Home Backend Development PHP Tutorial php几个常见有关问题

php几个常见有关问题

Jun 13, 2016 pm 12:03 PM
arr array gt merge

php几个常见问题

一、php浮点数的一个常见问题

? ? ?首先我们先看一个例子:

<?php $f = 0.58;    var_dump(intval($f * 100)); //为啥输出57?>
Copy after login

? ? ?为什么输出的是57?这是php的bug吗?有很多人经常会问这样的问题,为了搞懂这个问题,那必须要知道浮点数的表示。

? ? ?浮点数以64位的长度(双精度)为例:会采用1位符号位(E),11指数位(Q),52位尾数(M)表示(一共64位)。

? ? ?①符号位:最高位表示数据的正负,0表示正数,1表示负数。

? ? ?②指数位:表示数据以2为底的冪,指数采用偏移码表示;

? ? ?③尾数:表示数据小数后的有效数字。

? ? ?这里的关键点就在于,小数在二进制的表示,关于小数如何用二进制表示,如下:

? ? ?1、我们先讲解一下整数部分如何用二进制表示,如下图:


?就是用整数除以2查看它的余数,直到为0位置。整数部分计算非常简单,那下面查看小数部分如何计算

? ? ?2、小数部分用二进制表述计算方法如下:

? ? ?例如:5.6。整数部分5的二进制为101

? ? ?小数部分0.6,有这样一个公式:*2 如果1 就为1,基数=基数-1。

? ? ?则:0.6*2=1.2>0 ?那么就是1 ?基数=1.2-1=0.2

? ? ? ? ? ?0.2*2=0.4

? ? ? ? ? ?0.4*2=0.8

? ? ? ? ? ?0.8*2=1.6>1 ? 那么就为1 ?基数为1.6-1=0.6

? ? ? ? ? ?... ...

? ? ? ? ??所以5.6可以表示为:101.1001...

? ? ?3、0.58对于二进制表示来说,是无限长的值如下:

0.58的二进制表示基本上(52位)是: 00101000111101011100001010001111010111000010100011110.57的二进制表示基本上(52位)是: 0010001111010111000010100011110101110000101000111101
Copy after login

? ? ?至于0.58 * 100的具体浮点数乘法,?我们不考虑那么细,就以模糊的心意来看:0.58*100=57.999999999。那么intval一下,自然就是57了。

? ? ?可见, 这个问题的关键点就是: “你看似有穷的小数, 在计算机的二进制表示里却是无穷的”。

二、php数组合并

? ? 一提到数组合并,我们会想到array_merge函数或者array_merge_recursive函数,其实还有一种方式,这种方式大家平时很少想到,就是“+”运算符。我们分别看一下他们的用法:

? ? 1、array_merge在数组的下标为整数类型时:

$arr1 = array(1=>1,2=>2,3=>3);$arr2 = array(1=>2,2=>3,3=>4);$arr3 = array_merge($arr1,$arr2);print_r($arr3);//输出为Array ( [0] => 1 [1] => 2 [2] => 3 [3] => 2 [4] => 3 [5] => 4 ) 
Copy after login
? ?我们可以看到原有数组的下标丢弃,然后重新按照原来数组的顺序生成了一个新的合并后的数组。

? ? 2、array_merge在数组的下标为字符串类型时:

$arr1 = array('aa'=>1,'bb'=>2,'cc'=>3);$arr2 = array('bb'=>'b','cc'=>'c','dd'=>4);$arr3 = array_merge($arr1,$arr2);print_r($arr3);//输出为Array ( [aa] => 1 [bb] => b [cc] => c [dd] => 4 ) 
Copy after login
? ? 我们可以看到数组的下标保留,但是如果原数组中存在下标一致的情况,后面数组的值就覆盖前面数组的值。

? ?3、array_merge_recursive在数组的下标为整数类型时:

$arr1 = array(1=>1,2=>2,3=>3);$arr2 = array(1=>2,2=>3,3=>4);$arr3 = array_merge_recursive($arr1,$arr2);print_r($arr3);//输出为Array ( [0] => 1 [1] => 2 [2] => 3 [3] => 2 [4] => 3 [5] => 4 ) 
Copy after login

? ?发现效果与array_merge相同。

? ?4、array_merge_recursive在数组的下标为字符串类型时:

$arr1 = array('aa'=>1,'bb'=>2,'cc'=>3);$arr2 = array('bb'=>'b','cc'=>'c','dd'=>4);$arr3 = array_merge_recursive($arr1,$arr2);print_r($arr3);//输出为Array ( [aa] => 1 [bb] => Array ( [0] => 2 [1] => b ) [cc] => Array ( [0] => 3 [1] => c ) [dd] => 4 ) 
Copy after login

? ?我们发现下标保留,而且如果原数组有下标一致的情况,也不会出现数值覆盖的情况。而是生成一个数组存放这些值。

? ?5、当采用“+”运算符时:

$arr1 = array(1=>1,2=>2,3=>3);$arr2 = array(1=>2,2=>3,3=>4);$arr3 = $arr1+$arr2;print_r($arr3);//输出为Array ( [1] => 1 [2] => 2 [3] => 3 ) 
Copy after login
$arr1 = array('aa'=>1,'bb'=>2,'cc'=>3);$arr2 = array('bb'=>'b','cc'=>'c','dd'=>4);$arr3 = $arr1+$arr2;print_r($arr3);//输出为Array ( [aa] => 1 [bb] => 2 [cc] => 3 [dd] => 4 ) 
Copy after login

? ?这个我们就可以知道,不论下标类型为数字还是字符串,如果原数组存在下标一致的情况,那么就不在存储后面数组的数据。其实“+”的作用就是补全的功能,将前面数组中下标还未出现的数据由后面数组补全到这里面。

Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn

Hot AI Tools

Undresser.AI Undress

Undresser.AI Undress

AI-powered app for creating realistic nude photos

AI Clothes Remover

AI Clothes Remover

Online AI tool for removing clothes from photos.

Undress AI Tool

Undress AI Tool

Undress images for free

Clothoff.io

Clothoff.io

AI clothes remover

AI Hentai Generator

AI Hentai Generator

Generate AI Hentai for free.

Hot Article

R.E.P.O. Energy Crystals Explained and What They Do (Yellow Crystal)
2 weeks ago By 尊渡假赌尊渡假赌尊渡假赌
Repo: How To Revive Teammates
4 weeks ago By 尊渡假赌尊渡假赌尊渡假赌
Hello Kitty Island Adventure: How To Get Giant Seeds
4 weeks ago By 尊渡假赌尊渡假赌尊渡假赌

Hot Tools

Notepad++7.3.1

Notepad++7.3.1

Easy-to-use and free code editor

SublimeText3 Chinese version

SublimeText3 Chinese version

Chinese version, very easy to use

Zend Studio 13.0.1

Zend Studio 13.0.1

Powerful PHP integrated development environment

Dreamweaver CS6

Dreamweaver CS6

Visual web development tools

SublimeText3 Mac version

SublimeText3 Mac version

God-level code editing software (SublimeText3)

What are the differences between Huawei GT3 Pro and GT4? What are the differences between Huawei GT3 Pro and GT4? Dec 29, 2023 pm 02:27 PM

Many users will choose the Huawei brand when choosing smart watches. Among them, Huawei GT3pro and GT4 are very popular choices. Many users are curious about the difference between Huawei GT3pro and GT4. Let’s introduce the two to you. . What are the differences between Huawei GT3pro and GT4? 1. Appearance GT4: 46mm and 41mm, the material is glass mirror + stainless steel body + high-resolution fiber back shell. GT3pro: 46.6mm and 42.9mm, the material is sapphire glass + titanium body/ceramic body + ceramic back shell 2. Healthy GT4: Using the latest Huawei Truseen5.5+ algorithm, the results will be more accurate. GT3pro: Added ECG electrocardiogram and blood vessel and safety

Fix: Snipping tool not working in Windows 11 Fix: Snipping tool not working in Windows 11 Aug 24, 2023 am 09:48 AM

Why Snipping Tool Not Working on Windows 11 Understanding the root cause of the problem can help find the right solution. Here are the top reasons why the Snipping Tool might not be working properly: Focus Assistant is On: This prevents the Snipping Tool from opening. Corrupted application: If the snipping tool crashes on launch, it might be corrupted. Outdated graphics drivers: Incompatible drivers may interfere with the snipping tool. Interference from other applications: Other running applications may conflict with the Snipping Tool. Certificate has expired: An error during the upgrade process may cause this issu simple solution. These are suitable for most users and do not require any special technical knowledge. 1. Update Windows and Microsoft Store apps

How to Fix Can't Connect to App Store Error on iPhone How to Fix Can't Connect to App Store Error on iPhone Jul 29, 2023 am 08:22 AM

Part 1: Initial Troubleshooting Steps Checking Apple’s System Status: Before delving into complex solutions, let’s start with the basics. The problem may not lie with your device; Apple's servers may be down. Visit Apple's System Status page to see if the AppStore is working properly. If there's a problem, all you can do is wait for Apple to fix it. Check your internet connection: Make sure you have a stable internet connection as the "Unable to connect to AppStore" issue can sometimes be attributed to a poor connection. Try switching between Wi-Fi and mobile data or resetting network settings (General > Reset > Reset Network Settings > Settings). Update your iOS version:

Sort array using Array.Sort function in C# Sort array using Array.Sort function in C# Nov 18, 2023 am 10:37 AM

Title: Example of using the Array.Sort function to sort an array in C# Text: In C#, array is a commonly used data structure, and it is often necessary to sort the array. C# provides the Array class, which has the Sort method to conveniently sort arrays. This article will demonstrate how to use the Array.Sort function in C# to sort an array and provide specific code examples. First, we need to understand the basic usage of the Array.Sort function. Array.So

php提交表单通过后,弹出的对话框怎样在当前页弹出,该如何解决 php提交表单通过后,弹出的对话框怎样在当前页弹出,该如何解决 Jun 13, 2016 am 10:23 AM

php提交表单通过后,弹出的对话框怎样在当前页弹出php提交表单通过后,弹出的对话框怎样在当前页弹出而不是在空白页弹出?想实现这样的效果:而不是空白页弹出:------解决方案--------------------如果你的验证用PHP在后端,那么就用Ajax;仅供参考:HTML code

Simple and clear method to use PHP array_merge_recursive() function Simple and clear method to use PHP array_merge_recursive() function Jun 27, 2023 pm 01:48 PM

When programming in PHP, we often need to merge arrays. PHP provides the array_merge() function to complete array merging, but when the same key exists in the array, this function will overwrite the original value. In order to solve this problem, PHP also provides an array_merge_recursive() function in the language, which can merge arrays and retain the values ​​of the same keys, making the program design more flexible. array_merge

How to use the array_combine function in PHP to combine two arrays into an associative array How to use the array_combine function in PHP to combine two arrays into an associative array Jun 26, 2023 pm 01:41 PM

In PHP, there are many powerful array functions that can make array operations more convenient and faster. When we need to combine two arrays into an associative array, we can use PHP's array_combine function to achieve this operation. This function is actually used to combine the keys of one array as the values ​​of another array into a new associative array. Next, we will explain how to use the array_combine function in PHP to combine two arrays into an associative array. Learn about array_comb

Is watch4pro better or gt? Is watch4pro better or gt? Sep 26, 2023 pm 02:45 PM

Watch4pro and gt each have different features and applicable scenarios. If you focus on comprehensive functions, high performance and stylish appearance, and are willing to bear a higher price, then Watch 4 Pro may be more suitable. If you don’t have high functional requirements and pay more attention to battery life and reasonable price, then the GT series may be more suitable. The final choice should be decided based on personal needs, budget and preferences. It is recommended to carefully consider your own needs before purchasing and refer to the reviews and comparisons of various products to make a more informed choice.

See all articles