Source: Understand how to use packages, variables, and functions in Go

Main function

All executable programs in Go have this function because it’s the program’s starting point. You can only have one main() function in your program.

If you’re creating a Go package, you don’t need to write a main() function.

The main() function doesn’t have any parameters and returns nothing.

Command-line argumensts

If you need to access command-line arguments in Go, you can do it with the os package and the os.Args

package main

import (
    "fmt"
    "os"
    "strconv"
)

func main() {
    // os.Args are string
    number1, _ := strconv.Atoi(os.Args[1])
    number2, _ := strconv.Atoi(os.Args[2])
    fmt.Println("Sum:", number1+number2)
}

To execute the program:

> go run main.go 3 5

Custom functions

Syntax:

func name(parameters) (results) {
    body-content
}

Example:

func calc(number1 string, number2 string) (sum int, mul int) {
    int1, _ := strconv.Atoi(number1)
    int2, _ := strconv.Atoi(number2)
    sum = int1 + int2
    mul = int1 * int2
    return
}

If you define the return variables in the function, you can simply add a return line at the end. This is not recommended, as it can be unclear what the function is returning.

You can discard returned values by assigning value to the _ variable.

Change function parameters values (pointers)

When you pass a value to a function, every change in that function won’t affect the caller. Go takes that value and creates a local copy (new variable in memory).

In Go, there are two operators for working with pointers:

The & operator takes the address of the object that follows it.
The * operator dereferences a pointer. That is, it gives you access to the object at the address contained in the pointer.

package main

import "fmt"

func main() {
    firstName := "John"
    updateName(&firstName) # you are passing the address of the variable
    fmt.Println(firstName)
}

func updateName(name *string) {
    *name = "David" # you are modifiying the content of that variable
}