Switch.md

# Switch

Spec: http://golang.org/doc/go_spec.html#Switch_statements

Go's ` switch ` statements are pretty neat. For one thing, you don't need to break at the end of each case.

```
switch c {
case '&':
	esc = "&"
case '\'':
	esc = "'"
case '<':
	esc = "&lt;"
case '>':
	esc = "&gt;"
case '"':
	esc = "&quot;"
default:
	panic("unrecognized escape character")
}
```

[src/pkg/html/escape.go](http://golang.org/src/pkg/html/escape.go#L178)

## Not just integers

Switches work on values of any type.

```
switch syscall.OS {
case "windows":
	sd = &sysDir{
		Getenv("SystemRoot") + `\system32\drivers\etc`,
		[]string{
			"hosts",
			"networks",
			"protocol",
			"services",
		},
	}
case "plan9":
	sd = &sysDir{
		"/lib/ndb",
		[]string{
			"common",
			"local",
		},
	}
default:
	sd = &sysDir{
		"/etc",
		[]string{
			"group",
			"hosts",
			"passwd",
		},
	}
}
```

## Missing expression

In fact, you don't need to switch on anything at all. A switch with no value means "switch true", making it a cleaner version of an if-else chain, as in this example from Effective Go:

```
func unhex(c byte) byte {
	switch {
	case '0' <= c && c <= '9':
		return c - '0'
	case 'a' <= c && c <= 'f':
		return c - 'a' + 10
	case 'A' <= c && c <= 'F':
		return c - 'A' + 10
	}
	return 0
}
```

## Break

Go's ` switch ` statements ` break ` implicitly, but ` break ` is still useful:

```
command := ReadCommand()
argv := strings.Fields(command)
switch argv[0] {
case "echo":
	fmt.Print(argv[1:]...)
case "cat":
	if len(argv) <= 1 {
		fmt.Println("Usage: cat <filename>")
		break
	}
	PrintFile(argv[1])
default:
	fmt.Println("Unknown command; try 'echo' or 'cat'")
}
```

## Fall through

To fall through to a subsequent case, use the ` fallthrough ` keyword:

```
// Unpack 4 bytes into uint32 to repack into base 85 5-byte.
var v uint32
switch len(src) {
default:
	v |= uint32(src[3])
	fallthrough
case 3:
	v |= uint32(src[2]) << 8
	fallthrough
case 2:
	v |= uint32(src[1]) << 16
	fallthrough
case 1:
	v |= uint32(src[0]) << 24
}
```
[src/pkg/encoding/ascii85/ascii85.go](http://golang.org/src/pkg/encoding/ascii85/ascii85.go#L43)

The 'fallthrough' must be the last thing in the case; you can't write something like

```
switch {
case f():
	if g() {
		fallthrough // Does not work!
	}
	h()
default:
	error()
}
```
However, you can work around this by using a 'labeled' `fallthrough`:

```
switch {
case f():
	if g() {
		goto nextCase // Works now!
	}
	h()
    break
nextCase:
    fallthrough
default:
	error()
}
```
## Multiple cases

If you want to use multiple values in the same case, use a comma-separated list.

```
func letterOp(code int) bool {
	switch chars[code].category {
	case "Lu", "Ll", "Lt", "Lm", "Lo":
		return true
	}
	return false
}
```
## Type switch

With a type switch you can switch on the type of an interface value (only):

```
func typeName(v interface{}) string {
	switch v.(type) {
	case int:
		return "int"
	case string:
		return "string"
	default:
		return "unknown"
	}
}
```

You can also declare a variable and it will have the type of each ` case `:

```
func do(v interface{}) string {
	switch u := v.(type) {
	case int:
		return strconv.Itoa(u*2) // u has type int
	case string:
		mid := len(u) / 2 // split - u has type string
		return u[mid:] + u[:mid] // join
	}
	return "unknown"
}

do(21) == "42"
do("bitrab") == "rabbit"
do(3.142) == "unknown"
```