In PHP, sometimes we need to process a very large array, such as finding the median. But for very large arrays, using traditional sorting methods will be very time-consuming and memory-consuming. So, is there a more efficient way to find the median of a very large array? This article will introduce an efficient solution method based on the fast selection algorithm.
- Introduction to Quick Selection Algorithm
The quick selection algorithm is an improved algorithm based on the quick sort algorithm. Its main idea is to find items in an unordered array through rapid division. The kth smallest element. Its time complexity is O(n), which is more efficient than the time complexity of conventional sorting algorithm O(n log n).
The basic steps of the quick selection algorithm are as follows:
- Select a pivot element pivot (usually the first element in the array);
- Put the elements in the array The elements are divided into two parts: smaller than pivot and larger than pivot;
- If the number of elements smaller than pivot is less than k, continue to search for the k-th element among elements larger than pivot;
- If less than If the number of pivot elements is greater than or equal to k, then continue to search for the k-th element among elements smaller than pivot;
- Repeat the above steps until the k-th element is found.
- Find the median of a very large array
We now consider how to use the fast selection algorithm to find the median of a very large array. Suppose we have a very large array $nums$ and need to find its median. We first perform a quick division on $nums$, dividing the elements into two parts smaller than pivot and larger than pivot. If the pivot is exactly in the middle of the array, then it is the median. Otherwise, based on the location of the pivot, we can judge that the search should continue on the side where the pivot is located.
The following are the detailed algorithm steps:
- First, we need to determine the position of the median mid. If the number of elements $n$ in the array is an odd number, the median is $nums[(n-1)/2]$; if the number of elements $n$ is an even number, the median is $(nums[n /2-1] nums[n/2])/2$.
- Quickly divide the array $nums$ and record the pivot position $pos$.
- Based on the positional relationship between pos and mid, determine whether the median is on the left or right side of the pivot. If $pos=mid$, then the median is between positions $[0,pos-1]$ between.
- Repeat steps 2 and 3 until the median is found.
The following is the corresponding PHP code implementation:
function quickSelect($nums, $k) { $n = count($nums); $left = 0; $right = $n - 1; $mid = ($n - 1) / 2; while (true) { $pos = partition($nums, $left, $right); if ($pos == $mid) { if ($n % 2 == 0) { // 偶数个元素 return ($nums[$pos] + $nums[$pos + 1]) / 2; } else { // 奇数个元素 return $nums[$pos]; } } elseif ($pos = $pivot) { $j--; } while ($i <ol start="3"><li>Summary</li></ol><p>For very large arrays, using the traditional sorting method will bring very High time and space complexity. By using the fast selection algorithm to find the kth smallest element, we can complete the operation within O(n) time complexity, thereby achieving an efficient solution to the median of a very large array. It should be noted that in actual use, we also need to perform appropriate optimization according to different needs, such as pruning, etc. </p>
The above is the detailed content of How to find the median of a very large array in php. For more information, please follow other related articles on the PHP Chinese website!

The article compares ACID and BASE database models, detailing their characteristics and appropriate use cases. ACID prioritizes data integrity and consistency, suitable for financial and e-commerce applications, while BASE focuses on availability and

The article discusses securing PHP file uploads to prevent vulnerabilities like code injection. It focuses on file type validation, secure storage, and error handling to enhance application security.

Article discusses best practices for PHP input validation to enhance security, focusing on techniques like using built-in functions, whitelist approach, and server-side validation.

The article discusses strategies for implementing API rate limiting in PHP, including algorithms like Token Bucket and Leaky Bucket, and using libraries like symfony/rate-limiter. It also covers monitoring, dynamically adjusting rate limits, and hand

The article discusses the benefits of using password_hash and password_verify in PHP for securing passwords. The main argument is that these functions enhance password protection through automatic salt generation, strong hashing algorithms, and secur

The article discusses OWASP Top 10 vulnerabilities in PHP and mitigation strategies. Key issues include injection, broken authentication, and XSS, with recommended tools for monitoring and securing PHP applications.

The article discusses strategies to prevent XSS attacks in PHP, focusing on input sanitization, output encoding, and using security-enhancing libraries and frameworks.

The article discusses the use of interfaces and abstract classes in PHP, focusing on when to use each. Interfaces define a contract without implementation, suitable for unrelated classes and multiple inheritance. Abstract classes provide common funct


Hot AI Tools

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Undress AI Tool
Undress images for free

Clothoff.io
AI clothes remover

AI Hentai Generator
Generate AI Hentai for free.

Hot Article

Hot Tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Safe Exam Browser
Safe Exam Browser is a secure browser environment for taking online exams securely. This software turns any computer into a secure workstation. It controls access to any utility and prevents students from using unauthorized resources.

MantisBT
Mantis is an easy-to-deploy web-based defect tracking tool designed to aid in product defect tracking. It requires PHP, MySQL and a web server. Check out our demo and hosting services.

SecLists
SecLists is the ultimate security tester's companion. It is a collection of various types of lists that are frequently used during security assessments, all in one place. SecLists helps make security testing more efficient and productive by conveniently providing all the lists a security tester might need. List types include usernames, passwords, URLs, fuzzing payloads, sensitive data patterns, web shells, and more. The tester can simply pull this repository onto a new test machine and he will have access to every type of list he needs.

ZendStudio 13.5.1 Mac
Powerful PHP integrated development environment