Set/HashSet/KeySet

Last Updated: 2024-01-13

Abstractions

C++

Standard Library:

• std::set: Similar to absl::btree_set, provided by the standard but significantly less performant.
• std::unordered_set: Similar to absl::flat_hash_set, provided by the standard but significantly less performant.

Abseil:

• absl::flat_hash_set: an unordered collection of unique values.
• absl::node_hash_set: Similar to absl::flat_hash_set, but uses separate allocations for the entries, thus providing pointer stability;
• absl::btree_set: an ordered collection of unique values.

JavaScript

Set

Rust

std::collections::HashSet, std::collections::BTreeSet

Create

Java

Init EnumSet

EnumSet.of(Style.BOLD, Style.ITALIC);

JavaScript

const foo = new Set();

Python

Create Set:

>>> a = [1, 2, 2, 3, 3, 3]
>>> set(a)
{1, 2, 3}

Create FrozenSet:

>>> a = [1, 2, 2, 3, 3, 3]
>>> frozenset(a)
frozenset({1, 2, 3})

Hack

$k = keyset['foo', 'bar'];

Create: From Existing

Hack

$k = keyset($container);

Add / Remove Element

JavaScript

fooSet.add(1);
fooSet.delete(1);

Python

New elements can be added to set:

>>> s1 = set([1,2,3])
>>> s1.add(4)
>>> s1
{1, 2, 3, 4}

but not to frozenset

>>> s2 = frozenset([1,2,3])
>>> s2.add(4)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'frozenset' object has no attribute 'add'

Hack

$k[] = 'baz';

unset($k['baz']);

Contains

C++

absl::flat_hash_set<std::string> n = {"a", "b", "c"};
n.contains("a");

JavaScript

fooSet.has(1);

Python

Use in

>>> 1 in set([1,2,3])
True
>>> 4 in set([1,2,3])
False
>>> 4 not in set([1,2,3])
True

Hack

C\contains_key($k, 'foo')

Count

Hack

C\count($k)

Type Signature

Hack

keyset<TKey>

Type Of

Hack

is_keyset($k)

Join As String

Python

>>> "".join(s)
'ba'

Union

Intersection

Java

Use the retainAll() method of Set:

Set<String> s1;
Set<String> s2;
s1.retainAll(s2); // s1 now contains only elements in both sets

Find duplicates

Python

import collections

print([col for col, count in collections.Counter(header_trim).items() if count > 1])

Remove Duplicates

Java

Collection<Type> noDups = new HashSet<Type>(c);
Collection<Type> noDups = new LinkedHashSet<Type>(c);

Set<String> s = new HashSet<String>();
for (String a : args)
    if (!s.add(a))
        System.out.println("Duplicate detected: " + a);

System.out.println(s.size() + " distinct words: " + s);

Convert From Set

JavaScript

const arr = [1, 2, 3];
const set = new Set(arr);

Convert To Other Collections

Hack

keyset to Vector

new Vector($k)

keyset to Map

new Map($k)

keyset to Set

new Set($k)

keyset to array

Arrays::fromKeyset($v)