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 occurs only upon the first insertion.
Internally, the map is backed by an array of AssocList (buckets).
The array doubles in size when the expected bucket list size grows beyond a fixed threshold.
Certain operations, such as put, are amortized O(1) but can run in worst-case O(size) time.
These worst cases may exceed the cycle limit per message on large maps.
This analysis assumes that the hash function distributes keys uniformly.
Use caution when growing large maps and ensure good hash functions are used.
For maps without amortization, see TrieMap.
The initCapacity argument sets the initial number of buckets.
All runtime and space complexities assume that the equality and hash functions 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 | Space |
|---|---|
O(1) | 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 | Space |
|---|---|
O(1) | 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
| Runtime(worst) | Runtime(amortized) | Space |
|---|---|---|
O(size) | O(1) | 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
| Runtime(amortized) | Runtime(worst) | Space (amortized) | Space(worst) |
|---|---|---|---|
O(1) | O(size) | O(1) | O(size) |
This operation triggers the allocation of the underlying array if it is the first entry in the map.
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 | Worst Case Runtime | Expected Amortized Space | Worst Case Space |
|---|---|---|---|
O(1) | O(size) | O(1) | O(size) |
This operation triggers the allocation of the underlying array if it is the first entry in the map.
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 | Worst Case Runtime | Expected Space | Worst Case Space |
|---|---|---|---|
O(1) | O(size) | O(1) | 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 | Worst Case Runtime | Expected Space | Worst Case Space |
|---|---|---|---|
O(1) | O(size) | O(1) | 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 | Space |
|---|---|
O(size) | 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
| Runtime | Space |
|---|---|
O(size) | 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 | Space |
|---|---|
O(size) | 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
| Runtime(expected) | Runtime(worst) | Space(expected) | Space(worst) |
|---|---|---|---|
O(size) | O(size * size) | O(size) | 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
| Runtime(expected) | Runtime(worst) | Space(expected) | Space(worst) |
|---|---|---|---|
O(size) | O(size * size) | O(size) | 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)
| Runtime(expected) | Runtime(worst) | Space(expected) | Space(worst) |
|---|---|---|---|
O(size) | O(size * size) | O(size) | O(size) |
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
| Runtime(expected) | Runtime(worst) | Space(expected) | Space(worst) |
|---|---|---|---|
O(size) | O(size * size) | O(size) | O(size) |