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:

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

This creates a map that associates the first name of each Beatle with his last name. 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.

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 belief = {"nihilism": null}

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

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 prints 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 are iterated in when you use these methods. All it promises is that every entry will appear exactly once.