英文:
sign extending in shift operators in rust
问题
"Bit shifting is always sign-extending on signed integer types and zero-extending on unsigned integer types"
我的问题是什么是"always sign-extending"？如果可能的话，提供一个例子。在Rust中，如何处理移位操作的溢出和下溢。
谢谢您提前为您的时间和帮助。
英文:
I am reading rust programming language using book Programming Rust 2nd edition. Here in context of right shift operators and left shift operators mentioned as below
> Bit shifting is always sign-extending on signed integer types and zero-extending on unsigned integer types
My question is what is "always sign-extending"? If possible, provide an example. How overflowing and underflowing is handled in shift operators in Rust.
Thanks in advance for your time and help.
答案1
得分: 6
有了这种填充方式，对于负数，`>> 1` 永远等于 `/ 2` 是正确的。或者在我们的例子中，右移 `2` 等同于除以 `4`。左移 (`<<`) 总是用 `0` 填充。这导致 `<< 1` 等同于 `* 2`，对于负数也没有特殊情况。有关整数在二进制中如何表示的更多信息，请阅读有关[二进制补码](https://en.wikipedia.org/wiki/Two%27s_complement)的内容。
英文:
Signed:64 (0b01000000) >> 2 = 16 (0b00010000) // filled with 0, because sign is positive-64 (0b11000000) >> 2 = -16 (0b11110000) // filled with 1, because sign is negativeUnsigned:64 (0b01000000) >> 2 = 16 (0b00010000) // filled with 0, because unsigned192 (0b11000000) >> 2 = 48 (0b00110000) // filled with 0, because unsigned
With that way of padding, it is always true that a >> 1 is equal to a / 2, even for negative numbers. Or in our example, a right shift of 2 is equal to a division through 4.
Left shifts (<<) are always padded with 0. This again causes << 1 to be equal to * 2, no special case for negative numbers is needed here.
For more information about how integers are represented in binary, read about the two's complement.
Source:
fn main() {println!("Signed:");println!(" {} ({:#010b}) >> 2 = {} ({:#010b}) // filled with 0, because sign is positive",64i8,64i8,64i8 >> 2,64i8 >> 2);println!(" {} ({:#010b}) >> 2 = {} ({:#010b}) // filled with 1, because sign is negative",-64i8,-64i8,-64i8 >> 2,-64i8 >> 2);println!();println!("Unsigned:");println!(" {} ({:#010b}) >> 2 = {} ({:#010b}) // filled with 0, because unsigned",64u8,64u8,64u8 >> 2,64u8 >> 2);println!(" {} ({:#010b}) >> 2 = {} ({:#010b}) // filled with 0, because unsigned",(-64i8) as u8,(-64i8) as u8,(-64i8) as u8 >> 2,(-64i8) as u8 >> 2);}
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