计算php中两个负二进制数的和 7.1
Calculating sum of two negative binary numbers in php 7.1
我想了解如何在 PHP 7.1 中添加两个负数。所以，我在 stackeoverflow 中阅读了这些问题：
- When should I use a bitwise operator?
- PHP 用负数计算给出错误的值
我测试了这个脚本：
<?php
echo (~5) + (~7); // output: -14
但是我不明白为什么结果是-14。想手动解决，我是这样的：
~5 => (1011)
~7 => (1001)
(1011) + (1001) = 0100 => 8 != -14 the output of php script
哪里出错了？
阅读 php.net 中的几个示例后，我发现了一个非常好的演示：
The NOT or complement operator ( ~ ) and negative binary numbers can be confusing.
~2 = -3 because you use the formula ~x = -x - 1 The bitwise
complement of a decimal number is the negation of the number minus 1.
NOTE: just using 4 bits here for the examples below but in reality PHP
uses 32 bits.
Converting a negative decimal number (ie: -3) into binary takes 3
steps: 1) convert the positive version of the decimal number into
binary (ie: 3 = 0011) 2) flips the bits (ie: 0011 becomes 1100) 3) add
1 (ie: 1100 + 0001 = 1101)
You might be wondering how does 1101 = -3. Well PHP uses the method
"2's complement" to render negative binary numbers. If the left most
bit is a 1 then the binary number is negative and you flip the bits
and add 1. If it is 0 then it is positive and you don't have to do
anything. So 0010 would be a positive 2. If it is 1101, it is negative
and you flip the bits to get 0010. Add 1 and you get 0011 which equals
-3.
我想了解如何在 PHP 7.1 中添加两个负数。所以，我在 stackeoverflow 中阅读了这些问题：
- When should I use a bitwise operator?
- PHP 用负数计算给出错误的值
我测试了这个脚本：
<?php
echo (~5) + (~7); // output: -14
但是我不明白为什么结果是-14。想手动解决，我是这样的：
~5 => (1011)
~7 => (1001)
(1011) + (1001) = 0100 => 8 != -14 the output of php script
哪里出错了？
阅读 php.net 中的几个示例后，我发现了一个非常好的演示：
The NOT or complement operator ( ~ ) and negative binary numbers can be confusing.
~2 = -3 because you use the formula ~x = -x - 1 The bitwise complement of a decimal number is the negation of the number minus 1.
NOTE: just using 4 bits here for the examples below but in reality PHP uses 32 bits.
Converting a negative decimal number (ie: -3) into binary takes 3 steps: 1) convert the positive version of the decimal number into binary (ie: 3 = 0011) 2) flips the bits (ie: 0011 becomes 1100) 3) add 1 (ie: 1100 + 0001 = 1101)
You might be wondering how does 1101 = -3. Well PHP uses the method "2's complement" to render negative binary numbers. If the left most bit is a 1 then the binary number is negative and you flip the bits and add 1. If it is 0 then it is positive and you don't have to do anything. So 0010 would be a positive 2. If it is 1101, it is negative and you flip the bits to get 0010. Add 1 and you get 0011 which equals -3.