Often in Go a selection must be made based on the value of a variable. For an input, a special value must be returned. A switch handles this.
In Go we have versatile switch statements. We can match a variable to a constant value (even in a list). Or we can match based on expressions and logic.
To start, we use a switch with 3 cases. Each case matches an expression. So if height is less than or equal to 4, the first case is reached.
package main
import "fmt"
func main() {
height := 5
// Use switch on the height variable.
switch {
case height <= 4:
fmt.Println("Short")
case height <= 5:
fmt.Println("Normal")
case height > 5:
fmt.Println("Tall")
}
}NormalThis is another form of the switch construct. We specify a variable after the switch keyword. Then we use constants or lists of constants to match the variable.
switch case 10 is matched and the string "Even" is printed with fmt.package main
import "fmt"
func main() {
id := 10
// Use switch with multiple values in each case.
switch id {
case 10, 12, 14:
fmt.Println("Even")
case 11, 13, 15:
fmt.Println("Odd")
}
}EvenThis is a common code pattern: we use a switch to return a value in a func. We use return statements within the cases.
switch. If nothing matches, the final return is reached.package main
import "fmt"
func result(v int) int {
// Return a value based on a switch.
switch v {
case 10, 20, 30:
return v + 5
case 15, 25, 35:
return v - 5
}
return v
}
func main() {
// Call the method that uses a switch.
number := result(10)
fmt.Println(number)
number = result(25)
fmt.Println(number)
}15
20This keyword can be used in a case in a switch statement. When fallthrough is encountered, the next case is entered (even if the expression does not match).
Test() is called with the value of 0. The case 0 is reached in test(), and the func is done.test() method is called next with a value of 1. Case 1 is reached, and then case 0 is entered because control "falls through."package main
import "fmt"
func test(value int) {
switch value {
case 1:
// For 1, handle as 1 and fall-through to 0.
fmt.Println("One")
fallthrough
case 0:
// For 0, just print zero.
fmt.Println("Zero")
break
}
}
func main() {
fmt.Println(0)
test(0)
fmt.Println(1)
test(1)
}0
Zero
1
One
ZeroA switch's cases must be unique. Every constant, even in lists, is checked against the entire switch. A "duplicate case" error may be reported.
package main
import "fmt"
func main() {
value := 10
// Duplicate cases are not allowed.
switch value {
case 10:
fmt.Println(true)
case 10:
fmt.Println(true)
}
}C:\programs\file.go:14: duplicate case 10 in switch
previous case at C:\programs\file.go:12In some languages a float cannot be used in a switch. But in Go we can switch on floating-point numbers. This program shows a switch on a variable with value 2.5.
package main
import "fmt"
func main() {
value := 2.5
// Switch on a floating-point value.
switch value {
case 1.5:
fmt.Println("One point five")
case 2.5:
fmt.Println("Two point five")
}
}Two point fiveA switch can have a default case. This is reached when no other case matches. A switch can have 0 or 1 default cases, and the position in the switch statement is not important.
package main
import "fmt"
func main() {
value := 5
// Switch on the value.
switch value {
case 4:
fmt.Println("FOUR")
default:
// The default case is reached.
fmt.Println("UNKNOWN")
}
}UNKNOWNString switchThe Go language supports switch on strings. We can specify string literals in the case statements. The strings must exactly match.
package main
import "fmt"
func main() {
animal := "cat"
// Switch on string.
switch animal {
case "bird":
fmt.Println("MATCHED BIRD")
case "cat":
fmt.Println("MATCHED CAT")
}
}MATCHED CATswitchDoes the switch statement provide a huge performance boost over if? I tested a simple integer switch against an equivalent if-else chain.
switch statement. The cases in the switch are all encountered.if-statement instead of switch. The if-statements do the same thing as the switch.switch statement. On numbers, prefer switch when possible.package main
import (
"fmt"
"time"
)
func main() {
Version1()
Version2()
}
func Version1() {
result := 0
t0 := time.Now()
// Version 1: test switch statement.
for i := 0; i < 10000000; i++ {
for v := 0; v < 5; v++ {
switch v {
case 0:
result += 1
case 1, 2, 3:
result += 2
case 4:
result += 3
}
}
}
t1 := time.Now()
// Results.
fmt.Println(result)
fmt.Println(t1.Sub(t0))
}
func Version2() {
result := 0
t0 := time.Now()
// Version 2: test if-statement.
for i := 0; i < 10000000; i++ {
for v := 0; v < 5; v++ {
if v == 0 {
result += 1
} else if v == 1 || v == 2 || v == 3 {
result += 2
} else if v == 4 {
result += 3
}
}
}
t1 := time.Now()
// Results.
fmt.Println(result)
fmt.Println(t1.Sub(t0))
}100000000
53.8862ms Switch
100000000
82.807ms IfswitchWith a type switch we can detect what interfaces a variable implements. We use the (type) syntax to get the type and pass it to the switch. Then we have interface cases.
switch on an interface{} variable in the test() method. We then use (type).package main
import "fmt"
type Page interface {
PrintPage()
}
type HtmlPage struct {
// Implement Page interface.
Page
}
type Image interface {
PrintImage()
}
type ImagePage struct {
// Implement Image interface.
Image
}
func test(value interface{}) {
// Use type switch to test interface type.
// ... The argument is an interface.
switch value.(type) {
case nil:
fmt.Println("Is nil interface")
case Page:
fmt.Println("Is page interface");
case Image:
fmt.Println("Is image interface");
}
}
func main() {
// Create class that implements interface and pass to test func.
item1 := new(HtmlPage)
test(item1)
item2 := new(ImagePage)
test(item2)
}Is page interface
Is image interfaceIt is possible to use a lookup table in certain Go programs to improve performance over a switch. This can help hot loops.
Switches are emphasized in Go. We use them preferentially over if-statements. And most ifs can be rewritten elegantly with switch.