Switch Statement
This is tutorial number 10 in Golang tutorial series.
What is a switch statement?
A switch is a conditional statement that evaluates an expression and compares it against a list of possible matches and executes the corresponding block of code. It can be considered as an idiomatic way of replacing complex if else clauses.
Example
An example program is worth a hundred words. Let's start with a simple example which will take a finger number as input and outputs the name of that finger :) . For example, 1 is thumb, 2 is index, and so on.
package main
import (
"fmt"
)
func main() {
finger := 4
fmt.Printf("Finger %d is ", finger)
switch finger {
case 1:
fmt.Println("Thumb")
case 2:
fmt.Println("Index")
case 3:
fmt.Println("Middle")
case 4:
fmt.Println("Ring")
case 5:
fmt.Println("Pinky")
}
}
In the above program switch finger in line no. 10, compares the value of finger with each of the case statements. The cases are evaluated from top to bottom and the first case which matches the expression is executed. In this case, finger has a value of 4 and hence
Finger 4 is Ring
is printed.
Duplicate cases are not allowed
Duplicate cases with the same constant value are not allowed. If you try to run the program below, the compiler will complain ./prog.go:19:7: duplicate case 4 in switch previous case at ./prog.go:17:7
package main
import (
"fmt"
)
func main() {
finger := 4
fmt.Printf("Finger %d is ", finger)
switch finger {
case 1:
fmt.Println("Thumb")
case 2:
fmt.Println("Index")
case 3:
fmt.Println("Middle")
case 4:
fmt.Println("Ring")
case 4: //duplicate case
fmt.Println("Another Ring")
case 5:
fmt.Println("Pinky")
}
}
Default case
We have only 5 fingers in our hands. What will happen if we input an incorrect finger number? This is where the default case comes into the picture. The default case will be executed when none of the other cases match.
package main
import (
"fmt"
)
func main() {
switch finger := 8; finger {
case 1:
fmt.Println("Thumb")
case 2:
fmt.Println("Index")
case 3:
fmt.Println("Middle")
case 4:
fmt.Println("Ring")
case 5:
fmt.Println("Pinky")
default: //default case
fmt.Println("incorrect finger number")
}
}
In the above program finger is 8 and it does not match any of the cases and hence incorrect finger number in the default case is printed. It's not necessary that default should be the last case in a switch statement. It can be present anywhere in the switch.
You might also have noticed a small change in the declaration of finger. It is declared in the switch itself. A switch can include an optional statement that is executed before the expression is evaluated. In line no. 8, finger is first declared and then used in the expression. The scope of finger in this case is limited to the switch block.
Multiple expressions in case
It is possible to include multiple expressions in a case by separating them with comma.
package main
import (
"fmt"
)
func main() {
letter := "i"
fmt.Printf("Letter %s is a ", letter)
switch letter {
case "a", "e", "i", "o", "u": //multiple expressions in case
fmt.Println("vowel")
default:
fmt.Println("not a vowel")
}
}
The above program finds whether letter is a vowel or not. The code case "a", "e", "i", "o", "u": in line no. 11 matches any of the vowels. Since i is a vowel, this program prints
Letter i is a vowel
Expressionless switch
The expression in a switch is optional and it can be omitted. If the expression is omitted, the switch is considered to be switch true and each of the case expression is evaluated for truth and the corresponding block of code is executed.
package main
import (
"fmt"
)
func main() {
num := 75
switch { // expression is omitted
case num >= 0 && num <= 50:
fmt.Printf("%d is greater than 0 and less than 50", num)
case num >= 51 && num <= 100:
fmt.Printf("%d is greater than 51 and less than 100", num)
case num >= 101:
fmt.Printf("%d is greater than 100", num)
}
}
In the above program, the expression is absent in switch and hence it is considered as true and each of the cases is evaluated. case num >= 51 && num <= 100: in line no. 12 is true and the program prints
75 is greater than 51 and less than 100
This type of switch can be considered as an alternative to multiple if else clauses.
Fallthrough
In Go, the control comes out of the switch statement immediately after a case is executed. A fallthrough statement is used to transfer control to the first statement of the case that is present immediately after the case which has been executed.
Let's write a program to understand fallthrough. Our program will check whether the input number is less than 50, 100, or 200. For instance, if we input 75, the program will print that 75 is less than both 100 and 200. We will achieve this using fallthrough.
package main
import (
"fmt"
)
func number() int {
num := 15 * 5
return num
}
func main() {
switch num := number(); { //num is not a constant
case num < 50:
fmt.Printf("%d is lesser than 50\n", num)
fallthrough
case num < 100:
fmt.Printf("%d is lesser than 100\n", num)
fallthrough
case num < 200:
fmt.Printf("%d is lesser than 200", num)
}
}
Switch and case expressions need not be only constants. They can be evaluated at runtime too. In the program above num is initialized to the return value of the function number() in line no. 14. The control comes inside the switch and the cases are evaluated. case num < 100: in line no. 18 is true and the program prints 75 is lesser than 100. The next statement is fallthrough. When fallthrough is encountered the control moves to the first statement of the next case and also prints 75 is lesser than 200. The output of the program is
75 is lesser than 100
75 is lesser than 200
fallthrough should be the last statement in a case. If it is present somewhere in the middle, the compiler will complain that fallthrough statement out of place.
Fallthrough happens even when the case evaluates to false
There is a subtlety to be considered when using fallthrough. Fallthrough will happen even when the case evaluates to false.
Please consider the following program.
package main
import (
"fmt"
)
func main() {
switch num := 25; {
case num < 50:
fmt.Printf("%d is lesser than 50\n", num)
fallthrough
case num > 100:
fmt.Printf("%d is greater than 100\n", num)
}
}
In the above program, num is 25 which is less than 50 and hence the case in line no. 9 evaluates to true. A fallthrough is present in line no. 11. The next case case num > 100: in line no. 12 is false since num < 100. But fallthrough doesn't consider this. Fallthrough will happen even though the case evaluates to false.
The program above will print
25 is lesser than 50
25 is greater than 100
So be sure that you understand what you are doing when using fallthrough.
One more thing is fallthrough cannot be used in the last case of a switch since there are no more cases to fallthrough. If fallthrough is present in the last case, it will result in the following compilation error.
cannot fallthrough final case in switch
Breaking switch
The break statement can be used to terminate a switch early before it completes. Let's just modify the above example to a contrived one to understand how break works.
Let's add a condition that if num is less than 0 then the switch should terminate.
package main
import (
"fmt"
)
func main() {
switch num := -5; {
case num < 50:
if num < 0 {
break
}
fmt.Printf("%d is lesser than 50\n", num)
fallthrough
case num < 100:
fmt.Printf("%d is lesser than 100\n", num)
fallthrough
case num < 200:
fmt.Printf("%d is lesser than 200", num)
}
}
In the above program num is -5. When the control reaches the if statement in line no. 10, the condition is satisfied since num < 0. The break statement terminates the switch before it completes and the program doesn't print anything :).
Breaking the outer for loop
When the switch case is inside a for loop, there might be a need to terminate the for loop early. This can be done by labeling the for loop and breaking the for loop using that label inside the switch statement. Let's look at an example.
Let's write a program to generate a random even number.
We will create an infinite for loop and use a switch case to determine whether the generated random number is even. If it is even, the generated number is printed and the for loop is terminated using its label. The Intn function of the rand package is used to generate non-negative pseudo-random numbers.
package main
import (
"fmt"
"math/rand"
)
func main() {
randloop:
for {
switch i := rand.Intn(100); {
case i%2 == 0:
fmt.Printf("Generated even number %d", i)
break randloop
}
}
}
In the program above, the for loop is labeled randloop in line no. 9. A random number is generated between 0 and 99 (100 is not included) using the Intn function in line no. 11. If the generated number is even, the loop is broken in line no. 14 using the label.
This program prints,
Generated even number 18
Please note that if the break statement is used without the label, the switch statement will only be broken and the loop will continue running. So labeling the loop and using it in the break statement inside the switch is necessary to break the outer for loop.
This brings us to the end of this tutorial. There is one more type of switch called type switch. We will look into this when we learn about interfaces.
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