修改切片类型的地图值

Question

mp := map[string][]int{
"1" : {1,2,3,4},
}


for _,s := range mp {
  i:= 0
  for _,v:=range s{
    if v%2==0 {
      s[i] = v
      i++
    }
  }
  s = s[:i]
  // I tried mp[k] = s and it works fine
}

want := map[string][]int{
"1" : {2,4},
}

if !reflect.DeepEqual(mp,want) {
   fmt.Printf("not expected")
   fmt.Println(mp)
}

With the above code I'm not able to remove odd integers from the slice.

I changed this to

mp := map[string][]int{
"1" : {1,2,3,4},
}


for k,s := range mp {
  i:= 0
  for _,v:=range s{
    if v%2==0 {
      s[i] = v
      i++
    }
  }
  s = s[:i]
  mp[k] = s
}

want := map[string][]int{
"1" : {2,4},
}

And now it works. I am wondering what is the problem with the first piece of code. I think s's address is not changed.

Answer 1

for k,s := range mp {

Here s keeps the value of mp, which is []int.

If you change or modify the s in the loop it won't affect the value of mp, cause s is just a copy of the value of mp. If you want to affect the change into mp then modify with mp[k].