PHP manual中说:
以十进制能够精确表示的有理数如 0.1 或 0.7,无论有多少尾数都不能被内部所使用的二进制精确表示,因此不能在不丢失一点点精度的情况下转换为二进制的格式。
<code>// example1 $float = (0.1 + 0.7) * 10; echo (integer) $float; // 7 echo floor($float); // 7 // example2 echo (integer) (1.5+1.5); // 3 echo floor(1.5+1.5); // 3 // example3 echo (integer) (0.5*10); // 5 echo floor(0.5*10); // 5 </code>
为什么在例子2和例子3中,浮点数的加
和乘
运算能够保全精度呢?
回复内容:
PHP manual中说:
以十进制能够精确表示的有理数如 0.1 或 0.7,无论有多少尾数都不能被内部所使用的二进制精确表示,因此不能在不丢失一点点精度的情况下转换为二进制的格式。
<code>// example1 $float = (0.1 + 0.7) * 10; echo (integer) $float; // 7 echo floor($float); // 7 // example2 echo (integer) (1.5+1.5); // 3 echo floor(1.5+1.5); // 3 // example3 echo (integer) (0.5*10); // 5 echo floor(0.5*10); // 5 </code>
为什么在例子2和例子3中,浮点数的加
和乘
运算能够保全精度呢?
又是献上我博客的时候了:
- 代码之谜(四)- 浮点数(从惊讶到思考)
- 代码之谜(五)- 浮点数(谁偷了你的精度?)
0.1 + 0.7
的结果是 0.7999999999999999
0.5
1.5
可以用浮点数精确的表示。
0.1
的二进制:
<code>符号位 0 指数 01111011 (-4) 位数 1.10011001100110011001101 (1.60000002384185791015625) </code>
将这个数在转回十进制:0.10000000149011612
0.7
的二进制:
<code>符号位 0 指数 01111110 (-1) 位数 1.01100110011001100110011 (1.39999997615814208984375) </code>
将这个数在转回十进制:0.699999988079071

What’s still popular is the ease of use, flexibility and a strong ecosystem. 1) Ease of use and simple syntax make it the first choice for beginners. 2) Closely integrated with web development, excellent interaction with HTTP requests and database. 3) The huge ecosystem provides a wealth of tools and libraries. 4) Active community and open source nature adapts them to new needs and technology trends.

PHP and Python are both high-level programming languages that are widely used in web development, data processing and automation tasks. 1.PHP is often used to build dynamic websites and content management systems, while Python is often used to build web frameworks and data science. 2.PHP uses echo to output content, Python uses print. 3. Both support object-oriented programming, but the syntax and keywords are different. 4. PHP supports weak type conversion, while Python is more stringent. 5. PHP performance optimization includes using OPcache and asynchronous programming, while Python uses cProfile and asynchronous programming.

PHP is mainly procedural programming, but also supports object-oriented programming (OOP); Python supports a variety of paradigms, including OOP, functional and procedural programming. PHP is suitable for web development, and Python is suitable for a variety of applications such as data analysis and machine learning.

PHP originated in 1994 and was developed by RasmusLerdorf. It was originally used to track website visitors and gradually evolved into a server-side scripting language and was widely used in web development. Python was developed by Guidovan Rossum in the late 1980s and was first released in 1991. It emphasizes code readability and simplicity, and is suitable for scientific computing, data analysis and other fields.

PHP is suitable for web development and rapid prototyping, and Python is suitable for data science and machine learning. 1.PHP is used for dynamic web development, with simple syntax and suitable for rapid development. 2. Python has concise syntax, is suitable for multiple fields, and has a strong library ecosystem.

PHP remains important in the modernization process because it supports a large number of websites and applications and adapts to development needs through frameworks. 1.PHP7 improves performance and introduces new features. 2. Modern frameworks such as Laravel, Symfony and CodeIgniter simplify development and improve code quality. 3. Performance optimization and best practices further improve application efficiency.

PHPhassignificantlyimpactedwebdevelopmentandextendsbeyondit.1)ItpowersmajorplatformslikeWordPressandexcelsindatabaseinteractions.2)PHP'sadaptabilityallowsittoscaleforlargeapplicationsusingframeworkslikeLaravel.3)Beyondweb,PHPisusedincommand-linescrip

PHP type prompts to improve code quality and readability. 1) Scalar type tips: Since PHP7.0, basic data types are allowed to be specified in function parameters, such as int, float, etc. 2) Return type prompt: Ensure the consistency of the function return value type. 3) Union type prompt: Since PHP8.0, multiple types are allowed to be specified in function parameters or return values. 4) Nullable type prompt: Allows to include null values and handle functions that may return null values.


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