In the fifth line, where fmt.Println(v) occurs, can we access the element added via append inside fill and output it?

In Go, if inside the fill function you add an element to a slice using append, this element exists in the slice after the call to fill if the slice is passed by reference or returned.

Example:

package main

import "fmt"

func fill(s []int) []int {
    s = append(s, 1) // add element
    return s
}

func main() {
    var v []int
    v = fill(v)
    fmt.Println(v) // will output [1]
}

If the slice is passed to fill by value and not returned, then changes will not be saved. Therefore, to access the added element on line five, you need to either return the slice from fill or pass a pointer to the slice.

Thus, on line five, fmt.Println(v) can output the added one if the passing and returning of the slice are organized correctly.