wren

Maps

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:

{
  "maple":  "Sugar Maple (Acer Saccharum)",
  "larch":  "Alpine Larch (Larix Lyallii)",
  "oak":    "Red Oak (Quercus Rubra)",
  "fir":    "Fraser Fir (Abies Fraseri)"
}

This creates a map that associates a type of tree (key) to a specific tree within that family (value). 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 (not an instance of that class, the actual class itself).

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 using the subscript operator:

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:

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():

var capitals = {"Georgia": null}

System.print(capitals["Georgia"]) //> null (though key exists)
System.print(capitals["Idaho"])   //> null 
System.print(capitals.containsKey("Georgia")) //> true
System.print(capitals.containsKey("Idaho"))   //> false

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

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:

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

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

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, like with lists, you call clear():

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. You can use a regular for loop to iterate the contents, and map exposes two additional methods to access the contents: keys and values.

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

Regardless of how you iterate, the order that things are iterated in isn’t defined. Wren makes no promises about what order keys and values are iterated. All it promises is that every entry will appear exactly once.

Iterating with for(entry in map)
When you iterate a map with for, you’ll be handed an entry, which contains a key and a value field. That gives you the info for each element in the map.

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

for (bird in birds) {
  System.print("The state bird of %(bird.key) is %(bird.value)")
}

Iterating using the keys

You can also iterate over the keys and use each to look up its value:

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])
}