "Animalia"
"species" => "musculus"

Of course, this assumes that the keys and the values are in the correct order, because there is no way for Julia to know which value should be associated to each key.

The final data structure related to dictionaries is the Pair. A pair associates one key to one value:

pair_one_two = 1 => 2

1 => 2


We can use a different notation as well:

pair_one_two_take_2 = Pair(1,2)

1 => 2


The elements of a pair can be accessed using first and last:

first(pair_one_two)

1

last(pair_one_two)

2


Why are we talking about pairs here? Because dictionaries are simply a series of pairs wearing a trench coat:

eltype(collect(mus_musculus))

Pair{String, String}


We can collect a dictionary into a vector of Pair, but more importantly, we can create a dictionary from a vector of pairs. This is, indeed, what we have done when we created the mus_musculus variable.

A final noteworthy information is that we can expand dictionaries, by adding keys:

mus_musculus["IUCN"] = "least concern"

"least concern"


This is also more properly expressed as:

setindex!(mus_musculus, "IUCN", "least concern")

Dict{String, String} with 12 entries:
"family" => "Muridae"
"subgenus" => "Mus"
"genus" => "Mus"
"least concern" => "IUCN"
"vernacular" => "House mouse"
"IUCN" => "least concern"
"class" => "Mammalia"
"kingdom" => "Animalia"
"species" => "musculus"
"phylum" => "Chordata"
"authority" => "L. 1758"
"order" => "Rodentia"


And we can, similarly, remove information from a dictionary:

delete!(mus_musculus, "subgenus")

Dict{String, String} with 11 entries:
"family" => "Muridae"
"genus" => "Mus"
"least concern" => "IUCN"
"vernacular" => "House mouse"
"IUCN" => "least concern"
"class" => "Mammalia"
"kingdom" => "Animalia"
"species" => "musculus"
"phylum" => "Chordata"
"authority" => "L. 1758"
"order" => "Rodentia"


Note that this function does not throw an exception if we try to delete a key that does not exist. If we want to delete a key but keep track of its value, the right function to use is pop!:

mus_iucn_status = pop!(mus_musculus, "IUCN", nothing)

"least concern"


This will remove the key from the dictionary, but save the value to a variable. The third argument specifies what should be returned if the key doesn’t exist, in order to avoid an exception:

pop!(mus_musculus, "IUCN", nothing)
`

In summary, pairs and dictionaries are extremely useful data structures when there is a need to associate a value with a more memorable name. They have a lot of uses when, for example, storing the parameters of a model, or reading JSON files.