英文:
golang make 0 length slice and re-slicing slice not as expected
问题
我看到这段代码感到困惑，请帮我解释一下。
这段代码的作用是：
package main
import "fmt"
func main() {
s := make([]int, 0, 10)
s1 := s[0:1]
s1[0] = 1
println("-----")
fmt.Println(s1, len(s1), cap(s1)) //[1] 1 10
fmt.Println(s, len(s), cap(s)) //[] 0 10
fmt.Println(s, s[0:10]) // [] [1 0 0 0 0 0 0 0 0 0]
}
上述程序可以完美执行，但打印的值不符合我的预期，我有两个问题：
-
为什么在
fmt.Println(s1, len(s1), cap(s1))和fmt.Println(s, len(s), cap(s))中，s1 的打印值是 [1]，而 s 的打印值是 []。我认为 s1 的值应该等于 s 的值，都是 [1]。 -
为什么 s[0:10] 可能打印出 [1 0 0 0 0 0 0 0 0 0]，而 s 打印的是 []。我认为 s[0:10] 是对 s 进行重新切片，如果 s 是 []，那么 s[0:10] 也应该是 []。
英文:
I saw this code feel confusing，please help me explain.
the code:
package main
import "fmt"
func main() {
s := make([]int, 0, 10)
s1 := s[0:1]
s1[0] = 1
println("-----")
fmt.Println(s1, len(s1), cap(s1)) //[1] 1 10
fmt.Println(s, len(s), cap(s)) //[] 0 10
fmt.Println(s, s[0:10]) // [] [1 0 0 0 0 0 0 0 0 0]
}
The above program can be executed perfectly，While print value is not meet my expectations，I have two questions:
-
why s1 print value is: [1], while s print value is [] in the
fmt.Println(s1, len(s1), cap(s1))andfmt.Println(s, len(s), cap(s)). I think s1 value is equal to s value is [1]. -
why s[0:10] may print [1 0 0 0 0 0 0 0 0 0], while s print []. I think s[0:10] is re-slice of s，if s is [], s[0:10] is [] too.
答案1
得分: 1
s1打印出[1]，因为它是一个长度为1的切片，并且在这个长度内有元素1。它的长度为1，是因为它是从s的索引0到1的切片。因此，s1 := s[0:1]。s打印出[]，因为它是一个长度为0的切片。当长度为0时，没有值可以显示，所以在方括号中没有任何内容。它的长度为0是因为在make函数调用中指定了长度为0。在使用make创建切片时，第二个参数是切片的长度。s[0:10]可能打印出[1 0 0 0 0 0 0 0 0 0]，因为可以将现有的切片重新切片为其整个底层数组的容量，而不仅仅是长度。由于重新切片从0到10，它将包含10个值（切片的第二个边界是不包含的）。这就是为什么在打印输出中看到了所有10个值。- 作为额外的说明，
s[0:10]不能是[]，因为这意味着你指定了一个长度为10的切片，但返回的却是一个长度为0的切片。在 Go 语言中，切片s[a:b]的长度始终为b - a。如果这不可能，程序将会引发错误。它不会给你一个其他长度的切片。请参阅语言规范中关于这些规则的解释：https://go.dev/ref/spec#Slice_expressions
英文:
s1prints[1]because it is a slice of length 1, and it has the element 1 within that length. It is length 1 because it is a slice ofsfrom0to1. Hence,s1 := s[0:1]sprints the value[]because it is a slice of length 0. There are no values to show when the length is 0, so there is nothing between the brackets. It is length 0 because it was specified as such in themakecall. When usingmaketo make a slice, the second argument is the length of the slice.s[0:10]may print[1 0 0 0 0 0 0 0 0 0]because it is allowed to re-slice an existing slice into its whole underlying capacity, not just the length. Since the re-slice goes from0to10, it will include 10 values (the second bound of the slice is non-inclusive). That's why you see all 10 values in the printout.- As a bonus point,
s[0:10]cannot be[]because that would mean you specify a slice of length 10 and get in return a slice of length 0. In Go, the length of a slices[a:b]will always have a length ofb - a. If that's not possible, then the program will panic. It will not give you a slice of some other length. See the explanation of these rules in the language specification: https://go.dev/ref/spec#Slice_expressions
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