Challenge 3 — Solution

Task: Write a function double(n *int) that takes a pointer to an int
and doubles the value it points to (using dereferencing, not a
return value). Declare a variable, pass its address to double, and
print the variable afterward to confirm it changed.

package main

import "fmt"

func double(n *int) {
	*n = *n * 2
}

func main() {
	number := 7

	double(&number)

	fmt.Println(number)
}


Expected output:

14

Notes:
- &number passes the ADDRESS of number into double, not a copy of
  its value — this is what makes it possible for double to change
  the real variable rather than a throwaway duplicate.
- *n = *n * 2 reads as: dereference n to get the current value,
  double it, then dereference n again on the left side to store the
  new value back at that same address.
- This is the plain-function equivalent of what a pointer receiver
  does for a struct method (this chapter's main example) — the
  underlying mechanism (& and *) is identical either way.