Golang’s fallthrough keyword

Background

Go has only a handful of control-flow keywords. The everyday ones are break and continue; fallthrough is another, but it shows up far less often. This note walks through how it is actually used.

Official Doc

https://go.dev/ref/spec#FallthroughStmt

A "fallthrough" statement transfers control to the first statement of the next case clause in an expression "switch" statement. It may be used only as the final non-empty statement in such a clause.

FallthroughStmt = "fallthrough" .
The "fallthrough" statement is not permitted in a type switch.

That is the official explanation — let’s look at a few examples of how to use the keyword.

sample code

fallthrough must be the last statement; for example, the code below fails to compile:
```
switch {
case i > 1:
  fallthrough
  fmt.Println("I will print 1")
}
```
The error message is: ./prog.go:13:3: fallthrough statement out of place
fallthrough hands control to the next case clause. The trap to watch for is that it does not evaluate the next case’s condition. For example:
```
 i := 3
 switch {
 case i > 1:
     fmt.Println("print 1")
     fallthrough
 case i > 4:
     fmt.Println("print 4")
 default:
     fmt.Println("default print")
 }
 fmt.Println("done")
```
Here i is 3. Without fallthrough, it would only print print 1 and then exit. With the keyword: 1) the i > 1 branch is true first, so it runs the print; 2) on hitting fallthrough, the next statement is forced to run even though i = 3 > 4 is false; 3) after running the i > 4 branch, it exits the switch.

So the output is:

print 1
print 4
done

summary

fallthrough lets you force the next case branch to run after a matched switch case, so you must not rely on the next case’s condition being true. Rewriting the last sample, you could do:

	i := 3
	switch {
	case i > 1:
		fmt.Println("print 1")
		fallthrough
	case i > 4:
	        if i > 4 {
		  fmt.Println("print 4")
	        }
	default:
		fmt.Println("default print")
	}
	fmt.Println("done")

For cleaner code, put fallthrough in the case right before default, and add the condition check inside the default branch.

Golang's fallthrough keyword

practice

Background

Official Doc

sample code

summary

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