Watering the trees

An introduction to data structures

Ibrahim AshShohail
@ibrasho

Ibrahim AshShohail

  • Computer Science Graduate
  • Senior Developer at Tamkeen Technologies
  • Certified Zend Developer
  • +8 years of PHP experience
  • Open Source advocate, contributer to Laravel

What is a data structure?

  • Organization of data
  •  

Big O notation O( )

  • Describe the complexity of an algorithm
  • Time and space
  • Relative to the input size
  • For O(1), size does not effect the algorithm
     
  • For O(log n), size effects the algorithm logarithmically
     
  • For O(n), size effects the algorithm proportionally
     
  • For, O(n^2), size effects the algorithm exponentionally

Examples

$array = [1132, 2512, 4734, 5845, 7453];


// How long will this take?
$array[2];

function getIndex($array, $val) {
    foreach ($array as $i => $value) {
        if ($value == $val) {
            return $i;
        }
    }
}

// What about this?
getIndex($array, rand(1, 100000);


$array = range(1, 1000);

What if:

PHP Arrays!!!

An array in PHP is actually an ordered map. A map is a type that associates values to keys. This type is optimized for several different uses; it can be treated as an array, list (vector), hash table (an implementation of a map), dictionary, collection, stack, queue, and probably more. As array values can be other arrays, trees and multidimensional arrays are also possible.

http://php.net/manual/en/language.types.array.php

Trying to be a lot of things in the same time can be UGLY

Or COOL

PHP Arrays

  • Large waste of memory
  • 14,649,072 bytes
    = 13.97 MBs

SplFixedArray

  • Better for real arrays

  • 5,600,648 bytes
    =     5.34 MBs

$array = range(1, 1000000);
$array = new SplFixedArray(100000);
for ($i = 0; $i < 100000; ++$i) {
    $array[$i] = $i;
}

http://nikic.github.io/2011/12/12/How-big-are-PHP-arrays-really-Hint-BIG.html

Using an Array

  • Memory: 81.87 MBs
  • Time: 0.093 ms

Using a Class

  • Memory: 49.37 MBs
  • Time: 0.238 ms
$array[] = [
    'username' => 'username',
    'password' => 'password',
    'email' => 'e@mail.com',
    'age' => 25,
]
$array[] = new User(
    'username',
    'password',
    'e@mail.com',
    25
);

Creating 100,000 Users

Classes are 2.5 times slower than arrays

What are true arrays?

  • Fixed-size integer-indexed collections

0

1

3

2

When to use arrays?

  • Fixed size collection
  • Maintain order
  • Not linked
  • Memory efficient

SplFixedArray

class SplFixedArray implements Iterator , ArrayAccess , Countable {
	public __construct ([ int $size = 0 ] )
	public count() : int
	public current() : mixed
	public fromArray(array $array [, bool $save_indexes = true ] ) : static
	public getSize() : int
	public key() : int
	public next() : void
	public rewind() : void
	public setSize(int $size ) : int
	public toArray() : array
	public valid() : bool

}

Lists

Types of Lists

  • Singly-linked lists

 

 

 

  • Doubly-linked lists

SplDoublyLinkedList

class SplDoublyLinkedList implements Iterator , ArrayAccess , Countable {
    public __construct()
    public add(mixed $index , mixed $newval) : void
    public top() : mixed
    public bottom() : mixed
    public count() : int
    public current() : mixed
    public isEmpty() : bool
    public key() : mixed
    public next() : void
    public pop() : mixed
    public prev() : void
    public push(mixed $value) : void
    public rewind() : void
    public shift() : mixed
    public unshift(mixed $value) : void
    public valid() : bool
    public serialize() : string
    public unserialize(string $serialized) : void
}

Queues

Queue Representation

HEAD

TAIL

Dequeue

Enqueue

HEAD

TAIL

When to use a Queue?

  • FIFO
  • Sequential Processing
  • Size doesn't matter
  • Messaging
  • Priority Queues?
<?php

interface Queue
{
    // Enqueue an item
    public enqueue($item);

    // Dequeue the first item
    public dequeue();















}
    // (Optional) What is the first item
    public first();

    // (Optional) What is the last item
    public last();

    // (Optional) How can we know the number of elements?
    public count();

    // (Optional) What's the O() of this method?
    public contains();

SplQueue

http://php.net/manual/en/class.splqueue.php

SplQueue extends SplDoublyLinkedList implements Iterator , ArrayAccess , Countable {
    public __construct()
    public dequeue() : mixed
    public enqueue(mixed $value) : void
    public SplDoublyLinkedList::isEmpty() : bool
    public SplDoublyLinkedList::count() : int
    public SplDoublyLinkedList::add(mixed $index , mixed $newval) : void
    public SplDoublyLinkedList::push(mixed $value) : void
    public SplDoublyLinkedList::pop() : mixed
    public SplDoublyLinkedList::shift() : mixed
    public SplDoublyLinkedList::unshift(mixed $value) : void
    public SplDoublyLinkedList::top() : mixed
    public SplDoublyLinkedList::bottom() : mixed
    public SplDoublyLinkedList::current() : mixed
    public SplDoublyLinkedList::key() : mixed
    public SplDoublyLinkedList::next() : void
    public SplDoublyLinkedList::prev() : void
    public SplDoublyLinkedList::rewind() : void
    public SplDoublyLinkedList::valid() : bool
}

Stack

Stack Representation

TOP

POP

PUSH

TOP

TOP

Stack Overflow ?

When to use a Stack?

  • LIFO
  • Keeping record of progress
  • Syntax parsing (parenthesis)
<?php

interface Stack
{
    // Pop an item
    public pop();

    // Push an item to the stack
    public push($item);















}
    // (Optional) What is the first item
    public top();

    // (Optional) What is the last item
    public bottom();

    // (Optional) How can we know the if the stack is empty?
    public isEmpty();

    // (Optional) What's the O() of this method?
    public contains();

SplStack

http://php.net/manual/en/class.splstack.php

SplQueue extends SplDoublyLinkedList implements Iterator , ArrayAccess , Countable {
    public __construct()
    public SplDoublyLinkedList::isEmpty() : bool
    public SplDoublyLinkedList::count() : int
    public SplDoublyLinkedList::add(mixed $index , mixed $newval) : void
    public SplDoublyLinkedList::push(mixed $value) : void
    public SplDoublyLinkedList::pop() : mixed
    public SplDoublyLinkedList::shift() : mixed
    public SplDoublyLinkedList::unshift(mixed $value) : void
    public SplDoublyLinkedList::top() : mixed
    public SplDoublyLinkedList::bottom() : mixed
    public SplDoublyLinkedList::current() : mixed
    public SplDoublyLinkedList::key() : mixed
    public SplDoublyLinkedList::next() : void
    public SplDoublyLinkedList::prev() : void
    public SplDoublyLinkedList::rewind() : void
    public SplDoublyLinkedList::valid() : bool
}

Trees

Larry D. Moore CC BY-SA 3.0 http://en.wikipedia.org/wiki/The_Big_Tree,_Rockport#mediaviewer/File:Big_tree.jpg

Tree Representation

Types of trees

SplHeap

http://php.net/manual/en/class.splheap.php

http://php.net/manual/en/class.splmaxheap.php

http://php.net/manual/en/class.splminheap.php

abstract class SplHeap implements Iterator , Countable {
	public __construct()
	abstract protected int compare(mixed $value1 , mixed $value2)
	public count() :  int
	public current() :  mixed
	public extract() :  mixed
	public insert(mixed $value) :  void
	public isEmpty() :  bool
	public key() :  mixed
	public next() :  void
	public recoverFromCorruption() :  void
	public rewind() :  void
	public top() :  mixed
	public valid() :  bool
}

Traversal strategies

  • Pre-order
  • In-order
  • Post-order
  • Level-order

SPL alternatives?

Thank you

@ibrasho

ibra.sho@gmail.com

https://joind.in/13331

https://slides.com/ibrasho/watering-the-trees-confoo-2015

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