Variables

A variable gives a name to a value that your program can read and update. Go variables have types, even when the compiler infers those types for you.

Use var name string = "Mina" to state a variable’s name, type and value. You can omit the type when the initial value makes the type clear: var age = 12. Inside a function, := is a shorter declaration form, covered in the next lesson.

Assignment uses =, not :=, once the variable exists. You cannot assign a string to an int variable because the types do not match.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func main() {
	var name string = "Mina"
	var age int = 12
	fmt.Println(name, "is", age)
	age = 13
	fmt.Println("Next age:", age)
}

Output

Mina is 12
Next age: 13

Go allocates variables with specific types. The first printing call reads both variables. Updating age replaces its previous value without declaring a new variable, so the second line reflects 13.

• Use = to update an existing variable, not to declare a new one without var.
• A variable declared inside a function must be used; unused locals cause compilation failure.

Add an integer variable for the number of books the student owns and print it with the name.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Go Variables in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.