Fallthrough keyword in Switch Statement in Go (Golang)

fallthrough keyword is used in switch statement in golang. This keyword is used in switch case block. If the fallthrough keyword is present in the case block, then it will transfer control to the next case even though the current case might have matched.

To more understand this let’s first look at an example without fallthrough keyword

package main

import "fmt"

func main() {
    i := 45

    switch {
    case i < 10:
        fmt.Println("i is less than 10")
    case i < 50:
        fmt.Println("i is less than 50")
    case i < 100:
        fmt.Println("i is less than 100")
    }
}

Output:

i is less than 50

By default the switch statement matches goes through all the case statement from top to bottom and tries to find the first case expression that matches the switch expression. Once the matching case is found, it exits  and does not consider the other cases. This is what is happening in above example. Even though i is less than 100 but that case is never executed because the second case is matched and after that it exits

fallthrough keyword allows way around this limitation. See below code for fallthrough keyword example. In below example even though the second case matched it went through the third case because of fallthrough keyword

package main

import "fmt"

func main() {
    i := 45
    switch {
    case i < 10:
        fmt.Println("i is less than 10")
        fallthrough
    case i < 50:
        fmt.Println("i is less than 50")
        fallthrough
    case i < 100:
        fmt.Println("i is less than 100")
    }
}

Output

i is less than 50
i is less than 100

fallthrough needs to be final statement within the switch block. If it is not then compiler raise error

fallthrough statement out of place

Below program will raise the above error as we have fmt.Println after the fallthrough statement

package main

import "fmt"

func main() {
    i := 45
    switch {
    case i < 10:
        fmt.Println("i is less than 10")
        fallthrough
    case i < 50:
        fmt.Println("i is less than 50")
        fallthrough
        fmt.Println("Not allowed")
    case i < 100:
        fmt.Println("i is less than 100")
    }
}

Break statement

Below is the break statement example.

package main

import "fmt"

func main() {
    switch char := "b"; char {
    case "a":
        fmt.Println("a")
    case "b":
        fmt.Println("b")
        break
        fmt.Println("after b")
    default:
        fmt.Println("No matching character")
    }
}

Output

b

break statement will terminate the execution of the switch and below line below will never be executed

fmt.Println("after b")

Conclusion

This is all about how fallthrough keyword in used in switch statement in golang