Lookup for a Set should be O(1). This is true for sets and hashtables (eg maps) in any language.
The way this is possible is that sets store values differently than arrays.
In an array values are stored sequentially based on what their place is in the array and where that array is in memory, so to find your item you need to scan through the array sequentially to find your item (unless it's a sorted array, then you can use binary search at O(logn)).
Sets declare a block of memory, like an array, but instead of putting items in memory in sequence, like an array, they determine the index of the item to add by running the item through a hash function (essentially a function that takes in an object and returns an evenly distributed, very large random number), and then modulousing the result of that hash function by the size of the memory block they have.
So, when you call contains($needle, $mySetHaystack), php will take $needle, and feed it into a hashfunction, which will return a big number like 9283472378, then it takes the length of $mySetHaystack (let's say 31), and does 9283472378 % 31 = 28, so it checks the 28th index of $mySetHaystack to see if $needle is there. Everything in this list of operations is independent of the size of $mySetHaystack, hence the perf being O(1).
If a hash function returns the same value for two different items (a hash collision, which totally happens), or if the modulo of that value is the same, then an array of values is stored in the set at that index. Since sets don't allow duplicate values, this happens rarely and is negligible from a perf perspective.
You should check out the wikipedia page on hash tables (similar to sets), as there are lots of pictures that will make this concept easier to understand.
The Set class
(No version information available, might only be in Git)
소개
A Set is a sequence of unique values. This implementation uses the same hash table as Ds\Map, where values are used as keys and the mapped value is ignored.
Strengths
- Values can be any type, including objects.
- Supports array syntax (square brackets).
- Insertion order is preserved.
- Automatically frees allocated memory when its size drops low enough.
- add(), remove() and contains() are all O(1).
Weaknesses
- Doesn’t support push(), pop(), insert(), shift(), or unshift().
- get() is O(n) if there are deleted values in the buffer before the accessed index, O(1) otherwise.
클래스 개요
Ds\Set
implements
Ds\Collection
{
/* Constants */
/* 메소드 */
}예약 상수
Ds\Set::MIN_CAPACITY
Table of Contents
- Ds\Set::add — Adds values to the set.
- Ds\Set::allocate — Allocates enough memory for a required capacity.
- Ds\Set::capacity — Returns the current capacity.
- Ds\Set::clear — Removes all values.
- Ds\Set::__construct — Creates a new instance.
- Ds\Set::contains — Determines if the set contains all values.
- Ds\Set::copy — Returns a shallow copy of the set.
- Ds\Set::count — Returns the number of values in the set.
- Ds\Set::diff — Creates a new set using values that aren't in another set.
- Ds\Set::filter — Creates a new set using a callable to determine which values to include.
- Ds\Set::first — Returns the first value in the set.
- Ds\Set::get — Returns the value at a given index.
- Ds\Set::intersect — Creates a new set by intersecting values with another set.
- Ds\Set::isEmpty — Returns whether the set is empty
- Ds\Set::join — Joins all values together as a string.
- Ds\Set::jsonSerialize — Returns a representation that can be converted to JSON.
- Ds\Set::last — Returns the last value in the set.
- Ds\Set::merge — Returns the result of adding all given values to the set.
- Ds\Set::reduce — Reduces the set to a single value using a callback function.
- Ds\Set::remove — Removes all given values from the set.
- Ds\Set::reverse — Reverses the set in-place.
- Ds\Set::reversed — Returns a reversed copy.
- Ds\Set::slice — Returns a sub-set of a given range.
- Ds\Set::sort — Sorts the set in-place.
- Ds\Set::sorted — Returns a sorted copy.
- Ds\Set::sum — Returns the sum of all values in the set.
- Ds\Set::toArray — Converts the set to an array.
- Ds\Set::union — Creates a new set using values from the current instance and another set.
- Ds\Set::xor — Creates a new set using values in either the current instance or in another set, but not in both.
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User Contributed Notes 4 notes
Anonymous ¶
6 years ago
Sbastien ¶
2 years ago
One nice thing is the type-awarness. array_unique() lose values (whatever flag), while Ds\Set keep them.
<?php
$array = [true, false, null, '', 0, '0', 123, '123'];
var_dump(array_unique($array));
var_dump(new \Ds\Set($array));
/*
Gives :
array(4) {
[0]=> bool(true)
[1]=> bool(false)
[4]=> int(0)
[6]=> int(123)
}
object(Ds\Set)#1 (8) {
[0]=> bool(true)
[1]=> bool(false)
[2]=> NULL
[3]=> string(0) ""
[4]=> int(0)
[5]=> string(1) "0"
[6]=> int(123)
[7]=> string(3) "123"
}
*/
rtheunissen at php dot net ¶
6 years ago
Lookups by value are O(1), lookups by index are O(n) if there are deleted values before the index, O(1) otherwise.
