英文:
Invalid operation: shift of type float64
问题
我在使用Golang中的位移运算符<<时遇到了一个奇怪的问题。在我的最终代码中，位移值将是两个整数的绝对值。然而，Go语言包只为float64类型定义了Abs函数，所以我需要将参数转换为float64类型来使用它，然后再将结果转换回uint类型。
最后，这个值将被用作float64类型的参数，所以我在此之后将其转换回float64类型。
问题是返回值的转换似乎不像我预期的那样工作...
var test float64
// 以下所有行都按预期工作
test = float64(1 << 10)
test = float64(1 << uint(10))
test = float64(1 << uint(float64(11-1)))
test = float64(1 << uint(-float64(1-11)))
// 但是这一行不工作：编译时出错
test = float64(1 << uint(math.Abs(10)))
我收到的错误是：
invalid operation: 1 << uint(math.Abs(10)) (shift of type float64)
然而，似乎单独的转换操作是有效的：
var test = uint(math.Abs(10))
fmt.Println(reflect.Kind(test))
// uint32
这是一个示例：http://play.golang.org/p/36a8r8CCYL
英文:
I'm facing a strange issue using the shift operator << in Golang. In my final code, the shift value will be the absolute value of two integers. However, the Go package only defines the Abs function for float64 values, so I will need to cast the parameters to use it, and then cast the result back to uint.
Finally, this value will be used as a float64 parameter, so I cast it back to float64 after that.
The problem is that the return value's cast does not seem to work as I expected...
var test float64
// all the following lines are working as expected
test = float64(1 << 10)
test = float64(1 << uint(10))
test = float64(1 << uint(float64(11-1)))
test = float64(1 << uint(-float64(1-11)))
// but this one does not: error at compilation
test = float64(1 << uint(math.Abs(10)))
The error I'm receiving is:
invalid operation: 1 << uint(math.Abs(10)) (shift of type float64)
However, it seems that the cast operation alone works:
var test = uint(math.Abs(10))
fmt.Println(reflect.Kind(test))
// uint32
Is it a Golang issue ? A behaviour I did not find in the specifications ? A normal behaviour I simply don't understand ?
Here is a playground: http://play.golang.org/p/36a8r8CCYL
答案1
得分: 13
根据规范：
移位表达式中的右操作数必须具有无符号整数类型，或者是可以转换为无符号整数类型的无类型常量。如果非常量移位表达式的左操作数是无类型常量，则该常量的类型将与仅使用左操作数的移位表达式的类型相同。
因此，float64(1 << uint(math.Abs(10)))基本上与float64(1) << uint(math.Abs(10))是相同的，但会产生错误，因为不能简单地将浮点数进行移位操作。
英文:
From the spec:
>The right operand in a shift expression must have unsigned integer type or be an untyped constant that can be converted to unsigned integer type. If the left operand of a non-constant shift expression is an untyped constant, the type of the constant is what it would be if the shift expression were replaced by its left operand alone.
So float64(1 << uint(math.Abs(10))) is basically the same as float64(1) << uint(math.Abs(10)), which produces an error, since one does not simply shift a float.
答案2
得分: 0
你不应该使用math.Abs。在Go语言中，可以使用一个简单的小函数来解决这个问题。例如，
package main
import "fmt"
func shift(a, b int) uint {
s := a - b
if s < 0 {
s = -s
}
return uint(s)
}
func main() {
a, b := 24, 42
i := 1 << shift(a, b)
fmt.Printf("%X\n", i)
f := float64(i)
fmt.Println(f, i)
}
输出结果：
40000
262144 262144
英文:
You shouldn't be using math.Abs. In Go, solve the problem with a small, simple function. For example,
package main
import "fmt"
func shift(a, b int) uint {
s := a - b
if s < 0 {
s = -s
}
return uint(s)
}
func main() {
a, b := 24, 42
i := 1 << shift(a, b)
fmt.Printf("%X\n", i)
f := float64(i)
fmt.Println(f, i)
}
Output:
40000
262144 262144
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