英文:
uint(0) - uint(1) output maxValue of uint64
问题
有人能告诉我为什么输出不是-1而是18446744073709551615吗？
package main
import (
"fmt"
)
func main() {
a := uint(0)
b := uint(1)
fmt.Println(a - b)
}
输出结果为
18446744073709551615
目前，我的理解如下。
在计算机中，减法会被转换为加法。而b是负数，所以在加法中会使用a(0)和b(-1)的二进制补码。
所以计算过程如下：
00000000 00000000 00000000 00000001 <- 0的二进制补码
11111111 11111111 11111111 11111110 <- -1的二进制补码
11111111 11111111 11111111 11111111 <- (结果：0 + -1)的二进制补码
通常情况下，如果a和b是int类型，二进制补码会自动转换为二进制结果(10000000 00000000 00000000 00000001)，即-1。但是a和b是uint类型，所以不会进行二进制转换，它会将11111111 11111111 11111111 11111111(二进制补码)作为普通的二进制结果并输出。
这个解释正确吗？我的理解有什么遗漏吗？
另一个问题是，11111111 11111111 11111111 11111111应该是2的64次方减1，即18446744073709552000。
为什么我的输出只有18446744073709551615？
英文:
Can anybody tell me why the output is not -1 but 18446744073709551615?
package main
import (
"fmt"
)
func main() {
a := uint(0)
b := uint(1)
fmt.Println(a - b)
}
output
18446744073709551615
currently, my understanding is the following.
subtraction in pc will be converted into addition. And b is minus, so the 2's complement of a(0) and b(-1) will be used in addition.
so the calculation is like
00000000 00000000 00000000 00000001 <- 2's complement of 0
11111111 11111111 11111111 11111110 <- 2's complement of -1
11111111 11111111 11111111 11111111 <- 2's complement of (result: 0 + -1)
Usually, if a and b is a type of int, 2's complement will be converted to binary result(10000000 00000000 00000000 00000001) which is -1 automatically.
But a and b is the type of unit, so the convert into binary will not be taken, it will treat the result of 11111111 11111111 11111111 11111111(2's complement) as a normal binary result and output it.
← Does this explanation correct?
Does my understanding miss something?
Another question is that
11111111 11111111 11111111 11111111 should be pow(2,64) - 1 which is 18446744073709552000
Why my output is only 18446744073709551615?
答案1
得分: 1
uint64是所有无符号64位整数的集合。范围：0到18446744073709551615。
英文:
> uint64 is the set of all unsigned 64-bit integers. Range: 0 through 18446744073709551615.
答案2
得分: 1
只回答“另一个问题”：
2的64次方减1等于18446744073709551615。
如果你得到的是18446744073709552000，我猜测你可能是从一个使用十进制浮点表示法的计算器上得到的，而且精度不足以准确表示它。
（标准的二进制浮点格式会将18446744073709551616作为最接近的可表示答案。）
另外：2的64次方减1不可能等于18446744073709552000，因为显而易见，2的幂次方是偶数，所以2的N次方减1必定是奇数。
英文:
And just to answer the "another question":
> ... should be pow(2,64) - 1 which is 18446744073709552000
> Why my output is only 18446744073709551615?
2<sup>64</sup> - 1 is 18446744073709551615.
If you're getting 18446744073709552000, my guess would be you're getting it from a calculator that uses a decimal floating-point representation and not enough digits of precision to represent it exactly.
(Standard binary floating point formats would have 18446744073709551616 as the nearest representable answer.)
Aside: 2<sup>64</sup> - 1 could not possibly be equal to 18446744073709552000, because powers of two are even numbers for obvious reasons, so 2<sup>N</sup> - 1 must be odd.
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