msirovy / golang

GOlang

My GOlang journey from zero to ...

Tutorials

• Learn GO with tests
• GO by examples

Good to know

• Source codes ending with *_test.go should be tests.

CLI notes

go run source_code.go

# offline documentation for builtins, etc
go doc builtin	
go doc fmt.Println


# code normalization is there implemented
gofmt source_code.go > public_this.go

Types

• int - integer se znamenkem (could define 8 - 64)
• uint - integer bez znamenka (could define 8 - 64)
• float32/64
• complex32/64

package main

import "fmt"	

func main() {
	// declare a
	var a int = 109

	// change type and seve to b
	var b uint8 = a

	// > 109
	fmt.Printf("> %d\n", b)

	// >   109        add two spaces before the number
	fmt.Printf("> %5d\n", b)

	// > 00109				add two zeros before the number
	fmt.Printf("> %05d\n", b)

	// > 1101101			print it as a binary
	fmt.Printf("> %b\n", b)
}

• string - array of unicode chars (allows all utf8 chars)

var name string = "Karel"
fmt.Printf("%s", name)

// string is array of unicode bits
fmt.Println(name[3])	// prints 101
fmt.Println(string(x[3]))	// prints e

• array - the length and type must be defined during initialization

package main

import "fmt"

func main() {
	var arr1 [7]string

	arr1[0] = "go"
	arr1[2] = "is"
	arr1[4] = "the"
	arr1[6] = "best"

	for id, word := range arr1 {
		fmt.Println(id, " - ", word)
	}

}

• slice - let's say more flexible array (more informations)

slc := []string{
	"go",
	"is",
	"the",
	"best"}

// Println is cool and can print whole array or slice
fmt.Println(slc)

• map - hashtable

users := make(map[string]User)

users["kaja"] = User{"Karel", "Patočka", 42}
users["vilda"] = User{"Viliem", "Novak", 34}


value, exists := users["Paja"]
if exists {
	fmt.Println(value)
} else {
	fmt.Println("Given key is invalid")
}

• Own types and structures golang allows to define own structures and types and is very strictly (it increases safety)

package main

import (
	"fmt"
)

// define new type
type Datum string

// define new struc
type User struct {
	Name     string
	Surname  string
	Password string
	Age      uint8
}

/*
	structs can have methods ;-)
	- first brackets contains (self Struct) and than is definition common
*/
func (usr User) hi() {
	fmt.Println("Hi ", usr.Name)
}

// if I need to change data in the struct by the method I must use pointer!!!
func (usr *User) chpwd(pass string) {
  usr.Password = pass
}

// another method example
func (usr User) auth(pass string) bool {
	if usr.Password == pass {
		return true
	} else {
		return false
	}
}

func main() {
	// define date of type Datum
	var date Datum = "2020-11-02"

	// define slice of type User
	var users []User

	users = append(users, User{"Karel", "Patočka", "PasswordK", 42})
	users = append(users, User{"Vilda", "Novak", "PasswordV", 34})

	fmt.Println(date)

	for _, u := range users {
		if u.auth("PasswordV") {
			fmt.Println("Hi there ", u.Name)
		}
	}

}

Golang flow control

There is nothing special in go

package main

import (
	"fmt"
)

// stupid example but it works
func sum(x, y int) (sum int, err error) {
	if x == 0 && y == 0 {
		return 0, fmt.Errorf("Numbers shouldn't be zeros")
	} else {
		return x + y, nil
	}
}

func main() {
	/*
		I can allocate local variables in if,
		but sum variable will not be available out of if
	*/
	if sum, err := sum(10, 21); err == nil {
		fmt.Println(sum)
	}

	// define empty var
	var age uint

	// read input to age variable
	fmt.Println("Give me your age")
	fmt.Scanln(&age)

	// switch syntax
	switch {
	case age < 10:
		fmt.Println("You are children")
	case 10 < age && age < 18:
		fmt.Println("You are adolescent")
	case 18 < age:
		fmt.Println("You are adult")
	default:
		fmt.Println("Have you born already?")
	}

}

Basic program structure

This is example how the nice program could work

package main

// need to use everything imported
import (
	"fmt"
	"log"
)

func sayHello(message string) string {
	var err bool = false
	if len(message) == 0 {
		err = true
	}
	return message, err
}


func main() {
	// it is good to inform users that your program have started correctly
	log.Println("Program starting")

	msg, err := sayHello("Minimalism is a sexy!!!")	
	if err == nil {
		fmt.Println(msg)
	} else {
		log.Error("Something went wrong :-( ")
	}

}