英文:
Making multidimensional slice from multidimensional array does not work as expected
问题
我正在尝试从多维数组创建一个多维切片，但出现了一些奇怪的情况（至少对我来说是奇怪的）。我创建了一个多维数组 ma，并从中创建了三个切片 s1、s2、s3。下面的代码展示了这个过程：
package main
import (
"fmt"
)
func main() {
var ma [4][3]int32 = [4][3]int32{
{10, 20, 30},
{50, 60, 70},
{80, 90, 100},
{110, 120, 130},
}
var s1 [][3]int32 = ma[:] // 预期的结果
var s2 [][3]int32 = ma[:][:] // ????
var s3 [][]int32 = ma[0:2][0:2] // ????
}
var s1 [][3]int32 = ma[:] 的行为符合预期。ma[:] 创建了一个 [3]int32 数组的切片，因此我们有一个底层数组，每个元素的类型都是 [3]int32 数组。对我来说，这是完全符合预期的行为。
问题出现在定义 s1 和 s2 时。它们的行为与我预期的不同。var s2 [][3]int32 = ma[:][:] 的结果与 var s1 [][3]int32 = ma[:] 相同。我期望它创建一个切片的切片，而不是一个数组的切片。这是如何可能的？这两者怎么会得到相同的结果？
此外，我期望 var s3 [][]int32 = ma[0:2][0:2] 也会创建一个切片的切片。但实际上它给出了错误信息："cannot use ma[0:2][0:2] (value of type [][3]int32) as [][]int32 value in variable declaration compiler"。所以，用 ma[0:2][0:2] 得到的类型是 [][3]int32，而不是 [][]int32。这是怎么回事？
希望我已经解释清楚了我不明白的地方。
英文:
I'm trying to create a multidimensional slice from a multidimensional array and some strange things are happening (at least for me). I created a multidimensional array ma and made three slices s1, s2, s3 from it. The code below shows this:
package main
import (
"fmt"
)
func main() {
var ma [4][3]int32 = [4][3]int32{
{10, 20, 30},
{50, 60, 70},
{80, 90, 100},
{110, 120, 130},
}
var s1 [][3]int32 = ma[:] // expected
var s2 [][3]int32 = ma[:][:] // ????
var s3 [][]int32 = ma[0:2][0:2] // ????
}
var s1 [][3]int32 = ma[:] behaves as expected. ma[:] creates a slice of [3]int32 arrays, so we have an underlaying array with each element of type [3]int32 array. Tottally expected behaviour for me.
Problems arise when defining s1 and s2. It's not behaving as I expected. var s2 [][3]int32 = ma[:][:] gives the same result as var s1 [][3]int32 = ma[:]. I expected it to make a slice of slices, not a slice of arrays. How is it possible? How can the two give the same result?
Additionally, I expect var s3 [][]int32 = ma[0:2][0:2] to create a slice of slices as well. Instead it gives the error "cannot use ma[0:2][0:2] (value of type [][3]int32) as [][]int32 value in variable declaration compiler". So somehow, with ma[0:2][0:2] it gives the type [][3]int32, not [][]int32. How?
I hope I have explained what is not clear to me.
答案1
得分: 2
表达式ma[:][:]解析为(ma[:])[:]。该表达式在ma的切片表达式的结果上进行切片表达式的求值。外部切片表达式是一个空操作。
Go语言没有多维切片表达式。
英文:
The expression ma[:][:] parses as (ma[:])[:]. The expression evaluates to a slice expression on the result of the slice expression on ma. The outer slice expression is a noop.
Go does not have multidimensional slice expressions.
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