为什么追加修改通过的切片

Question

How could I iterate through the slice and pass somewhere the slice except the current element? Seems append() function modifies the underlying slice as we could see in documentation. But anyway I still don't know how to reach this.

func main() {
    args := []string{ "2", "3", "8" }

    for i, _ := range args {
        fmt.Println(append(args[:i], args[i+1:]...)) // or pass to function
    }

    fmt.Println(args)
}

result:

[3 8]
[3 8]
[3 8]
[3 8 8] // it is args now

what I expect:

 [3 8]
 [2 8]
 [2 3]

I already saw this Why does append() modify the provided slice? (See example)

but what is the Capacity of the slice is the secret for me, and I dont understand why did I exceed it.

Answer 1

You can imagine a slice as a struct composed of an array of fixed size and a counter for the number of elements in it. The capacity of the slice is the size of the underlying array, while the lenght of the slice is the counter.

Append is defined that way : func append(slice []Type, elems ...Type) []Type (godoc), which essentially means that you will be appending the elem variadic argument to the slice argument. If len(elems) + len(slice) > cap(slice), then udnerlying array will need to be changed for a bigger one (with a bigger capacity), which mean (in go) a new slice (hence the return parameter).

In your case, you didn't exceeded the capacity of the slice. You just modified its content.

One simple (albeit slightly ugly) trick would be to nested two appends to an empty slice :

package main

import "fmt"

func main() {
    args := []string{ "2", "3", "8" }

    for i, _ := range args {
        fmt.Println(append(append([]string{}, args[:i]...), args[i+1:]...)) 
    }

    fmt.Println(args)
}

Or, if you want to pass a copy of the slice to a method (and do what you want after that), you can use the copy function…