2.1. Variables

Basics

Go is a statically typed language. This means that each variable gets a type on declaration which can’t be changed later.

Commonly used data types are:

• int and uint
• float32 and float64
• bool
• string
• byte (alias for uint8)
• error to return errors from functions

All Go’s predeclared identifiers are defined in the builtin package.

Declaration

The short assignment statement := declares a variable and assigns a value to it. The type of variable is inferred from the value (type inference).

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package main

import "fmt"

func main() {
	// type string inferred from "foo.txt"
	name := "foo.txt"

	// type int inferred from 42
	size := 42

	// type bool inferred from true
	isFile := true

	fmt.Println(name, size, isFile)
}

Output:
foo.txt 42 true

Once a variable is declared you can assign values to it with a regular assignment =.

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package main

import "fmt"

func main() {
	name := "foo.txt"

	name = "bar.csv"

	fmt.Println(name)
}

Output:
bar.csv

With the var keyword you can declare a variable without assigning a value to it.

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package main

import "fmt"

func main() {
	// single variable
	var name string

	// multiple variables
	var (
		size   int
		isFile bool
	)

	fmt.Println(name, size, isFile)
}

Output:
 0 false

Zero values

If you declare a variable and do not assign a value it is initialized with the zero value of its type.

The zero values are:

• 0 for numeric types
• false for booleans
• "" (empty string) for strings