Variables, basic types and zero values

What a variable is

A variable is a name for a value your program keeps in memory, such as a price or a student's name. In Go every variable has a type, and the type never changes. A type says what kind of value fits: whole numbers, decimal numbers, text or true/false.

Two ways to declare

package main

import "fmt"

var shopName = "Sharma Stores" // package level: must use var

func main() {
	var price int = 499    // full form: var name type = value
	var qty = 3            // type inferred as int
	total := price * qty   // short form, only inside functions
	var discount int       // no value: gets the zero value 0
	fmt.Println(shopName, total, discount)
}

• var name type = value: the full form. Works everywhere.
• var name = value: Go infers (works out) the type from the value.
• name := value: the short variable declaration. Only inside functions. It is what you will write most often.
• Assignment with = changes an existing variable: total = 2000. := creates a new one.

Basic types

Zero values: no garbage, ever

In C, a variable you forget to set may hold random leftover memory. In Go every variable starts with its zero value, so it is always safe to read.

var n int        // 0
var f float64    // 0
var s string     // "" (empty string)
var ok bool      // false
var p *int       // nil (no pointer)
var list []int   // nil slice, length 0
fmt.Println(n, f, s == "", ok, p, list)

Several at once

a, b := 10, 20
a, b = b, a            // swap in one line
name, age := "Asha", 21
fmt.Println(a, b, name, age)

Integer and float maths

fmt.Println(7 / 2)     // 3   (int / int drops the fraction)
fmt.Println(7 % 2)     // 1   (remainder)
fmt.Println(7.0 / 2)   // 3.5 (a float is involved)

var small int8 = 127
small++
fmt.Println(small)     // -128 (it wrapped around)

The rules that surprise beginners

• A local variable that is declared but never used is a compile error: "declared and not used: x". Delete it, or use it.
• If you really must ignore a value, assign it to the blank identifier _ (underscore): _ = x.
• := needs at least one NEW variable on the left. x := 1 then x := 2 in the same scope fails with "no new variables on left side of :=". Use x = 2.
• Shadowing: := inside an if or for block makes a NEW variable that hides the outer one. Changes to it do not change the outer one.
• := cannot be used outside functions. At package level, write var.

count := 0
if true {
	count := 5        // a new, inner count (shadows the outer one)
	fmt.Println(count) // 5
}
fmt.Println(count)    // 0, the outer count was never changed