Dictionaries

A list finds items by position. A dictionary finds them by a key you choose, such as a name. Each entry pairs a key with a value, written key: value:

const ages = ["Ada": 36, "Grace": 85]
print(ages)
print(ages.count)

Output

["Ada": 36, "Grace": 85]
2

Like a list, a dictionary holds one type of key and one type of value. ages is a Dict[String, Int]: keys that are strings, values that are whole numbers.

Looking up a value

Square brackets look a value up by its key:

const ages = ["Ada": 36, "Grace": 85]
print(ages["Ada"])
print(ages["Bob"])

Output

36
nothing

A key that isn’t there gives nothing, so a lookup is optional: its type is Int?. That means you have to decide what a missing key means before using the value:

const ages = ["Ada": 36]
print(ages["Ada"] + 1)

Output

dictionaries.em:2:7: addition needs numbers, but this is Int? and Int
  print(ages["Ada"] + 1)
        ^^^^^^^^^^^^^^^
One of these may be absent. Give it a fallback with `.or(0)`, or check it against `nothing` first.

Often a fallback is exactly right:

const ages = ["Ada": 36]
print(ages["Ada"].or(0) + 1)
print(ages["Bob"].or(0))

Output

37
0

contains_key?("Bob") asks whether a key is there without looking up its value.

Adding and changing entries

A dictionary declared with var can change. Assigning to a key adds the entry, or replaces its value if the key is already there; remove takes one out:

var ages = ["Ada": 36]
ages["Grace"] = 85     # add
ages["Ada"] = 37       # replace
print(ages)
ages.remove("Ada")
print(ages)

Output

["Ada": 37, "Grace": 85]
["Grace": 85]

A key appears at most once, so assigning to an existing key never makes a second entry.

An empty dictionary

An empty dictionary is written [], with a type saying what it will hold. It prints as [:], so you can tell it from an empty list:

var stock: Dict[String, Int] = []
print(stock)
stock["apples"] = 3
print(stock)

Output

[:]
["apples": 3]

Going through every entry

A for loop visits each entry, giving the key and the value names of their own:

const ages = ["Ada": 36, "Grace": 85]
for (name, age) in ages {
    print("#{name} is #{age}")
}

Output

Ada is 36
Grace is 85

Entries come out in the order they were added. keys() and values() give lists of just the keys or just the values.

A dictionary that counts

Dictionaries are the natural way to count things. Here each word is a key, and its value is how many times it has appeared so far:

const words = "the cat and the hat".split(" ")
var counts: Dict[String, Int] = []
for word in words {
    counts[word] = counts[word].or(0) + 1
}
print(counts)

Output

["the": 2, "cat": 1, "and": 1, "hat": 1]

counts[word].or(0) is the count so far, or 0 the first time a word is seen.

List or dictionary?

• Use a list when order matters and you think of items by position: a queue, a leaderboard, the lines of a file.
• Use a dictionary when you look things up by a name or other key: a phone book, stock levels, settings.

The Dict reference describes every method.

What you’ve learned

• A dictionary, ["Ada": 36], maps keys to values, one type of each.
• dict[key] looks a value up and might be nothing; .or(fallback) handles a missing key.
• dict[key] = value adds or replaces an entry, and remove(key) takes one out.
• An empty dictionary is [] with a type, and prints as [:].
• for (key, value) in dict visits the entries in the order they were added.

Next, sets: collections where each value appears once.