How to make the unit added through append visible on the fifth line as well? Without changing the fill function's signature.

In Go, if a function fill takes a slice by value and adds an element inside using append, the outer slice will not change, because append may create a new array.

To make the unit added via append visible on the fifth line without changing the signature of fill, you can pass the slice by pointer or return a new slice from the function. But since the signature cannot be changed, you can use a slice with sufficient capacity so that append does not create a new array, and changes will be visible outside.

Example:

func fill(s []int) {
    s = append(s, 1) // if capacity is enough, the addition will modify the original array
}

func main() {
    s := make([]int, 4, 5) // length 4, capacity 5
    fill(s)
    fmt.Println(s) // now the added 1 will be visible
}

If the capacity is insufficient, append will create a new array, and changes will not be visible. Therefore, it is important that the slice's capacity is sufficient before calling fill.