Golang – commands with flag

Flag is a golang library to parse commands on the command line.

For example, let’s create an Hello World program that gets a name as parameter and returns the string:

package main

import (
	"flag"
	"fmt"
	"os"
)

func main() {

	name := flag.String("name", "", "The name to create the hello world message")
	flag.Parse()

	if *name == "" {
		fmt.Println("Enter your name:")
		flag.Usage()
		os.Exit(1)
	}

	fmt.Println("Hello,", *name)
}

Ton run the command and see the “Hello, Laura” message, assuming the file is called “flags.go”, run:

go run flags.go -name Laura

Golang – using Maps

Maps can be defined in Golang with the keywork map.

The type is defined in the square brackets, followed by the value:

For example:

package main

import "fmt"

func main() {

	//variable lenght arrays
	empty_map := map[string]string{}

	fmt.Println(empty_map)

	location_map := map[string]string{
		"city":    "Frankfurt",
		"state":   "Hessen",
		"country": "Germany",
	}

	fmt.Println(location_map)

	coordinates_map := map[string]float32{
		"latitude":  56.7,
		"longitude": 45.6,
	}

	fmt.Println(coordinates_map)

	//print specific values
	fmt.Println(coordinates_map["latitude"])
}

Golang – Slices are Arrays!

Arrays in Golang are called slices:

package main

import "fmt"

func main() {

	//variable lenght arrays
	slice := []int{1, 2, 3, 4}

	slice = append(slice, 4)

	fmt.Println(slice)

	//fixed lenght arrays
	fixed_slice := [4]int{1, 2, 3, 4}
	//cannot append another element - compiler error!
	//fixed_slice = append(slice, 4)

	fmt.Println(fixed_slice)

	//another way to define a slice
	another_slice := make([]int, 10)

	//initially filled with zeros
	fmt.Println(another_slice)

	//add a different number in the first position
	another_slice[0] = 45

	fmt.Println(another_slice)
}

Golang – using pointers

Golang also uses pointers, but it doesn’ need any memory allocation code or anything.

It’s very straightforward.

The ampersand & creates a pointer, and the asteryx * allows you to get the value referenced by the pointer.

Simply:

package main

import (
	"fmt"
)

func main() {
	s := "this is a string"

	string_pointer := &s

	fmt.Println(s)

	//reference the pointer with asteryx
	fmt.Println("reference by pointer: ", *string_pointer)
	//reference the pointer with asteryx
	fmt.Println("address: ", string_pointer)
}

Error Handling with Golang

In Golang you have to specify an error handling.

For example, you can use the panic keyword:

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

func main() {

	var sum int

	//exclude for argument from Args
	for _, a := range os.Args[1:] {

		i, err := strconv.Atoi(a)

		if err != nil {
			panic(fmt.Sprintf("Invalid value : %v", err))
		}

		sum += i

	}

	fmt.Printf("Sum = %v\n", sum)

}

As you can see, if you run the code with string parameters you get an error:

go run error_handling.go 1 a c

Then you get:

panic: Invalid value : strconv.Atoi: parsing "a": invalid syntax

goroutine 1 [running]:
main.main()
        C:/Users/lliparul/Desktop/go/3_ERROR_HANDLING/error_handling.go:19 +0x159
exit status 2

Inspired by the course “Getting started with Go Programming Language” by Matthew Spaulding (Packt Publishing)

Introduction to Golang

Go was developed by some developers at Google and meant to be a better system programming language than C and C++.

It’s compiled and not interpreted.

It’s fast and suitable for concurrency.

Go uses vendoring for depedepency management. It means that the cofe of the dependencies is included in the project, once the dependency is downloaded.

Hello world program

A simple Hello world program (a file called “hello_world.go”) looks like this:

package main

import "fmt"

func main() {
	fmt.Println("Hello, World!")
}

As you can see the main function is the entry point to run golang applications.

Once golang is installed on your machine, you can simply run:

go run hello_world.go

The package fmt is a fundamental library to deal with strings, etc.

Assignments, data types, control structures

To assign values, columns and equal simbols are used. You can use data types like int, but also omit them.

a := 5  
var b int = 4

Strings always use double quotes.

fmt.Println("a is negative!")

Of course you have conditional clauses like, if and switch:

if a < 0 {
		fmt.Println("a is negative!")
	} else if a > b {
		fmt.Println("a is bigger than b")
	}

switch a {
	case 10:
		fmt.Println("It's 10")
	case 5:
		fmt.Print("It's five")
	}

And three types of for loops.

Simple loop:


	for i := 0; i < 4; i++ {
		for i := 0; i < 4; i++ {
		fmt.Println(i)
	}

Range loop:

for _, i := range []int{1, 2, 3, 4} {
		fmt.Println(i)
	}

Infinite loop:

	for {
		fmt.Println("this is infinite!")
	 }
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