[Go] switch - fallthrough

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.

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

ackage 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

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") // ERROR
    case i < 100:
        fmt.Println("i is less than 100")
    }
}

error as we have fmt.Println after the fallthrough statement

[Notice] be careful when use fallthrough, it seems that fallthrough will enter next case block no matter the condition. That might cause suprise bug in code;

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 < 20:
		fmt.Println("i is less than 20")
	}
}

// i is less than 50
// i is less than 20