07-switch-statement.md

Lab: Switch Statement

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All the code fragments in this lab are complete mini-programs, so you can paste them into the editor and run them to see the results:

Running the code fragments

1. Right click in Explorer pane to create a new file, e.g. test.go
2. Paste the question code snippet into the editor pane
3. Open the terminal window and execute go run test.go
4. Re-use your test.go file by replacing the content with that of the next question.

Note that switch statements are nothing more than a shorthand way of writing if - else if - else if - else, where default is the final else

Questions

1. What would be the output of the following program -

   package main
   
   import "fmt"
   
   func main() {
           var a, b = 100, 5
           switch {
           case a/b == 10:
                   fmt.Println("10")
           case a/b == 20:
                   fmt.Println("20")
           case a/b == 10:
                   fmt.Println("30")
           default:
                   fmt.Println("default")
           }
   
   }

   • 30
   • 40
   • 10
   • 20

   Reveal

   20

   1. Work out a/b. 100/5 = 20
   2. Examine the case statements for the one that matches the above evaluation.
2. What would be the output of the following program -

   package main
   
   import "fmt"
   
   func main() {
           day := "sunday"
           switch day {
           case "monday":
                   fmt.Println("monday")
           case "tuesday":
                   fmt.Println("tuesday")
           case "wednesday":
                   fmt.Println("wednesday")
           case "thursday":
                   fmt.Println("thursday")
           case "friday":
                   fmt.Println("friday")
           case "saturday", "sunday":
                   fmt.Println("weekend")
           default:
                   fmt.Println("default")
           }
   }

   • friday
     sunday
   • weekend
     default
   • weekend
   • saturday
     weekend

   Reveal

   weekend

   • It will switch on the value of day which is sunday
   • It will match on the final case, which is checking for saturday or sunday
   • No default case will be selected, as we already have a match.
3. What would be the output of the following program -

   package main
   
   import "fmt"
   
   func main() {
           day := "wednesday"
           switch day {
           case "monday":
                   fmt.Println("monday")
           case "tuesday":
                   fmt.Println("tuesday")
           case "wednesday":
                   fmt.Println("wednesday")
                   fallthrough
           case "thursday":
                   fmt.Println("thursday")
                   fallthrough
           case "friday":
                   fmt.Println("friday")
           case "saturday", "sunday":
                   fmt.Println("weekend")
           default:
                   fmt.Println("default")
           }
   }

   • wednesday
     thursday
   • wednesday
     thursday
     friday
     default
   • wednesday
     thursday
     friday
   • wednesday
     friday
     default

   Reveal

   wednesday
   thursday
   friday

   • It will switch on the value of day which is wednesday
   • It will match case "wednesday"
   • It then encounters fallthrough, which means run the code in the case that follows irrespective of whether the case condition matches, thus thursday is printed. Similar for friday, but there's no fallthrough in friday's case, so default is not printed.
4. What would be the output of the following program -

   package main
   
   import "fmt"
   
   func main() {
           var a, b = 100, 5
           switch a {
           case a/b == 10:
                   fmt.Println("10")
           case a/b == 20:
                   fmt.Println("20")
           case a/b == 10:
                   fmt.Println("30")
           default:
                   fmt.Println("default")
           }
   }

   • 30
   • Error
   • 10
   • 20

   Reveal

   Error

   At first glance this looks like Q1, but there is a subtle difference! This time it's not an unbounded switch, it is switching on the value of a, and a is an int.

   The comparisons for the cases are all boolean which can't be tested against the integer switch variable a, so you will get a compilation error.

5. What would be the output of the following program -

   Note: When using switch with conditionals - we don't specify expression after switch. This is what the error was in the last question.

   package main
   
   import "fmt"
   
   func main() {
           var i, j = 10, 50
   
           switch {
           case i+j == 60:
                   fmt.Println("equal to 60")
           case i+j <= 60:
                   fmt.Println("less than or equal to 60")
                   fallthrough
           default:
                   fmt.Println("greater than 60")
           }
   }

   • less than or equal to 60
     greater than 60
   • equal to 60
   • less than or equal to 60
     equal to 60
   • less than or equal to 60
     greater than 60

   Reveal

   equal to 60

   1. Evaluate i+j - 10+50 = 60
   2. The first case matches, so equal to 60 is printed.
   3. There is no fallthrough on this case, so the switch is complete and no other case is considered.