英文:
why `1<<32 - 1` is 4294967295 in go?
问题
我在Go语言中发现了一个奇怪的结果，你能帮我看看吗？
fmt.Print(1<<32 - 1)
结果是4294967295。但是在C语言中：
printf("%lld", 0x1ll<<32 - 1);
结果是2147483648。
英文:
I found a weird result in Go, could you help me?
fmt.Print( 1<<32 - 1)
it's 4294967295
But in C
printf("%lld",0x1ll<<32 - 1);
it's 2147483648
答案1
得分: 17
在Go语言中，1<<32-1被解析为(1 << 32) - 1（通过gofmt插入的空格有助于提示）。而在C语言中，它被解析为1 << (32 - 1)。这是因为在C语言中，移位操作的优先级低于加法和减法，而在Go语言中则相反。
你可以从输出中看到这一点，在Go语言中结果是奇数，为4294967295，即2^32 - 1。而在C语言中结果是2147483648，即2^31。
Go语言的运算符优先级（来自https://golang.org/ref/spec#Operators）：
优先级 运算符
5 * / % << >> & &^
4 + - | ^
3 == != < <= > >=
2 &&
1 ||
C语言的运算符优先级：https://en.cppreference.com/w/c/language/operator_precedence
英文:
In go, 1<<32-1 parses as (1 << 32) - 1 (which gofmt helpfully hints at with the spaces it inserts). In C, it parses as 1 << (32 - 1). This is because in C, shifts have lower precedence than + and -, and in go, it's the other way round.
You can see this in the output, in go it's odd, and is 4294967295 which is 2^32 - 1. In C it's 2147483648 which is 2^31.
Go operator precendence (from https://golang.org/ref/spec#Operators)
Precedence Operator
5 * / % << >> & &^
4 + - | ^
3 == != < <= > >=
2 &&
1 ||
In C: https://en.cppreference.com/w/c/language/operator_precedence
答案2
得分: 4
C和Go之间的关键区别在于运算符优先级。
在C中，-的优先级比<<高，所以1 << 32 - 1等于1 << (32 - 1)，即1 << 31。
而在Go中，情况正好相反，所以1 << 32 - 1等于(1 << 32) - 1。
这更加令人困惑的是，当你搜索"go operator precedence"时，第一个谷歌搜索结果并不是我提供的规范链接，而是tutorialspoint.com上的教程（我不会提供链接），它完全错误地解释了这一点 - 实际上，它似乎直接从其C教程中抄袭了C运算符优先级部分，包括包含许多在Go中甚至不存在的运算符的优先级表。
[1] https://en.cppreference.com/w/c/language/operator_precedence
[2] https://golang.org/ref/spec#Operator_precedence
英文:
The key difference between C and Go here is operator precedence.
In C, - has higher precedence[1] than <<, so 1 << 32 - 1 is 1 << (32 - 1) i.e. 1 << 31.
In Go, it's the other way round[2], so 1 << 32 - 1 is (1 << 32) - 1.
This is made more confusing because the first Google result when you search for "go operator precedence" is not the spec I linked, but the tutorial at tutorialspoint.com (which I won't link), which gets this completely wrong - in fact, it appears to have copied the C operator precedence section verbatim from its C tutorial, including the precedence table, which contains lots of operators that do not even exist in Go.
[1] https://en.cppreference.com/w/c/language/operator_precedence
[2] https://golang.org/ref/spec#Operator_precedence
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库，让每个人都能够通过互相帮助和分享经验来进步。
评论