Maps

A Map(K, V) is an insertion-ordered dictionary. K is Int or String. Lookup, insertion and update are O(1): the entries stay a dense array in arrival order, with a hash index over it.

main.luc

func main():
    var ages = new Map(String, Int)
    ages["ada"] = 36
    ages["grace"] = 45
    ages["alan"] = 41
    ages["ada"] += 1                 # the key is evaluated once

    for name, age in ages:
        print(f"{name} is {age}")

    print(f"{len(ages)} people")
    print(f"has grace: {ages.has("grace")}")
    print(f"get nobody: {ages.get("nobody", -1)}")

    ages.remove("alan")
    ages.remove("nobody")            # absent: a no-op, not an error
    print(f"{len(ages)} left")

Output

ada is 37
grace is 45
alan is 41
3 people
has grace: true
get nobody: -1
2 left

Indexing a key that is not there is a trap, on purpose. Asking a map for something you did not put in it is a bug in the program, not news from the world — so guard with has, or use get(key, default), which cannot trap.

main.luc

func main():
    var ages = new Map(String, Int)
    ages["ada"] = 36
    print(str(ages["nobody"]))

Output — the program traps

loom: trap: key not found in map [key_missing]
    at main (main.luc:4:5)

Counting#

The Map counting idiom, which is what most real uses are:

main.luc

import std.strings

func main():
    let text = "the cat sat on the mat the end"
    var counts = new Map(String, Int)
    for word in text.split(" "):
        counts[word] = counts.get(word, 0) + 1

    for word, seen in counts:
        if seen > 1:
            print(f"{word}: {seen}")

Output

the: 3

keys() and values() hand back fresh lists the receiver owns.

main.luc

func main():
    var stock = new Map(Int, String)
    stock[3] = "fig"
    stock[1] = "pear"

    let ids = stock.keys()
    let names = stock.values()
    print(f"{len(ids)} keys, first {ids[0]}")
    print(f"{len(names)} values, first {names[0]}")

Output

2 keys, first 3
2 values, first fig