Passing slice to Scanf

The snippet:

package main
import "fmt"

func main() {
    mapIsh := make([]int64, 5)
    fmt.Scanf("%v %v %v %v %v", &mapIsh[0], &mapIsh[1], &mapIsh[2], &mapIsh[3], &mapIsh[4])

    fmt.Print(mapIsh[0], mapIsh[1], mapIsh[2], mapIsh[3], mapIsh[4])
}

I crate a slice and want to save the contents of a read operation. The above works as expected. What i don't get is why I have to pass pointers to the array elements(I assumed the values are passed by reference for slices/arrays).

Also if I don't make one of the arguments a pointer the subsequent values wont be read. So can someone explain the following:

Input data: 1 2 3 4 5

1. For:

   fmt.Scanf("%v %v %v %v %v", mapIsh[0], mapIsh[1], mapIsh[2], mapIsh[3], mapIsh[4])

I get: 0 0 0 0 0

2. For:

    fmt.Scanf("%v %v %v %v %v", &mapIsh[0], &mapIsh[1], mapIsh[2], &mapIsh[3], &mapIsh[4])
   

I get: 1 2 0 0 0

Arguments are always passed by value in Go, never by reference.

You're right that passing a slice to a function would allow the function to mutate its elements. But you're not passing a slice into Scanf; your two examples are passing in the int64 elements versus passing in the addresses of those elements. I think you're getting confused by the indexing syntax and thinking mapIsh[0] somehow means you're passing in something the function will be able to mutate. But mapIsh[0] really is just an int64, the first element of mapIsh.

As for why you're getting zeros, that's because Go initializes mapIsh to the zero-value, which for an array of int64s is all 0s. When Scanf can't read into the value you forgot the & for, it was not able to continue. This is why you should check the error returned from Scanf, which will indicate the error.

(Note that pass-by-value means that passing an array to a function creates a copy of that array, so the function will not be able to mutate the elements like it can for a slice. That isn't actually relevant here, as described above.)

First of all, values are not passed by reference for slices/arrays.

At the same time, if you were passing a slice value to some function, the receiver would be able to modify the same underlying array, because a slice value contains a pointer.

In your case, you are not passing a slice itself, but rather copies of the dereferenced int64 values. And fmt.Scanf is not updating mapIsh. Only passing addresses are you able to use the scanned values.