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Understanding Functions.

Objective

To understand functions in Go.

Functions are first class citizens in Go, in fact you have already been using functions, we have been declaring "main" function and using it from our first program.

General syntax is:

func funcName(optional parameters) return parameters {
    do something
}

Structure

Navigate to our code folder

code/basic/

For our program create a new folder '09_functions'

code/basic/09_functions/

And lets create a file 'functions.go' in it, finally the structure would look like this:

code/basic/09_functions/functions.go

Code

The code will be divided into four parts:

1.)

Functions
    1 package main
    2
    3 import "fmt"
    4
    5 // sayHello prints "Hello, Octy!"
    6 func sayHello() {
    7 fmt.Println("In func sayHello()...")
    8 fmt.Println("Hello, Octy!")
    9 fmt.Println()
    10 }
    11

Review

Line 6 defines a function "sayHello" which prints "Hello, Octy!" on screen

func sayHello()

2.)

Functions
    12 // function with passing values
    13 func sayHelloTo(name string) {
    14 fmt.Println("In func sayHelloTo()...")
    15 fmt.Println("Hello,", name)
    16 fmt.Println()
    17 }
    18

Review

On line 13 we declare a function "sayHelloTo" with accepts a parameter.

func sayHelloTo(name string)

sayHelloTo accepts a string "name" as a parameter and then print it on line 15.

3.)

Functions
    19 // function with return values
    20 func printNums(n int) error {
    21 fmt.Println("In func printNums()...")
    22 var err error
    23 for i := 0; i <= n; i++ {
    24 _, err = fmt.Println(i)
    25 }
    26 return err
    27 }
    28

Review

Line 20 we define a function "printNums" with accepts a parameter and returns a parameter of type "error".

func printNums(n int) error

On line 22 we define a variable "err" of type "error", till now we have seen data types of integer, float, boolean, string, similarly Go offers a custom type "error".

var err error

Line 24 is a little special, Println() returns two values, "number of bytes written" and an "error", as we don't want number of bytes written we use a underscore "_", Go compiler ignores underscore and throws away its value.

_, err = fmt.Println(i)

We catch the error value in the variable "err" and return it on line 26.

return err

4.)

Functions
    29 // main function
    30 func main() {
    31 fmt.Println("In func main()")
    32 fmt.Println("Now calling func sayHello()...")
    33 fmt.Println()
    34 // calling function sayHello()
    35 sayHello()
    36 // calling a function with passing values
    37 name := "Gopher"
    38 sayHelloTo(name)
    39 // calling a function with return parameters
    40 err := printNums(10)
    41 if err == nil {
    42 fmt.Println("There are no errors!")
    43 } else {
    44 fmt.Println("Error with printing:", err)
    45 }
    46 }

Review

We call individual functions on lines 35, 38 & 40.

sayHello()

sayHelloTo(name)

We pass a value "name" along with the function.

err := printNums(10)

We call printNums along with a value "10" and assign it to the variable "err", the return error gets stored in the variable "err"

Full Code

Complete Functions Code
    package main

    import "fmt"

    // sayHello prints "Hello, Octy!"
    func sayHello() {
        fmt.Println("In func sayHello()...")
        fmt.Println("Hello, Octy!")
        fmt.Println()
    }

    // function with passing values
    func sayHelloTo(name string) {
        fmt.Println("In func sayHelloTo()...")
        fmt.Println("Hello,", name)
        fmt.Println()
    }

    // function with return values
    func printNums(n int) error {
        fmt.Println("In func printNums()...")
        var err error
        for i := 0; i <= n; i++ {
            _, err = fmt.Println(i)
        }
        return err
    }

    // main function
    func main() {
        fmt.Println("In func main()")
        fmt.Println("Now calling func sayHello()...")
        fmt.Println()
        // calling function sayHello()
        sayHello()
        // calling a function with passing values
        name := "Gopher"
        sayHelloTo(name)
        // calling a function with return parameters
        err := printNums(10)
        if err == nil {
            fmt.Println("There are no errors!")
        } else {
            fmt.Println("Error with printing:", err)
        }
    }

Running your code

Open your terminal and navigate to our folder

code/basic/09_functions/

Once in the folder type the following command

go run main.go

Output

Output
In func main()
Now calling func sayHello()...

In func sayHello()...
Hello, Octy!

In func sayHelloTo()...
Hello, Gopher

In func printNums()...
0
1
2
3
4
5
6
7
8
9
10
There are no errors!

Github

Just in case you have some errors with your code, you can check out the code at github repo

Github Repo

Golang Playground

You can also run the code at playground

Golang Playground

Next

In the next chapter we will learn about packages in Go.

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