提取与位运算符位 [英] Extracting bits with bitwise operators

问题描述

我想学习如何使用位运算符在给定的输入，但我没有多少运气搞清楚如何使用它们。

I'm trying to learn how to use bitwise operators on a given input but am not having much luck figuring out how to use them.

让我们说我有这八位如下：

Let's say I have this following octet:

11(01)0000

我将如何提取括号之间的位？

How would I extract the bits between the braces?

推荐答案

您需要：

1. 创建只有那些你是哪里需要字节（你只需要编写二进制数并转换为十六进制例如把它的C程序中）合适的面具。在你的中的括号11（01）0000 是你的指示在何处放置的那些在你的面具。

1. create a suitable mask with ones only where are the bytes you need (you just need to write the number in binary and convert to e.g. hex to put it inside the C program). The parentheses in your 11(01)0000 are your indication to where to put the ones in your mask.

另外，创建由尽可能多的，你有兴趣的比特块（面具你的情况两个1，即 11 二进制，即 3 十进制）和左移它它移动到你需要的位置，它（向左移位运算符：＆LT;＆LT; ）。如果你的位窗口的位置，只在运行时已知此方法可能是有用的。

Alternatively, create a mask made of as many ones as the chunk of bits you are interested in (in your case two ones, i.e. 11 in binary, i.e. 3 in decimal) and left shift it to move it to the position where you need it (left shift operator: <<). This approach can be useful if the position of your "bit window" is known only at runtime.

执行你的电话号码和掩码之间的按位与操作（按位和操作是＆安培; ）。

Perform a bitwise-and operation between your number and the mask (the bitwise and operator is &).

按位，只留下为1，在这两个操作数是1位，所以效果是过滤与掩码位源编号：对应于那些在掩码只有位让流过吧，所有其他位保持为零。

The bitwise and only leaves as 1 the bits that are 1 in both the operands, so the effect is "filtering" the source number with the bits of the mask: only the bits that correspond to ones in the mask are let "flow through" it, all the other bits are left as zero.

现在你已经提取你感兴趣的位，但他们还是在数量内原来的位置。如果你想/需要它，那么你可以用鼠标右键将它们转移到它们右对齐。（使用右移位运算符：＆GT;＆GT; ）

Now you have extracted the bits of your interest, but they are still in their original position inside the number. If you want/need it, you can then right shift them to "align them to the right" (use the right shift operator: >>).

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