How to pass slice of struct as pointer to a function and modify it? - go

Question

I have a slice of struct []student, and I want to modify its content with function.
type student struct {
name string
age int
}
students := []student{
{"Doraemon", 30},
{"King Kong", 25},
}
Thus, I decided to pass it as a pointer. May I know how to pass the slice as a reference to a function?
func addAge (s *[]student) error { //this code has error
//everyone add 2 years old
for i, e := range *s {
s[i].age = s[i].age + 2
}
//make the first student much older
s[0].age = s[0].age + 5
return nil
}
I keep playing with Go Playground, but it gives many complains, such as
cannot range over s (type *[]student)
invalid operation: s[i] (type *[]student does not support indexing)
invalid indirect of s
...
How to precisely pass the reference of a slice of struct to a function? How to range the slice of struct? And how to change the value of the struct (modify the same struct in THE slice)?
I keep getting error while playing with s *[]student, range *s, s []student, s *[]*student ... so hard to get it correct...
sorry for my NEWBIE question, still learning GO... trying hard

Answer 1

Slices are passed by reference, so as long as you are modifying the existing slice content you should not explicitly pass a pointer.
package main
import (
"fmt"
)
type student struct {
name string
age int
}
func main() {
students := []student{
{"Doraemon", 30},
{"King Kong", 25},
}
err := addAge (students)
fmt.Println(students)
if err != nil {
fmt.Println("error")
}
}
func addAge (s []student) error {
for i, _ := range s {
s[i].age = 3
}
return nil
}
Now, for your addAdditinalStudent function you should actually use the append function. Plus, have in mind
..., since the slice header is always updated by a call to
append, you need to save the returned slice after the call. In fact,
the compiler won't let you call append without saving the result.
Slices#append
// add student
students = append(students, student{"Test", 33})
Go Playground

Answer 2

in Go you can pass items by value ([]student) or by reference ([]*student). When you want to operate on the values of a struct{} you should pass it to a function with its reference (the pointer).
So you can do something like this:
type student struct {
name string
age int
}
func addTwoYearsToAll(students []*student){
for _, s := range students {
s.age += 2
}
}
This way you're working with the same exact items you build when appending to the slice. Playground example.
Also take a look at Are Golang function parameter passed as copy-on-write?