Using our knowledge of addresses, pointers, and dereferencing, let’s return to our initial problem: How can we change the value of a variable when we’re in a different scope? Let’s take a look at the code again:

func addHundred(num int) {
  num += 100
}

func main() {
  x := 1
  addHundred(x)
  fmt.Println(x) // Prints 1
}

Even though we call addHundred(x), the value of x doesn’t change! Why is that?

Remember, Go is a pass-by-value language. When we call addHundred(x) we’re providing addHundred() with a value of 1. We’re not actually providing the address of x for addHundred() to go in and change the value stored there.

If we want to change the value of x using a function, we’re going to need to first change our function:

func addHundred (numPtr *int) {
  *numPtr += 100
}

Our new function now has a parameter of a pointer for an integer. By passing the value of a pointer (which is an address) to addHundred(), we can also dereference the address and add 100 to its value. But now that addHundred() expects a pointer for an argument, we’re also going to need to change our main()! The complete code is as follows:

func addHundred (numPtr *int) {
  *numPtr += 100
}

func main() {
  x := 1
  addHundred(&x)
  fmt.Println(x) // Prints 101
}

Since addHundred() expects a pointer (and pointers are variables that store an address) the final touch was to provide addHundred() the address of x. With that, x is now 101!