Functions in

About Functions

A function allows you to group code into a reusable unit. It consists of the func keyword, the name of the function, and a comma-separated list of zero or more parameters and types in round brackets.

Function Parameters

All parameters must be explicitly typed; there is no type inference for parameters. There are no default values for parameters so all function parameters are required.

import "fmt"

// No parameters
func PrintHello() {
    fmt.Println("Hello")
}

// One parameter
func PrintHelloName(name string) {
  fmt.Println("Hello " + name)
}

Parameters of the same type can be declared together, followed by a single type declaration.

import "fmt"

func PrintGreetingName(greeting, name string) {
  fmt.Println(greeting + " " + name)
}

Parameters vs. Arguments

Let's quickly cover two terms that are often confused together: parameters and arguments. Function parameters are the names defined in the function's signature, such as greeting and name in the function PrintGreetingName above. Function arguments are the concrete values passed to the function parameters when we invoke the function. For instance, in the example below, "Hello" and "Katrina" are the arguments passed to the greeting and name parameters:

PrintGreetingName("Hello", "Katrina")

Return Values

The function parameters are followed by zero or more return values which must also be explicitly typed. Single return values are left bare, multiple return values are wrapped in parenthesis. Values are returned to the calling code from functions using the return keyword. There can be multiple return statements in a function. The execution of the function ends as soon as it hits one of those return statements. If multiple values are to be returned from a function, they are comma separated. More information about idiomatic use of multiple return values can be found in the linked concept.

func Hello(name string) string {
  return "Hello " + name
}

func HelloAndGoodbye(name string) (string, string) {
  return "Hello " + name, "Goodbye " + name
}

Invoking Functions

Invoking a function is done by specifying the function name and passing arguments for each of the function's parameters in parenthesis.

import "fmt"

// No parameters, no return value
func PrintHello() {
    fmt.Println("Hello")
}
// Called like this:
PrintHello()

// One parameter, one return value
func Hello(name string) string {
  return "Hello " + name
}
// Called like this:
greeting := Hello("Dave")

// Multiple parameters, multiple return values
func SumAndMultiply(a, b int) (int, int) {
    return a+b, a*b
}
// Called like this:
aplusb, atimesb := SumAndMultiply(a, b)

Named Return Values and Naked Return

As well as parameters, return values can optionally be named. If named return values are used, a return statement without arguments will return those values. This is known as a 'naked' return.

func SumAndMultiplyThenMinus(a, b, c int) (sum, mult int) {
    sum, mult = a+b, a*b
    sum -= c
    mult -= c
    return
}

Pass by Value vs. Pass by Reference

It is also important to clarify the concept of passing by value and passing by reference.

First, let's clarify passing by value. In the example below, when we pass the variable val to the function MultiplyByTwo, we passed a copy of val. Because of this, newVal has the updated value 4 but the original variable val is still 2. Behind the scene, Go essentially makes a copy of the original value so that only this copy, a.k.a. v, is modified by the function.

val := 2
func MultiplyByTwo(v int) int {
    v = v * 2
    return v
}
newVal := MultiplyByTwo(val)
// newval is 4, val is still 2 because only a copy of its value was passed into the function

Strictly speaking, all arguments are passed by value in Go, i.e. a copy is made of the value or data provided to the function. But if you don't want to make a copy of the data that is passed to a function and want to change the data in the function, then you should use pointers as arguments, a.k.a. pass by reference.

Pointers

We use a pointer to achieve passing by reference. By passing pointer arguments into a function, we could modify the underlying data passed into the function instead of only operating on a copy of the data.

For now, it is sufficient to know that pointer types can be recognized by the * in front of the type in the function signature.

func HandlePointers(x, y *int) {
    // Some logic to handle integer values referenced by pointers x and y
}

If the concept of pointer is confusing, no worries. We have a dedicated section later in the syllabus to help you master pointers.

Exceptions

Note that slices and maps are exceptions to the above-mentioned rule. When we pass a slice or a map as arguments into a function, they are treated as pointer types even though there is no explicit * in the type. This means that if we pass a slice or map into a function and modify its underlying data, the changes will be reflected on the original slice or map.

Other types of functions

Functions in Go are considered first class citizens making them very powerful. There are a number of other concepts around functions like methods and anonymous functions which you will meet later in your journey.