二进制补码和fmt.Printf [英] Two's complement and fmt.Printf

问题描述

因此，计算机使用二进制补码在内部表示带符号的整数.即-5表示为^ 5 +1 ="1111 1011".

So computers use Two's complement to internally represent signed integers. I.e., -5 is represented as ^5 + 1 = "1111 1011".

但是，尝试打印二进制表示形式，例如以下代码:

However, trying to print the binary representation, e.g. the following code:

var i int8 = -5
fmt.Printf("%b", i)

输出 -101 .不完全是我所期望的.格式是否不同?还是不使用二进制补码?

Outputs -101. Not quite what I'd expect. Is the formatting different or is it not using Two's complement after all?

有趣的是，转换为无符号int会导致正确"位模式:

Interestingly, converting to an unsigned int results in the "correct" bit pattern:

var u uint8 = uint(i)
fmt.Printf("%b", u)

输出为 11111011 -恰好是 -5 的2s补码.

Output is 11111011 - exactly the 2s complement of -5.

所以在我看来，该值实际上是在内部使用Two's补码的格式，但是格式是打印未签名的 5 并在-前面.

So it seems to me the value is internally the really using Two's complement, but the formatting is printing the unsigned 5 and prepending a -.

有人可以澄清吗?

推荐答案

我相信答案在于 fmt 模块如何格式化二进制数字，而不是内部格式.

I believe the answer lies in how the fmt module formats binary numbers, rather than the internal format.

如果您查看 fmt.integer ，该函数首先执行的操作之一就是将负有符号整数转换为正整数:

If you take a look at fmt.integer, one of the very first actions that the function does is to convert the negative signed integer to a positive one:

   165      negative := signedness == signed && a < 0
   166      if negative {
   167          a = -a
   168      }

然后有逻辑将-附加在输出的字符串之前

There's then logic to append - in front of the string that's output here.

IOW -101 实际上是将-二进制附加到 5 上.

IOW -101 really is - appended to 5 in binary.

注意: fmt.integer 是从pp.fmtInt64 调用的rel ="noreferrer"> print.go ，它本身是通过 pp.printArg 在同一函数中调用的.

Note: fmt.integer is called from pp.fmtInt64 in print.go, itself called from pp.printArg in the same function.

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