Question

On a quest to expand my programming prowess, I’ve delved ever-so-slightly into The Standard PHP Library. This led to my discovery of the SplDoublyLinkedList class. From there I read the descriptions of Linked Lists and Doubly Linked Lists on Wikipedia.

I understand how they work… But I cannot conceive of a reason WHY we need it—or better yet a practical example of SplDoublyLinkedList since we have indexed and associative arrays in PHP.

How are Linked Lists normally used in-and-out of PHP?

Answer 1

The SPL data structures reduce memory consumption and improve performance. Good explanations:

Data structures are inherently language-independent and exist as a set of logical concepts based in mathematics. These containers use different algorithms as appropriate to maximize efficiency.

For example, if you don’t need the hash map capabilities of an associative array — that is, if you aren’t using the array key for a specific purpose and only need an enumerated array — SplFixedArray (formerly SplFastArray, currently undocumented) may be a suitable replacement. The only caveat is that the size of the array is fixed, meaning that you must specify the size when you instantiate the class and an error will occur if you attempt to store more than that number of elements. This is the reason that, on average, it performs better than standard PHP arrays.

http://web.archive.org/web/20130805120049/http://blueparabola.com/blog/spl-deserves-some-reiteration

Within the C code that makes up the PHP interpreter, arrays are implemented as a data structure called a hash table or hash map. When a value contained within an array is referenced by its index, PHP uses a hashing function to convert that index into a unique hash representing the location of the corresponding value within the array.

This hash map implementation enables arrays to store an arbitrary number of elements and provide access to all of those elements simultaneously using either numeric or string keys. Arrays are extremely fast for the capabilities they provide and are an excellent general purpose data structure.

In computer science, a list is defined as an ordered collection of values. A linked list is a data structure in which each element in the list includes a reference to one or both of the elements on either side of it within the list. The term “doubly-linked list” is used to refer to the latter case. In the SPL, this takes the form of the class SplDoublyLinkedList…. It makes sense to use lists when the number of elements to be stored is not known in advance and the elements only need to be accessed by sequential position.

http://matthewturland.com/2010/05/20/new-spl-features-in-php-5-3/