HashMap

Class HashMap<K, V> provides a hashmap from keys of type K to values of type V. The class is parameterized by the key's equality and hash functions, and an initial capacity. However, the underlying allocation happens only when the first key-value entry is inserted.

Internally, the map is represented as an array of AssocList (buckets). The growth policy of the underyling array is very simple, for now: double the current capacity when the expected bucket list size grows beyond a certain constant.

WARNING: Certain operations are amortized O(1) time, such as put, but run in worst case O(size) time. These worst case runtimes may exceed the cycles limit per message if the size of the map is large enough. Further, this runtime analysis assumes that the hash functions uniformly maps keys over the hash space. Grow these structures with discretion, and with good hash functions. All amortized operations below also list the worst case runtime.

For maps without amortization, see TrieMap.

Note on the constructor: The argument initCapacity determines the initial number of buckets in the underyling array. Also, the runtime and space anlyses in this documentation assumes that the equality and hash functions for keys used to construct the map run in O(1) time and space.

Example:

import HashMap "mo:base/HashMap";
import Text "mo:base/Text";

let map = HashMap.HashMap<Text, Nat>(5, Text.equal, Text.hash);

Runtime: O(1)

Space: O(1)

Class HashMap<K, V>

class HashMap<K, V>(initCapacity : Nat, keyEq : (K, K) -> Bool, keyHash : K -> Hash.Hash)

Function size

func size() : Nat

Returns the current number of key-value entries in the map.

Example:

map.size() // => 0

Runtime: O(1)

Space: O(1)

Function get

func get(key : K) : (value : ?V)

Returns the value assocaited with key key if present and null otherwise.

Example:

map.put("key", 3);
map.get("key") // => ?3

Expected Runtime: O(1), Worst Case Runtime: O(size)

Space: O(1)

Function put

func put(key : K, value : V)

Insert the value value with key key. Overwrites any existing entry with key key.

Example:

map.put("key", 3);
map.get("key") // => ?3

Expected Amortized Runtime: O(1), Worst Case Runtime: O(size)

Expected Amortized Space: O(1), Worst Case Space: O(size)

Note: If this is the first entry into this map, this operation will cause the initial allocation of the underlying array.

Function replace

func replace(key : K, value : V) : (oldValue : ?V)

Insert the value value with key key. Returns the previous value associated with key key or null if no such value exists.

Example:

map.put("key", 3);
ignore map.replace("key", 2); // => ?3
map.get("key") // => ?2

Expected Amortized Runtime: O(1), Worst Case Runtime: O(size)

Expected Amortized Space: O(1), Worst Case Space: O(size)

Note: If this is the first entry into this map, this operation will cause the initial allocation of the underlying array.

Function delete

func delete(key : K)

Deletes the entry with the key key. Has no effect if key is not present in the map.

Example:

map.put("key", 3);
map.delete("key");
map.get("key"); // => null

Expected Runtime: O(1), Worst Case Runtime: O(size)

Expected Space: O(1), Worst Case Space: O(size)

Function remove

func remove(key : K) : (oldValue : ?V)

Deletes the entry with the key key. Returns the previous value associated with key key or null if no such value exists.

Example:

map.put("key", 3);
map.remove("key"); // => ?3

Expected Runtime: O(1), Worst Case Runtime: O(size)

Expected Space: O(1), Worst Case Space: O(size)

Function keys

func keys() : Iter.Iter<K>

Returns an Iterator (Iter) over the keys of the map. Iterator provides a single method next(), which returns keys in no specific order, or null when out of keys to iterate over.

Example:

map.put("key1", 1);
map.put("key2", 2);
map.put("key3", 3);

var keys = "";
for (key in map.keys()) {
  keys := key # " " # keys
};
keys // => "key3 key2 key1 "

Cost of iteration over all keys:

Runtime: O(size)

Space: O(1)

Function vals

func vals() : Iter.Iter<V>

Returns an Iterator (Iter) over the values of the map. Iterator provides a single method next(), which returns values in no specific order, or null when out of values to iterate over.

Example:

map.put("key1", 1);
map.put("key2", 2);
map.put("key3", 3);

var sum = 0;
for (value in map.vals()) {
  sum += value;
};
sum // => 6

Cost of iteration over all values:

Runtime: O(size)

Space: O(1)

Function entries

func entries() : Iter.Iter<(K, V)>

Returns an Iterator (Iter) over the key-value pairs in the map. Iterator provides a single method next(), which returns pairs in no specific order, or null when out of pairs to iterate over.

Example:

import Nat "mo:base/Nat";

map.put("key1", 1);
map.put("key2", 2);
map.put("key3", 3);

var pairs = "";
for ((key, value) in map.entries()) {
  pairs := "(" # key # ", " # Nat.toText(value) # ") " # pairs
};
pairs // => "(key3, 3) (key2, 2) (key1, 1)"

Cost of iteration over all pairs:

Runtime: O(size)

Space: O(1)

Function clone

func clone<K, V>(map : HashMap<K, V>, keyEq : (K, K) -> Bool, keyHash : K -> Hash.Hash) : HashMap<K, V>

Returns a copy of map, initializing the copy with the provided equality and hash functions.

Example:

map.put("key1", 1);
map.put("key2", 2);
map.put("key3", 3);

let map2 = HashMap.clone(map, Text.equal, Text.hash);
map2.get("key1") // => ?1

Expected Runtime: O(size), Worst Case Runtime: O(size * size)

Expected Space: O(size), Worst Case Space: O(size)

Function fromIter

func fromIter<K, V>(iter : Iter.Iter<(K, V)>, initCapacity : Nat, keyEq : (K, K) -> Bool, keyHash : K -> Hash.Hash) : HashMap<K, V>

Returns a new map, containing all entries given by the iterator iter. The new map is initialized with the provided initial capacity, equality, and hash functions.

Example:

let entries = [("key3", 3), ("key2", 2), ("key1", 1)];
let iter = entries.vals();

let map2 = HashMap.fromIter<Text, Nat>(iter, entries.size(), Text.equal, Text.hash);
map2.get("key1") // => ?1

Expected Runtime: O(size), Worst Case Runtime: O(size * size)

Expected Space: O(size), Worst Case Space: O(size)

Function map

func map<K, V1, V2>(hashMap : HashMap<K, V1>, keyEq : (K, K) -> Bool, keyHash : K -> Hash.Hash, f : (K, V1) -> V2) : HashMap<K, V2>

Creates a new map by applying f to each entry in hashMap. Each entry (k, v) in the old map is transformed into a new entry (k, v2), where the new value v2 is created by applying f to (k, v).

map.put("key1", 1);
map.put("key2", 2);
map.put("key3", 3);

let map2 = HashMap.map<Text, Nat, Nat>(map, Text.equal, Text.hash, func (k, v) = v * 2);
map2.get("key2") // => ?4

Expected Runtime: O(size), Worst Case Runtime: O(size * size)

Expected Space: O(size), Worst Case Space: O(size)

*Runtime and space assumes that f runs in O(1) time and space.

Function mapFilter

func mapFilter<K, V1, V2>(hashMap : HashMap<K, V1>, keyEq : (K, K) -> Bool, keyHash : K -> Hash.Hash, f : (K, V1) -> ?V2) : HashMap<K, V2>

Creates a new map by applying f to each entry in hashMap. For each entry (k, v) in the old map, if f evaluates to null, the entry is discarded. Otherwise, the entry is transformed into a new entry (k, v2), where the new value v2 is the result of applying f to (k, v).

map.put("key1", 1);
map.put("key2", 2);
map.put("key3", 3);

let map2 =
  HashMap.mapFilter<Text, Nat, Nat>(
    map,
    Text.equal,
    Text.hash,
    func (k, v) = if (v == 2) { null } else { ?(v * 2)}
);
map2.get("key3") // => ?6

Expected Runtime: O(size), Worst Case Runtime: O(size * size)

Expected Space: O(size), Worst Case Space: O(size)

*Runtime and space assumes that f runs in O(1) time and space.