c++ - 代码中的＆符号，它有什么作用

Question

我在网上找到了我想理解的这段代码。但是，谷歌搜索并没有在以下代码中找到任何关于 & 符号含义的结果

return ( 1.0 - ( (x * (x * x * 15731 + 789221) + 1376312589) & 7fffffff) / 1073741824.0); 

我从以下页面得到它：http: //freespace.virgin.net/hugo.elias/models/m_perlin.htm

是的，有人指出它不是真正的 perlin，但我不在乎，我现在想知道基础知识。

问候

Answer 1

The ampersand is a bitwise AND. It means that you're comparing on the bit level. For each bit, the resulting bit is 1 if and only if the 2 incoming bits are 1.

1 & 2 = 0

Because :

1 = 00000001

2 = 00000010

But

2 & 3 = 2

Because we have :

2 = 000000 1 0

3 = 000000 1 1

result = 000000 1 0

In your case, the bitwise AND is used to force a 0 on the first bit of the result (if the the result is in 32 bits, which is the case in your example), because :

7fffffff = (0111) (1111) (1111) etc...

So no matter what you "AND" it with, the result will start with a 0 and then be the unchanged.

Given that the result is a signed integer, the effect of putting the first bit to 0 is to ensure that the result is always positive.

This is due to the fact that in cpp, the first bit of a signed integer is used to set the sign. A 1 means the number is negative, a 0 means it is positive.

Answer 2

&是位运算and符。

0 & 0 == 0
1 & 0 == 0
0 & 1 == 0
1 & 1 == 1