Random Access¶
Random access is the ability to read or write any element of a data structure, or any byte of a file, directly, in constant time, without having to traverse the elements that precede it. It is opposed to sequential access, where reaching the n-th element requires visiting every element before it first.
Arrays and hash maps typically offer random access: a PHP array, whether indexed or associative, is actually an ordered hash map under the hood, and supports O(1) average-time access to any key, be it $array[5] or $array['name']. Linked lists, by contrast, only offer sequential access: reaching the n-th node means walking the list from its head.
Random access also applies to files: fopen() combined with fseek() lets a program jump directly to any offset of a file and read or write from there, instead of reading it sequentially from the start. This is essential for large files, or for formats such as fixed-length record files, where the position of a given record can be computed directly.
<?php
$array = ['a', 'b', 'c', 'd'];
echo $array[2]; // random access: 'c', without reading 'a' and 'b' first
$handle = fopen('data.bin', 'rb');
fseek($handle, 100); // jump directly to byte 100
$chunk = fread($handle, 10);
?>
See also Random access.
Related : Array, [], Hash Table,
