Control flow with switch statements

8 minutes

Like other programming languages, Go supports switch statements. You use switch statements to avoid chaining multiple if statements. By using switch statements, you avoid the difficulty of maintaining and reading code that includes many if statements. These statements also make complicated conditions easier to construct. Take a look at switch statements in the following sections.

Basic switch syntax

Like the if statement, the switch condition doesn't require parentheses. In its simplest form, a switch statement looks like this:

package main

import (
    "fmt"
    "math/rand"
    "time"
)

func main() {
    sec := time.Now().Unix()
    rand.Seed(sec)
    i := rand.Int31n(10)

    switch i {
    case 0:
        fmt.Print("zero...")
    case 1:
        fmt.Print("one...")
    case 2:
        fmt.Print("two...")
    }

    fmt.Println("ok")
}

If you run the preceding code several times, you'll see a different output every time. (But if you run the code in the Go Playground, you'll get the same result every time. That's one of the service's limitations.)

Go compares each case of the switch statement until it finds a match for the condition. But notice that the previous code doesn't cover all possible cases of num variable values. If num ends up being 5, the program output is ok.

You can also be more specific about the default use case and include it like this:

switch i {
case 0:
    fmt.Print("zero...")
case 1:
    fmt.Print("one...")
case 2:
    fmt.Print("two...")
default:
    fmt.Print("no match...")
}

Notice that for the default case, you don't write a validating expression. The value of the i variable is validated against the case statements, and the default case handles any unvalidated values.

Use multiple expressions

Occasionally, more than one expression matches only one case statement. In Go, if you want a case statement to include more than one expression, separate the expressions by using commas (,). This technique allows you to avoid duplicate code.

The following code sample shows how to include multiple expressions.

package main

import "fmt"

func location(city string) (string, string) {
    var region string
    var continent string
    switch city {
    case "Delhi", "Hyderabad", "Mumbai", "Chennai", "Kochi":
        region, continent = "India", "Asia"
    case "Lafayette", "Louisville", "Boulder":
        region, continent = "Colorado", "USA"
    case "Irvine", "Los Angeles", "San Diego":
        region, continent = "California", "USA"
    default:
        region, continent = "Unknown", "Unknown"
    }
    return region, continent
}
func main() {
    region, continent := location("Irvine")
    fmt.Printf("John works in %s, %s\n", region, continent)
}

Notice that the values you include in the expressions for the case statement correspond to the data type of the variable that the switch statement validates. If you include an integer value as a new case statement, the program won't compile.

Invoke a function

A switch can also invoke a function. From that function, you can write case statements for possible return values. For example, the following code calls the time.Now() function. The output that it prints depends on the current weekday.

package main

import (
    "fmt"
    "time"
)

func main() {
    switch time.Now().Weekday().String() {
    case "Monday", "Tuesday", "Wednesday", "Thursday", "Friday":
        fmt.Println("It's time to learn some Go.")
    default:
        fmt.Println("It's the weekend, time to rest!")
    }

    fmt.Println(time.Now().Weekday().String())
}

When you call a function from a switch statement, you can modify its logic without changing the expression because you always validate what the function returns.

Also, you can call a function from a case statement. Use this technique, for instance, to match a particular pattern by using a regular expression. Here's an example:

package main

import "fmt"

import "regexp"

func main() {
    var email = regexp.MustCompile(`^[^@]+@[^@.]+\.[^@.]+`)
    var phone = regexp.MustCompile(`^[(]?[0-9][0-9][0-9][). \-]*[0-9][0-9][0-9][.\-]?[0-9][0-9][0-9][0-9]`)

    contact := "foo@bar.com"

    switch {
    case email.MatchString(contact):
        fmt.Println(contact, "is an email")
    case phone.MatchString(contact):
        fmt.Println(contact, "is a phone number")
    default:
        fmt.Println(contact, "is not recognized")
    }
}

Notice that the switch block has no validating expression. Let's talk about that concept in the next section.

Omit a condition

In Go, you can omit a condition in a switch statement like you do in an if statement. This pattern is like comparing a true value as if you were forcing the switch statement to run all the time.

Here's an example of how to write a switch statement without a condition:

package main

import (
    "fmt"
    "math/rand"
    "time"
)

func main() {
    rand.Seed(time.Now().Unix())
    r := rand.Float64()
    switch {
    case r > 0.1:
        fmt.Println("Common case, 90% of the time")
    default:
        fmt.Println("10% of the time")
    }
}

The program always runs this type of switch statement because the condition is always true. A conditional switch block can be easier to maintain than a long chain of if and else if statements.

Make the logic fall through to the next case

In some programming languages, you write a break keyword at the end of every case statement. But in Go, when the logic falls into one case, it exits the switch block unless you explicitly stop it. To make the logic fall through to the next immediate case, use the fallthrough keyword.

To understand this pattern better, look at the following code sample.

package main

import (
    "fmt"
)

func main() {
    switch num := 15; {
    case num < 50:
        fmt.Printf("%d is less than 50\n", num)
        fallthrough
    case num > 100:
        fmt.Printf("%d is greater than 100\n", num)
        fallthrough
    case num < 200:
        fmt.Printf("%d is less than 200", num)
    }
}

Run the code and analyze the output:

15 is less than 50
15 is greater than 100
15 is less than 200

Do you see anything wrong?