How To Write Your First Program in Go

Introduction

The “Hello, World!” program is a classic and time-honored tradition in computer programming. It’s a simple and complete first program for beginners, and it’s a good way to make sure your environment is properly configured.

This tutorial will walk you through creating this program in Go. However, to make the program more interesting, you’ll modify the traditional “Hello, World!” program so that it asks the user for their name. You’ll then use the name in the greeting. When you’re done with the tutorial, you’ll have a program that looks like this when you run it:


Output
Please enter your name. Sammy Hello, Sammy! I'm Go!

Prerequisites

Before you begin this tutorial, you will need a local Go development environment set up on your computer. You can set this up by following one of these tutorials:

Step 1 — Writing the Basic “Hello, World!” Program

To write the “Hello, World!” program, open up a command-line text editor such as nano and create a new file:


  • nano hello.go

Once the text file opens up in the terminal window, you’ll type out your program:

hello.go
package main

import "fmt"

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

Let’s break down the different components of the code.

package is a Go keyword that defines which code bundle this file belongs to. There can be only one package per folder, and each .go file has to declare the same package name at the top of its file. In this example, the code belongs to the main package.

import is a Go keyword that tells the Go compiler which other packages you want to use in this file. Here you import the fmt package that comes with the standard library. The fmt package provides formatting and printing functions that can be useful when developing.

fmt.Println is a Go function, found in the fmt package, that tells the computer to print some text to the screen.

You follow the fmt.Println function by a sequence of characters, like "Hello, World!", enclosed in quotation marks. Any characters that are inside of quotation marks are called a string. The fmt.Println function will print this string to the screen when the program runs.

Save and exit nano by typing CTRL + X, when prompted to save the file, press Y.

Now you can try your program.

Step 2 — Running a Go Program

With your “Hello, World!” program written, you’re ready to run the program. You’ll use the go command, followed by the name of the file you just created.


  • go run hello.go

The program will execute and display this output:


Output
Hello, World!

Let’s explore what actually happened.

Go programs need to compile before they run. When you call go run with the name of a file, in this case hello.go, the go command will compile the application and then run the resulting binary. For programs written in compiled programming languages, a compiler will take the code from a program and generate another type of lower-level code (such as source code or machine code) to produce an executable program.

Go applications require a main package and exactly one main() function that serves as the entry point for the application. The main function takes no arguments and returns no values. Instead it tells the Go compiler that the package should be compiled as an executable package.

Once compiled, the code executes by entering the main() function in the main package. It executes the line fmt.Println("Hello, World!") by calling the fmt.Println function. The string value of Hello, World! is then passed to the function. In this example, the string Hello, World! is also called an argument since it is a value that is passed to a method.

The quotes that are on either side of Hello, World! are not printed to the screen because you use them to tell Go where your string begins and ends.

In this step, you’ve created a working “Hello, World!” program with Go. In the next step, you will explore how to make the program more interactive.

Step 3 — Prompting for User Input

Every time you run your program, it produces the same output. In this step, you can add to your program to prompt the user for their name. You’ll then use their name in the output.

Instead of modifying your existing program, create a new program called greeting.go with the nano editor:


  • nano greeting.go

First, add this code, which prompts the user to enter their name:

greeting.go
package main

import (
    "fmt"
)

func main() {
  fmt.Println("Please enter your name.")
}

Once again, you use the fmt.Println function to print some text to the screen.

Now add the highlighted line to store the user’s input:

greeting.go
package main

import (
    "fmt"
)

func main() {
  fmt.Println("Please enter your name.")
  var name string
}

The var name string line will create a new variable using the var keyword. You name the variable name, and it will be of type string.

Then, add the highlighted line to capture the user’s input:

greeting.go
package main

import (
    "fmt"
)

func main() {
  fmt.Println("Please enter your name.")
  var name string
  fmt.Scanln(&name)
}

The fmt.Scanln method tells the computer to wait for input from the keyboard ending with a new line or (n), character. This pauses the program, allowing the user to enter any text they want. The program will continue when the user presses the ENTER key on their keyboard. All of the keystrokes, including the ENTER keystroke, are then captured and converted to a string of characters.

You want to use those characters in your program’s output, so you save those characters by writing them into the string variable called name. Go stores that string in your computer’s memory until the program finishes running.

Finally, add the following highlighted line in your program to print the output:

greeting.go
package main

import (
    "fmt"
)

func main() {
  fmt.Println("Please enter your name.")
  var name string
  fmt.Scanln(&name)
  fmt.Printf("Hi, %s! I'm Go!", name)
}

This time, instead of using the fmt.Println method again, you’re using fmt.Printf. The fmt.Printf function takes a string, and using special printing verbs, (%s), it injects the value of name into the string. You do this because Go does not support string interpolation, which would let you take the value assigned to a variable and place it inside of a string.

Save and exit nano by pressing CTRL + X, and press Y when prompted to save the file.

Now run the program. You’ll be prompted for your name, so enter it and press ENTER. The output might not be exactly what you expect:


Output
Please enter your name. Sammy Hi, Sammy ! I'm Go!

Instead of Hi, Sammy! I'm Go!, there’s a line break right after the name.

The program captured all of our keystrokes, including the ENTER key that we pressed to tell the program to continue. In a string, pressing the ENTER key creates a special character that creates a new line. The program’s output is doing exactly what you told it to do; it’s displaying the text you entered, including that new line. It’s not what you expected the output to be, but you can fix it with additional functions.

Open the greeting.go file in your editor:


  • nano greeting.go

Locate this line in your program:

greeting.go
...
fmt.Scanln(&name)
...

Add the following line right after it:

greeting.go
name = strings.TrimSpace(name)

This uses the TrimSpace function, from Go’s standard library strings package, on the string that you captured with fmt.Scanln. The strings.TrimSpace function removes any space characters, including new lines, from the start and end of a string. In this case, it removes the newline character at the end of the string created when you pressed ENTER.

To use the strings package you need to import it at the top of the program.

Locate these lines in your program:

greeting.go
import (
    "fmt"
)

Add the following line to import the strings package:

greeting.go
import (
    "fmt"
  "strings"
)

Your program will now contain the following:

greeting.go
package main

import (
        "fmt"
        "strings"
)

func main() {
    fmt.Println("Please enter your name.")
    var name string
    fmt.Scanln(&name)
    fmt.Printf("Hi, %s! I'm Go!", name)
    name = strings.TrimSpace(name)
}

Save and exit nano. Press CTRL + X, then press Y when prompted to save the file.

Run the program again:


  • go run greeting.go

This time, after you enter your name and press ENTER, you get the expected output:


Output
Please enter your name. Sammy Hi, Sammy! I'm Go!

You now have a Go program that takes input from a user and prints it back to the screen.

Conclusion

In this tutorial, you wrote a “Hello, World!” program that takes input from a user, processes the results, and displays the output. Now that you have a basic program to work with, try to expand your program further. For example, ask for the user’s favorite color, and have the program say that its favorite color is red. You might even try to use this same technique to create a simple Mad-Lib program.

Originally posted on DigitalOcean Community Tutorials
Author: DigitalOcean

Deja un comentario

Tu dirección de correo electrónico no será publicada. Los campos obligatorios están marcados con *