重新分配slice参数的行为有所不同

Question

package main

import "fmt"

func main() {
    paths := []string{"hello", "world", "mars"}
    var result = delete(paths, 1)
    fmt.Println(result)
    fmt.Println(paths)
}

func delete(paths []string, index int) []string {
    paths = append(paths[:index], paths[index+1:]...)
    return paths
}

The result of the code above is the following:

[hello mars]

[hello mars mars]

As you see, the second fmt.Println(paths) obviously uses the modified slice but does not use the reassigned value. Why is that? I was expecting it to print [hello mars] like in the print before.

I know that paths being passed is not the same slice as paths parameter in the delete() function expect for referencing the same underlying array. But I still don't understand how I changed the underlying array of the paths being passed to delete function as it prints [hello mars mars] instead of [hello world mars].

Answer 1

Because, as you said, the same underlying array is in use. When you do the append, paths[:1] is a slice of length 1 and capacity 3, and paths[2:] is a slice of length 1, so there is enough room in the underlying array of the first slice to append the new value without allocating a new array. paths in main is still a slice of length 3 since it was never modified, but since the underlying array was modified (specifically element 1), you see the value that you see.

You may want to take a look at https://blog.golang.org/go-slices-usage-and-internals if you haven't already.