Array

public func init<X>(size : Nat, initValue : X) : [var X]

Create a mutable array with size copies of the initial value.

motoko include=import
let array = Array.init<Nat>(4, 2);

| Runtime | Space | |-----------|-----------| | O(size) | O(size) |

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

Create 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);

| Runtime | Space | |-----------|-----------| | O(size) | O(size) |

public func tabulateVar<X>(size : Nat, generator : Nat -> X) : [var X]

Create a mutable array of size size. Each element at index i is created by applying generator to i.

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

| Runtime | Space | |-----------|-----------| | O(size) | O(size) |

public func freeze<X>(varArray : [var X]) : [X]

Transforms a mutable array into an immutable array.

motoko include=import

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

| Runtime | Space | |-----------|-----------| | O(size) | O(1) |

public func thaw<A>(array : [A]) : [var A]

Transforms an immutable array into a mutable array.

motoko include=import

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

| Runtime | Space | |-----------|-----------| | O(size) | O(1) |

public func equal<X>(
  array1 : [X],
  array2 : [X],
  equal : (X, X) -> 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 | Space | |-----------|-----------| | O(size1 + size2) | O(size1 + size2) |

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

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 | Space | |-----------|-----------| | O(size) | O(1) |

public func append<X>(array1 : [X], array2 : [X]) : [X]

Create a new array by appending the values of array1 and array2.

:::note Efficient appending

Array.append copies its arguments and runs in linear time. For better performance in loops, consider using Buffer and Buffer.append instead.

:::

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

| Runtime | Space | |-----------|-----------| | O(size1 + size2) | O(size1 + size2) |

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

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 | Space | |-----------|-----------| | O(size * log(size)) | O(size) |

public func sortInPlace<X>(array : [var X], compare : (X, X) -> Order.Order)

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

motoko include=import
import {compare} "mo:base/Nat";
let array = [var 4, 2, 6];
Array.sortInPlace(array, compare);
array

| Runtime | Space | |-----------|-----------| | O(size * log(size)) | O(size) |

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

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

motoko include=import
let array = [10, 11, 12];
Array.reverse(array)

| Runtime | Space | |-----------|-----------| | O(size) | O(1) |

public func map<X, Y>(array : [X], f : X -> Y) : [Y]

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 array = [0, 1, 2, 3];
Array.map<Nat, Nat>(array, func x = x * 3)

| Runtime | Space | |-----------|-----------| | O(size) | O(size) |

public func filter<X>(array : [X], predicate : X -> Bool) : [X]

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 | Space | |-----------|-----------| | O(size) | O(size) |

public func mapEntries<X, Y>(array : [X], f : (X, Nat) -> Y) : [Y]

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 | Space | |-----------|-----------| | O(size) | O(size) |

public func mapFilter<X, Y>(array : [X], f : X -> ?Y) : [Y]

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.mapFilter<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 | Space | |-----------|-----------| | O(size) | O(size) |

public func mapResult<X, Y, E>(array : [X], f : X -> Result.Result<Y, E>) : Result.Result<[Y], 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 | Space | |-----------|-----------| | O(size) | O(size) |

public func chain<X, Y>(array : [X], k : X -> [Y]) : [Y]

Creates a new array by applying k to each element in array, and concatenating the resulting arrays in order. This operation is similar to what in other functional languages is known as monadic bind.

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

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

| Runtime | Space | |-----------|-----------| | O(size) | O(size) |

public func foldLeft<X, A>(
  array : [X],
  base : A,
  combine : (A, X) -> 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 | Space | |-----------|-----------| | O(size) | O(1) |

public func foldRight<X, A>(
  array : [X],
  base : A,
  combine : (X, 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 | Space | |-----------|-----------| | O(size) | O(1) |

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

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

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

| Runtime | Space | |-----------|-----------| | O(n) | O(n) |

public func make<X>(element : X) : [X]

Create an array containing a single value.

motoko include=import
Array.make(2)

| Runtime | Space | |-----------|-----------| | O(1) | O(1) |

public func vals<X>(array : [X]) : I.Iter<X>

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

:::note Alternative approach

Alternatively, you can use array.size() to achieve the same result. See the example below. :::

motoko include=import
let array = [10, 11, 12];
var sum = 0;
for (element in array.vals()) {
 sum += element;
};
sum

| Runtime | Space | |-----------|-----------| | O(1) | O(1) |

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

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

:::note Alternative approach 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 | Space | |-----------|-----------| | O(1) | O(1) |

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

Returns the size of array.

:::note Alternative approach

Alternatively, you can use array.size() to achieve the same result. See the example below. :::

motoko include=import
let array = [10, 11, 12];
let size = Array.size(array);

| Runtime | Space | |-----------|-----------| | O(1) | O(1) |

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

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

:::note 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 | Space | |-----------|-----------| | O(length) | O(length) |

public func indexOf<X>(
  element : X,
  array : [X],
  equal : (X, X) -> 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 | Space | |-----------|-----------| | O(array.size()) | O(1) |

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

Returns the index of the next occurrence 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 | Space | |-----------|-----------| | O(array.size()) | O(1) |

public func lastIndexOf<X>(
  element : X,
  array : [X],
  equal : (X, X) -> 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 | Space | |-----------|-----------| | O(array.size()) | O(1) |

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

Returns the index of the previous occurrence 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 | Space | |-----------|-----------| | O(array.size()) | O(1) |

public func slice<X>(
  array : [X],
  fromInclusive : Nat,
  toExclusive : Nat
) : I.Iter<X>

Returns an iterator over a slice of the given array.

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

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

| Runtime | Space | |-----------|-----------| | O(1) | O(1) |

public func take<T>(array : [T], length : Int) : [T]

Returns a new subarray of given length from the beginning or end of the given array.

Returns the entire array if the length is greater than the size of the array.

motoko include=import
let array = [1, 2, 3, 4, 5];
assert Array.take(array, 2) == [1, 2];
assert Array.take(array, -2) == [4, 5];
assert Array.take(array, 10) == [1, 2, 3, 4, 5];
assert Array.take(array, -99) == [1, 2, 3, 4, 5];

| Runtime | Space | |-----------|-----------| | O(length) | O(length) |