for和foreach那个效率高?原因是什么?
PHP中for和foreach效率高的是foreach
for循环遍历方法:
public function getForTime(){ $big_Array = range(0,1000000,1); /* for循环遍历数组示例 */ $start_For_Time = $this->microtime_float(); //$array_Count = count($big_Array); for ($i=0;$i<count($big_Array);$i++) { $i; } $end_For_Time = $this->microtime_float(); $for_Time = $end_For_Time - $start_For_Time; echo 'for循环遍历耗时:'.$for_Time.'<br>'; }
foreach循环遍历方法:
public function getForeachTime(){ $big_Array = range(0,1000000,1); /* foreach循环遍历数组示例 */ $start_Foreach_Time = $this->microtime_float(); foreach ($big_Array as $key=>$val) { $key; } $end_Foreach_Time = $this->microtime_float(); $foreach_Time = $end_Foreach_Time - $start_Foreach_Time; echo 'foreach循环遍历耗时:'.$foreach_Time; }
时间计算方法:
/** * 时间统计函数 */ private function microtime_float($time = null) { list($usec, $sec) = explode(' ', $time ? $time : microtime()); return ((float)$usec + (float)$sec); }
看一下两种方式耗时
/* * 输出结果:第一种情况:先count在for循环遍历耗时:0.028002023696899 秒 * foreach循环遍历耗时:0.003000020980835 秒 * 第二种情况:在for循环条件中做count遍历耗时:0.095005035400391 秒 * foreach循环遍历耗时:0.0040009021759033 秒 * */
从上面的测试中我们可以明显的得出两条结论:
1、for循环遍历的效率是低于foreach循环遍历
2、for循环在外部做count和在条件中做count相比较,第一种效率更高
效率高的原因是什么呢?在寻找这个答案之前我们先探讨原理分别是什么。
for 循环:
每次从$i开始,每次循环都需要判断$i是否小于count,这占用了很大一部分时间
小于继续,否则终止循环
foreach:
foreach 依赖 IEnumerable.
第一次 var a in GetList() 时 调用 GetEnumerator 返回第一个对象 并 赋给a,
以后每次再执行 var a in GetList() 的时候 调用 MoveNext.直到循环结束.
期间GetList()方法只执行一次.
从上面是分析我们明显可以得出结论:php 的foreach循环效率是大大高于for循环。也许有很大的一个原因是for 要进行很多次条件判断。所以以后能用foreach的地方就用foreach,可以提高1倍的效率。
BUT:事实真的是这样吗?有人会说这个例子已经很明显了啊,结论一目了然,难道还有其他的可能吗?
我觉得事实没这么简单,如果真的是这样,for循环存在的意义是什么呢?
既然foreach效率高于for这么多倍,就直接都用foreach不就行了吗?个人觉得我测试的这个例子有一定的局限性,并不能作为评估两个循环方式效率高低的绝对依据。
不过,对于我们phper来说,正常工作当中还是使用foreach循环遍历比较好,至于编译层是如何工作的没必要涉及太深,如果有兴趣可以深度研究一下。
更多相关知识,请访问 PHP中文网!!

In PHP, trait is suitable for situations where method reuse is required but not suitable for inheritance. 1) Trait allows multiplexing methods in classes to avoid multiple inheritance complexity. 2) When using trait, you need to pay attention to method conflicts, which can be resolved through the alternative and as keywords. 3) Overuse of trait should be avoided and its single responsibility should be maintained to optimize performance and improve code maintainability.

Dependency Injection Container (DIC) is a tool that manages and provides object dependencies for use in PHP projects. The main benefits of DIC include: 1. Decoupling, making components independent, and the code is easy to maintain and test; 2. Flexibility, easy to replace or modify dependencies; 3. Testability, convenient for injecting mock objects for unit testing.

SplFixedArray is a fixed-size array in PHP, suitable for scenarios where high performance and low memory usage are required. 1) It needs to specify the size when creating to avoid the overhead caused by dynamic adjustment. 2) Based on C language array, directly operates memory and fast access speed. 3) Suitable for large-scale data processing and memory-sensitive environments, but it needs to be used with caution because its size is fixed.

PHP handles file uploads through the $\_FILES variable. The methods to ensure security include: 1. Check upload errors, 2. Verify file type and size, 3. Prevent file overwriting, 4. Move files to a permanent storage location.

In JavaScript, you can use NullCoalescingOperator(??) and NullCoalescingAssignmentOperator(??=). 1.??Returns the first non-null or non-undefined operand. 2.??= Assign the variable to the value of the right operand, but only if the variable is null or undefined. These operators simplify code logic, improve readability and performance.

CSP is important because it can prevent XSS attacks and limit resource loading, improving website security. 1.CSP is part of HTTP response headers, limiting malicious behavior through strict policies. 2. The basic usage is to only allow loading resources from the same origin. 3. Advanced usage can set more fine-grained strategies, such as allowing specific domain names to load scripts and styles. 4. Use Content-Security-Policy-Report-Only header to debug and optimize CSP policies.

HTTP request methods include GET, POST, PUT and DELETE, which are used to obtain, submit, update and delete resources respectively. 1. The GET method is used to obtain resources and is suitable for read operations. 2. The POST method is used to submit data and is often used to create new resources. 3. The PUT method is used to update resources and is suitable for complete updates. 4. The DELETE method is used to delete resources and is suitable for deletion operations.

HTTPS is a protocol that adds a security layer on the basis of HTTP, which mainly protects user privacy and data security through encrypted data. Its working principles include TLS handshake, certificate verification and encrypted communication. When implementing HTTPS, you need to pay attention to certificate management, performance impact and mixed content issues.


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

Atom editor mac version download
The most popular open source editor

SAP NetWeaver Server Adapter for Eclipse
Integrate Eclipse with SAP NetWeaver application server.

Zend Studio 13.0.1
Powerful PHP integrated development environment

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.

SublimeText3 Chinese version
Chinese version, very easy to use