# Variables, Constants and Zero Values — Go (Golang)

Source: https://www.skillbyai.com/en/go/b-vars

> Every type has a useful default.

## var, :=, const

Declare variables with `var name type` or, inside functions, the short form `name := value`, which infers the type. Variables declared without a value get their type's **zero value**: 0 for numbers, "" for strings, false for booleans, nil for pointers, slices, maps, channels and interfaces, and a struct with all fields zeroed. Constants (`const`) are fixed at compile time and can be untyped. Designing types whose zero value is useful (like `sync.Mutex` or `bytes.Buffer`) is idiomatic Go.

## Types, control flow, functions, collections

Go's basics are small but have important details: zero values, multiple returns, and slice aliasing.

![Four ideas: variables and zero values, control flow, functions, slices and maps.](assets/figures/go/section-2-map.svg) — Figure 2.1 — Variables, control flow, functions and collections.

## Zero values and type inference, run

I ran this with Go 1.27.1 (`go run .` in a module named demo, standard library only). Uninitialised variables hold zero values; the struct prints with %+v showing field names, and its nil slice is still usable. %T shows inferred types.

```go
package main

import "fmt"

type User struct {
	Name   string
	Age    int
	Active bool
	Tags   []string
}

const MaxRetries = 3 // untyped constant

func main() {
	var count int
	var price float64
	var label string
	var u User
	fmt.Printf("int=%d float=%v string=%q\n", count, price, label)
	fmt.Printf("struct=%+v tags-nil=%v\n", u, u.Tags == nil)
	city := "Pune" // short declaration infers the type
	fmt.Printf("%T %T %d\n", city, 2.5, MaxRetries)
}
```

Output:

```
int=0 float=0 string=""
struct={Name: Age:0 Active:false Tags:[]} tags-nil=true
string float64 3
```

## Prefer := inside functions

The short form is idiomatic for local variables; use var when you want the zero value or an explicit type.

**Quiz:** What is the zero value of a string in Go?

- [ ] nil
- [x] The empty string ""
- [ ] "0"
- [ ] undefined

*Answer:* The empty string "". Every type has a defined zero value.