Lesson 2 · 2 of 20

Variables and types · Go

Store data in Go variables with var and :=, meet the basic types (int, float64, string and bool) and the zero value of each one.

Go · Variables and types in DMarlon Studio: the example's code and its output in the terminal

A variable is a name for a piece of data. var declares one with its type: var city string = "Valencia" creates a string and var population int = 800000 a whole number. In Go the type goes after the name.

The short form := declares the variable and works out the type from the value: temperature := 21.5 is a float64 (a number with decimals) and isCoastal := true is a bool (true or false). It only works inside a function, and it is the form you will see most.

A variable declared without a value gets its zero value: 0 for numbers, "" (empty string) for string and false for bool. That is why visitors is 0 and note comes out empty between the brackets.

%T inside fmt.Printf prints the type of each value; lesson 4 covers the rest of these verbs. At the end, = (without the colon) gives an existing variable a new value, always of the same type, and visitors += 1500 adds 1500 to it.

Go does not let you declare a variable and leave it unused: the program would not compile. It is one of the rules the language uses to keep code tidy.

Code · main.go

package mainimport "fmt"func main() {	// var declares a variable with its type.	var city string = "Valencia"	var population int = 800000	// := declares it and takes the type from the value.	temperature := 21.5	isCoastal := true	// Without a value, a variable starts at its zero value.	var visitors int	var note string	fmt.Println("City:", city)	fmt.Println("Population:", population)	fmt.Println("Temperature:", temperature)	fmt.Println("Coastal:", isCoastal)	fmt.Println("Visitors:", visitors, "Note: ["+note+"]")	// %T prints the type of each value.	fmt.Printf("Types: %T %T %T %T\n", city, population, temperature, isCoastal)	// = gives a new value of the same type.	population = 810000	visitors += 1500	fmt.Println("Now:", population, "people and", visitors, "visitors")}

Output · go run main.go

City: Valencia
Population: 800000
Temperature: 21.5
Coastal: true
Visitors: 0 Note: []
Types: string int float64 bool
Now: 810000 people and 1500 visitors

Key points

var name type = value declares with the type in sight.
name := value declares and infers the type.
Basic types: int, float64, string and bool.
Without a value, every variable starts at its zero value.
:= creates the variable; = changes its value.

Exercise

Add country := "Spain" and area := 134.6 (the area in km²), print them with Println and check with %T which type Go inferred for each one.