wren/doc/site/maps.markdown

^title Maps ^category types

A map is an associative collection. It holds a set of entries, each of which maps a key to a value. The same data structure has a variety of names in other languages: hash table, dictionary, association, table, etc.

You can create a map by placing a series of comma-separated entries inside curly braces. Each entry is a key and a value separated by a colon:

:::dart
{
  "George": "Harrison",
  "John": "Lennon",
  "Paul": "McCartney",
  "Ringo": "Starr"
}

This creates a map that maps the first names of the Beatles to their last names. Syntactically, in a map literal, keys can be any literal, a variable name, or a parenthesized expression. Values can be any expression. Here, we're using string literals for both keys and values.

Semantically, values can be any object, and multiple keys may map to the same value. Keys have a few limitations. They must be one of the immutable built-in value types in Wren. That means a number, string, range, bool, or null. You can also use a class object as a key.

In addition, even though they aren't strictly immutable, fibers can be used as map keys. This is handy for storing data that's roughly "thread-local" by using the current fiber as a map key.

The reason for this limitation—and the reason maps are called "hash tables" in other languages—is that each key is used to generate a numeric hash code. This lets a map locate the value associated with a key in constant time, even in very large maps. Since Wren only knows how to hash certain built-in types, only those can be used as keys.

Adding entries

You add new key-value pairs to the map by using the subscript operator:

:::dart
var capitals = {}
capitals["Georgia"] = "Atlanta"
capitals["Idaho"] = "Boise"
capitals["Maine"] = "Augusta"

If the key isn't already present, this adds it and associates it with the given value. If the key is already there, this just replaces its value.

Looking up values

To find the value associated with some key, again you use your friend the subscript operator:

:::dart
System.print(capitals["Idaho"]) // "Boise".

If the key is present, this returns its value. Otherwise, it returns null. Of course, null itself can also be used as a value, so seeing null here doesn't necessarily mean the key wasn't found.

To tell definitively if a key exists, you can call containsKey():

:::dart
var belief = {"nihilism": null}

System.print(belief["nihilism"]) // "null" though key exists.
System.print(belief["solipsism"]) // Also "null".
System.print(belief.containsKey("nihilism")) // "true".
System.print(belief.containsKey("solipsism")) // "false".

You can see how many entries a map contains using count:

:::dart
System.print(capitals.count) // "3".

Removing entries

To remove an entry from a map, call remove() and pass in the key for the entry you want to delete:

:::dart
capitals.remove("Maine")
System.print(capitals.containsKey("Maine")) // "false".

If the key was found, this returns the value that was associated with it:

:::dart
System.print(capitals.remove("Georgia")) // "Atlanta".

If the key wasn't in the map to begin with, remove() just returns null.

If you want to remove everything from the map, just like with lists, you can just call clear():

:::dart
capitals.clear()
System.print(capitals.count) // "0".

Iterating over the contents

The subscript operator works well for finding values when you know the key you're looking for, but sometimes you want to see everything that's in the map. For that, map exposes two methods: keys and values.

The first returns a Sequence that iterates over all of the keys in the map, and the second returns one that iterates over the values.

If you want to see all of the key-value pairs in a map, the easiest way is to iterate over the keys and use each to look up its value:

:::dart
var birds = {
  "Arizona": "Cactus wren",
  "Hawaii": "Nēnē",
  "Ohio": "Northern Cardinal"
}

for (state in birds.keys) {
  System.print("The state bird of " + state + " is " + birds[state])
}

This program will print the three states and their birds. However, the order that they are printed isn't defined. Wren makes no promises about what order keys and values will be iterated in when you use these methods. All it promises is that every entry will appear exactly once.