Hello, World!

Lesson 1 of 24 · Part 1 · Getting started · about 15 minutes

Your first Go program does one thing: it shows a line of text on the screen. Every piece of code on this page runs right in your browser: press Run, change something, and run it again.

1package main2 3import "fmt"4 5func main() {6	fmt.Println("Hello, World!")7}

Hello, World!

Here is what each part does:

There is no semicolon at the end of the line: Go adds them for you.

Line by line

The statements in main run from top to bottom, one after another. Press Step to watch it go line by line.

1package main2 3import "fmt"4 5func main() {6	fmt.Println("First line")7	fmt.Println("Second line")8	fmt.Println("Third line")9}

First line
Second line
Third line

Println and Print

fmt.Println can show several things at once: put commas between them and it puts a space between each, numbers included. fmt.Print shows them without starting a new line at the end:

1package main2 3import "fmt"4 5func main() {6	fmt.Println("Apples:", 3)7	fmt.Println("Total:", 3+4)8	fmt.Print("No new line, ")9	fmt.Print("so this comes right after\n")10}

Apples: 3
Total: 7
No new line, so this comes right after

\n inside quotes is a new line, so Print("…\n") ends the line just as Println does.

1package main2 3import "fmt"4 5func main() {6	fmt.Print("A")7	fmt.Println("B")8	fmt.Println("C")9}

What compiling is

Go is a compiled language: your code first goes to the compiler (go build), which turns it into a program the computer can run. Only then does it run. Here, when you press Run, the page does both for you, with the same results as Go 1.24.

The compiler is strict. Import a package and never use it, and it refuses to build:

1package main2 3import (4	"fmt"5	"strings"6)7 8func main() {9	fmt.Println("Hello")10}

# command-line-arguments
./prog.go:5:2: "strings" imported and not used

Read the line with the file name: ./prog.go:5:2 is the file, line 5, character 2, and after it comes what is wrong. Unused packages and unused variables are errors in Go, not warnings: this keeps programs clean. Delete the "strings" line and run it again.

Braces on the same line

In Go the opening brace { must be on the same line as func main(). Put it on the next line and the program doesn't compile:

1package main2 3import "fmt"4 5func main()6{7	fmt.Println("Hello")8}

# command-line-arguments
./prog.go:6:1: syntax error: unexpected semicolon or newline before {

Go added a semicolon at the end of func main(), so it sees a function with no body. Every Go program is laid out the same way; the gofmt tool even rewrites your code into this layout, with tabs for indentation.

Comments

Comments are for people; the compiler skips them. From // to the end of the line is a comment, and so is everything between /* and */.

1package main2 3import "fmt"4 5func main() {6	// say hello first7	fmt.Println("Hello")8	/* not this one for now:9	fmt.Println("Goodbye")10	*/11}

Hello

Practice

Write the code, then press Check. A hint costs nothing; the answer is there when you need it.

Hello, Go!

1package main2 3import "fmt"4 5func main() {6 7}

Price: 25 Count: 4

1package main2 3import "fmt"4 5func main() {6	fmt.Println("Price:")7}

Ready

1package main2 3import (4	"fmt"5	"strings"6)7 8func main() {9	fmt.Println("Ready")10}

What you learned

• A Go program is package main with a func main(); it starts running at main.
• import "fmt" brings in fmt.Println and fmt.Print; Println puts spaces between its values and ends the line.
• Go adds the semicolons, and the { stays on the same line.
• An unused package or variable is a compile error; the error gives the file, line and character.
• // and /* */ are comments.