Set: keep unique values¶

2 min read

A Set<T> stores each distinct value once. Use it to remove duplicates or check membership.

Import: import system.collections; · Availability: desktop, browser, and Node.

import system;
import system.collections;

fun main() {
    let names: Set<string> = {
        "Ada", "Sam"
    };
    names.add("Ada");
    System.println(names.length);
    System.println(names.contains("Sam"));
}

Output: 2, then true on a new line. Adding an existing value does not add a second copy.

Add, check, and remove¶

add returns true only when the value was new, so it also tells you whether you have seen a value before. A for loop visits each value once, in no guaranteed order.

Member	Returns	Effect
`Set<T>(capacity = 8)`	`Set<T>`	Empty set sized for `capacity` values.
`Set(items)`	`Set<T>`	New set of the array's distinct values. Set literals work the same way.
`add(value)`	`bool`	Adds the value; `false` when it was already present.
`add_all(items)`	`void`	Adds every value of an array, ignoring duplicates.
`contains(value)`	`bool`	Whether the value is present.
`remove(value)`	`bool`	Removes the value; `false` when it was missing.
`clear()`	`void`	Removes every value and keeps storage.
`.length`	`int`	Number of values.
`.capacity`	`int`	Current table size. The set grows before the table is three-quarters full.
`is_empty()`	`bool`	`true` when the set has no values.
`reserve(additional)`	`Result<Unit, CapacityError>`	Makes room for `additional` more values; `Err` for a negative or unsupported count.
`to_array()`	`T[]`	New array of the values.
`to_string()`	`string`	Values as text in the form `{a, b}`.

For a Set<string>, contains also accepts a ReadOnlySpan<char>, so you can test text you have not turned into a string.

Compare sets¶

union(other) includes values from either set. intersection(other) keeps values in both. difference(other) keeps values only in this set. is_subset and is_disjoint answer relationship questions.

Member	Returns	Effect
`union(other)`	`Set<T>`	New set of values in either set.
`intersection(other)`	`Set<T>`	New set of values in both sets.
`difference(other)`	`Set<T>`	New set of values in this set but not `other`.
`is_subset(other)`	`bool`	Whether every value of this set is in `other`.
`is_disjoint(other)`	`bool`	Whether the sets share no values.

Sets also have the query helpers such as filter, map, and any.

Use a list if positions or duplicate values matter. See all Set signatures.