Array

motoko name=import
import Array "mo:base/Array";

public func empty<T>() : [T]

Creates an empty array (equivalent to []).

public func repeat<T>(item : T, size : Nat) : [T]

Creates an array containing item repeated size times.

motoko include=import
let array = Array.repeat<Nat>("Echo", 3);
assert array == [var "Echo", "Echo", "Echo"];

Runtime: O(size)

Space: O(size)

public func tabulate<T>(size : Nat, generator : Nat -> T) : [T]

Creates an immutable array of size size. Each element at index i is created by applying generator to i.

motoko include=import
let array : [Nat] = Array.tabulate<Nat>(4, func i = i * 2);
assert array == [0, 2, 4, 6];

Runtime: O(size)

Space: O(size)

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

public func fromVarArray<T>(varArray : [var T]) : [T]

Transforms a mutable array into an immutable array.

motoko include=import

let varArray = [var 0, 1, 2];
varArray[2] := 3;
let array = Array.fromVarArray<Nat>(varArray);

Runtime: O(size)

Space: O(1)

public func toVarArray<T>(array : [T]) : [var T]

Transforms an immutable array into a mutable array.

motoko include=import

let array = [0, 1, 2];
let varArray = Array.toVarArray<Nat>(array);
varArray[2] := 3;
varArray

Runtime: O(size)

Space: O(1)

public func equal<T>(
  array1 : [T],
  array2 : [T],
  equal : (T, T) -> Bool
) : Bool

Tests if two arrays contain equal values (i.e. they represent the same list of elements). Uses equal to compare elements in the arrays.

motoko include=import
// Use the equal function from the Nat module to compare Nats
import {equal} "mo:base/Nat";

let array1 = [0, 1, 2, 3];
let array2 = [0, 1, 2, 3];
Array.equal(array1, array2, equal)

Runtime: O(size1 + size2)

Space: O(1)

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

public func find<T>(array : [T], predicate : T -> Bool) : ?T

Returns the first value in array for which predicate returns true. If no element satisfies the predicate, returns null.

motoko include=import
let array = [1, 9, 4, 8];
Array.find<Nat>(array, func x = x > 8)

Runtime: O(size)

Space: O(1)

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

public func concat<T>(array1 : [T], array2 : [T]) : [T]

Create a new array by concatenating the values of array1 and array2. Note that Array.append copies its arguments and has linear complexity.

motoko include=import
let array1 = [1, 2, 3];
let array2 = [4, 5, 6];
Array.concat<Nat>(array1, array2)

Runtime: O(size1 + size2)

Space: O(size1 + size2)

public func sort<T>(array : [T], compare : (T, T) -> Order.Order) : [T]

Sorts the elements in the array according to compare. Sort is deterministic and stable.

motoko include=import
import Nat "mo:base/Nat";

let array = [4, 2, 6];
Array.sort(array, Nat.compare)

Runtime: O(size * log(size))

Space: O(size) *Runtime and space assumes that compare runs in O(1) time and space.

public func reverse<T>(array : [T]) : [T]

Creates a new array by reversing the order of elements in array.

motoko include=import

let array = [10, 11, 12];

Array.reverse(array)

Runtime: O(size)

Space: O(1)

public func forEach<T>(array : [T], f : T -> ())

Calls f with each element in array. Retains original ordering of elements.

motoko include=import
import Debug "mo:base/Debug";

let array = [0, 1, 2, 3];
Array.forEach<Nat>(array, func(x) {
  Debug.print(debug_show x)
})

Runtime: O(size)

Space: O(size)

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

public func map<T, R>(array : [T], f : T -> R) : [R]

Creates a new array by applying f to each element in array. f "maps" each element it is applied to of type X to an element of type Y. Retains original ordering of elements.

motoko include=import

let array1 = [0, 1, 2, 3];
let array2 = Array.map<Nat, Nat>(array1, func x = x * 3)
assert array2 == [0, 2, 4, 6];

Runtime: O(size)

Space: O(size)

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

public func filter<T>(array : [T], f : T -> Bool) : [T]

Creates a new array by applying predicate to every element in array, retaining the elements for which predicate returns true.

motoko include=import
let array = [4, 2, 6, 1, 5];
let evenElements = Array.filter<Nat>(array, func x = x % 2 == 0);

Runtime: O(size)

Space: O(size) *Runtime and space assumes that predicate runs in O(1) time and space.

public func filterMap<T, R>(array : [T], f : T -> ?R) : [R]

Creates a new array by applying f to each element in array, and keeping all non-null elements. The ordering is retained.

motoko include=import
import {toText} "mo:base/Nat";

let array = [4, 2, 0, 1];
let newArray =
  Array.filterMap<Nat, Text>( // mapping from Nat to Text values
    array,
    func x = if (x == 0) { null } else { ?toText(100 / x) } // can't divide by 0, so return null
  );

Runtime: O(size)

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

public func mapResult<T, R, E>(array : [T], f : T -> Result.Result<R, E>) : Result.Result<[R], E>

Creates a new array by applying f to each element in array. If any invocation of f produces an #err, returns an #err. Otherwise returns an #ok containing the new array.

motoko include=import
let array = [4, 3, 2, 1, 0];
// divide 100 by every element in the array
Array.mapResult<Nat, Nat, Text>(array, func x {
  if (x > 0) {
    #ok(100 / x)
  } else {
    #err "Cannot divide by zero"
  }
})

Runtime: O(size)

Space: O(size)

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

public func mapEntries<T, R>(array : [T], f : (T, Nat) -> R) : [R]

Creates a new array by applying f to each element in array and its index. Retains original ordering of elements.

motoko include=import

let array = [10, 10, 10, 10];
Array.mapEntries<Nat, Nat>(array, func (x, i) = i * x)

Runtime: O(size)

Space: O(size)

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

public func flatMap<T, R>(array : [T], k : T -> Iter.Iter<R>) : [R]

Creates a new array by applying k to each element in array, and concatenating the resulting arrays in order.

motoko include=import
import Nat "mo:base/Nat";

let array = [1, 2, 3, 4];
Array.flatMap<Nat, Int>(array, func x = [x, -x])

Runtime: O(size)

Space: O(size) *Runtime and space assumes that k runs in O(1) time and space.

public func foldLeft<T, A>(
  array : [T],
  base : A,
  combine : (A, T) -> A
) : A

Collapses the elements in array into a single value by starting with base and progessively combining elements into base with combine. Iteration runs left to right.

motoko include=import
import {add} "mo:base/Nat";

let array = [4, 2, 0, 1];
let sum =
  Array.foldLeft<Nat, Nat>(
    array,
    0, // start the sum at 0
    func(sumSoFar, x) = sumSoFar + x // this entire function can be replaced with `add`!
  );

Runtime: O(size)

Space: O(1)

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

public func foldRight<T, A>(
  array : [T],
  base : A,
  combine : (T, A) -> A
) : A

Collapses the elements in array into a single value by starting with base and progessively combining elements into base with combine. Iteration runs right to left.

motoko include=import
import {toText} "mo:base/Nat";

let array = [1, 9, 4, 8];
let bookTitle = Array.foldRight<Nat, Text>(array, "", func(x, acc) = toText(x) # acc);

Runtime: O(size)

Space: O(1)

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

public func join<T>(arrays : Iter.Iter<[T]>) : [T]

Combines an iterator of arrays into a single array. Retains the original ordering of the elements.

Consider using Array.flatten() where possible for better performance.

motoko include=import

let arrays = [[0, 1, 2], [2, 3], [], [4]];
Array.join<Nat>(Array.fromIter(arrays))) // => [0, 1, 2, 2, 3, 4]

Runtime: O(number of elements in array)

Space: O(number of elements in array)

public func flatten<T>(arrays : [[T]]) : [T]

Combines an array of arrays into a single array. Retains the original ordering of the elements.

This has better performance compared to Array.join().

motoko include=import

let arrays = [[0, 1, 2], [2, 3], [], [4]];
Array.flatten<Nat>(arrays)) // => [0, 1, 2, 2, 3, 4]

Runtime: O(number of elements in array)

Space: O(number of elements in array)

public func singleton<T>(element : T) : [T]

Create an array containing a single value.

motoko include=import
var array = Array.singleton(2);
assert array == [2];

Runtime: O(1)

Space: O(1)

public func size<T>(array : [T]) : Nat

Returns the size of an array. Equivalent to array.size().

public func isEmpty<T>(array : [T]) : Bool

Returns whether an array is empty, i.e. contains zero elements.

public func fromIter<T>(iter : Iter.Iter<T>) : [T]

Converts an iterator to an array.

public func keys<T>(array : [T]) : Iter.Iter<Nat>

Returns an iterator (Iter) over the indices of array. An iterator provides a single method next(), which returns indices in order, or null when out of index to iterate over.

Note: You can also use array.keys() instead of this function. See example below.

motoko include=import
let array = [10, 11, 12];

var sum = 0;
for (element in array.keys()) {
  sum += element;
};
sum

Runtime: O(1)

Space: O(1)

public func values<T>(array : [T]) : Iter.Iter<T>

Iterator provides a single method next(), which returns elements in order, or null when out of elements to iterate over.

Note: You can also use array.values() instead of this function. See example below.

motoko include=import
let array = [10, 11, 12];

var sum = 0;
for (element in array.values()) {
  sum += element;
};
sum

Runtime: O(1)

Space: O(1)

public func enumerate<T>(array : [var T]) : Iter.Iter<(Nat, T)>

Iterator provides a single method next(), which returns pairs of (index, element) in order, or null when out of elements to iterate over.

motoko include=import
let array = [10, 11, 12];

var sum = 0;
for ((index, element) in Array.enumerate(array)) {
  sum += element;
};
sum // => 33

Runtime: O(1)

Space: O(1)

public func all<T>(array : [T], predicate : T -> Bool) : Bool

Returns true if all elements in array satisfy the predicate function.

motoko include=import
let array = [1, 2, 3, 4];
Array.all<Nat>(array, func x = x > 0) // => true

Runtime: O(size)

Space: O(1)

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

public func any<T>(array : [T], predicate : T -> Bool) : Bool

Returns true if any element in array satisfies the predicate function.

motoko include=import
let array = [1, 2, 3, 4];
Array.any<Nat>(array, func x = x > 3) // => true

Runtime: O(size)

Space: O(1)

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

public func subArray<T>(
  array : [T],
  start : Nat,
  length : Nat
) : [T]

Returns a new sub-array from the given array provided the start index and length of elements in the sub-array.

Limitations: Traps if the start index + length is greater than the size of the array

motoko include=import

let array = [1, 2, 3, 4, 5];
let subArray = Array.subArray<Nat>(array, 2, 3);

Runtime: O(length)

Space: O(length)

public func indexOf<T>(
  element : T,
  array : [T],
  equal : (T, T) -> Bool
) : ?Nat

Returns the index of the first element in the array.

motoko include=import
import Char "mo:base/Char";
let array = ['c', 'o', 'f', 'f', 'e', 'e'];
assert Array.indexOf<Char>('c', array, Char.equal) == ?0;
assert Array.indexOf<Char>('f', array, Char.equal) == ?2;
assert Array.indexOf<Char>('g', array, Char.equal) == null;

Runtime: O(array.size())

Space: O(1)

public func nextIndexOf<T>(
  element : T,
  array : [T],
  fromInclusive : Nat,
  equal : (T, T) -> Bool
) : ?Nat

Returns the index of the next occurence of element in the array starting from the from index (inclusive).

motoko include=import
import Char "mo:base/Char";
let array = ['c', 'o', 'f', 'f', 'e', 'e'];
assert Array.nextIndexOf<Char>('c', array, 0, Char.equal) == ?0;
assert Array.nextIndexOf<Char>('f', array, 0, Char.equal) == ?2;
assert Array.nextIndexOf<Char>('f', array, 2, Char.equal) == ?2;
assert Array.nextIndexOf<Char>('f', array, 3, Char.equal) == ?3;
assert Array.nextIndexOf<Char>('f', array, 4, Char.equal) == null;

Runtime: O(array.size())

Space: O(1)

public func lastIndexOf<T>(
  element : T,
  array : [T],
  equal : (T, T) -> Bool
) : ?Nat

Returns the index of the last element in the array.

motoko include=import
import Char "mo:base/Char";
let array = ['c', 'o', 'f', 'f', 'e', 'e'];
assert Array.lastIndexOf<Char>('c', array, Char.equal) == ?0;
assert Array.lastIndexOf<Char>('f', array, Char.equal) == ?3;
assert Array.lastIndexOf<Char>('e', array, Char.equal) == ?5;
assert Array.lastIndexOf<Char>('g', array, Char.equal) == null;

Runtime: O(array.size())

Space: O(1)

public func prevIndexOf<T>(
  element : T,
  array : [T],
  fromExclusive : Nat,
  equal : (T, T) -> Bool
) : ?Nat

Returns the index of the previous occurence of element in the array starting from the from index (exclusive).

motoko include=import
import Char "mo:base/Char";
let array = ['c', 'o', 'f', 'f', 'e', 'e'];
assert Array.prevIndexOf<Char>('c', array, array.size(), Char.equal) == ?0;
assert Array.prevIndexOf<Char>('e', array, array.size(), Char.equal) == ?5;
assert Array.prevIndexOf<Char>('e', array, 5, Char.equal) == ?4;
assert Array.prevIndexOf<Char>('e', array, 4, Char.equal) == null;

Runtime: O(array.size()); Space: O(1);

public func range<T>(
  array : [T],
  fromInclusive : Int,
  toExclusive : Int
) : Iter.Iter<T>

Returns an iterator over a slice of the given array.

motoko include=import
let array = [1, 2, 3, 4, 5];
let s = Array.range<Nat>(array, 3, array.size());
assert s.next() == ?4;
assert s.next() == ?5;
assert s.next() == null;

let s = Array.range<Nat>(array, 0, 0);
assert s.next() == null;

Runtime: O(1)

Space: O(1)

public func toText<T>(array : [T], f : T -> Text) : Text

Converts the array to its textual representation using f to convert each element to Text.

motoko include=import
import Nat "mo:base/Nat";
let array = [1, 2, 3];
Array.toText<Nat>(array, Nat.toText) // => "[1, 2, 3]"

Runtime: O(size)

Space: O(size)

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

public func compare<T>(
  array1 : [T],
  array2 : [T],
  compare : (T, T) -> Order.Order
) : Order.Order

Compares two arrays using the provided comparison function for elements. Returns #less, #equal, or #greater if array1 is less than, equal to, or greater than array2 respectively.

If arrays have different sizes but all elements up to the shorter length are equal, the shorter array is considered #less than the longer array.

motoko include=import
import Nat "mo:base/Nat";
let array1 = [1, 2, 3];
let array2 = [1, 2, 4];
Array.compare<Nat>(array1, array2, Nat.compare) // => #less

let array3 = [1, 2];
let array4 = [1, 2, 3];
Array.compare<Nat>(array3, array4, Nat.compare) // => #less (shorter array)

Runtime: O(min(size1, size2))

Space: O(1)

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