In the last article ( "Arrays and Loops in Go" ), we created a programme that outputted our name, age, profession, favouriteAnimal and a list of our hobbies.

In this article we're going to extend our programme to act differently depending on the age variable. Before we get started, let's remove some code we won't be needing in this article:

package main

import "log"

func main() {

    // Start of our code

    name := "Simon"
    // age := 29

    hobbies := [4]string{"example1", "mountain biking", "example2", "example3"}

    log.Printf("Hello World. My name is %s.", name)

    log.Printf("I have %d hobbies, and they are: ", len(hobbies))

    for i := 0; i < len(hobbies); i++ {
        log.Println(hobbies[i])
    }

    // End of our code

}

Note: We've commented out the age parameter. One of the rules of the Go compiler is you must not declare any unused variables. If we had declared it ( age := 29 ) but not referenced it - it's considered unused and you will see an error when running the programme.

Running the application now, you should see the following output:

$ go run first.go 
2019/06/04 07:19:37 Hello World. My name is Simon.
2019/06/04 07:19:37 I have 4 hobbies, and they are: 
2019/06/04 07:19:37 example1
2019/06/04 07:19:37 mountain biking
2019/06/04 07:19:37 example2
2019/06/04 07:19:37 example3

We're now going to bring back the age variable and, depending on its value, we'll print one of the following:

• If the age is less than 10, we will print "I am x years young".
• If the age is more than 10, we will print "I am x years old".

To do this we're going to use an if statement. The syntax for declaring an if statement in Go is simple:

if expression {
    // Run this code if expression evaluates to true
}

To perform an action dependant on age, we can simply add two if statements to our code ( Note: we have omitted all of the code not related to age, at this stage, to focus on what we're discussing )

package main

import "log"

func main() {

    // Start of our code

    age := 29

    if age < 10 {
        log.Printf("I am %d years young", age)
    }

    if age > 10 {
        log.Printf("I am %d years old", age)
    }

    // End of our code

}

We are now performing two checks:

• Is age less than ( < ) 10? If it is, we print out the first message.
• Is age greater than ( > ) 10? If it is, we print out the second message.

Perfect! Actually, unfortunately not. There are two things wrong with this code snippet.

Problem 1

We're checking if age is less than ( < ) and greater than ( > ) 10, but what if age is 10? It doesn't call either Print statements.

$ go run conditionals.go

To fix this we first need to decide what statement we should print if the persons age is 10. I think 10 is still pretty young so let's print out the first statement. If you disagree, see if you can make it print out the second statement.

We then need to change our "less than" ( < ) condition, to be "less than or equal to" ( <= ).

package main

import "log"

func main() {

    // Start of our code

    age := 10

    if age <= 10 {
        log.Printf("I am %d years young", age)
    }

    if age > 10 {
        log.Printf("I am %d years old", age)
    }

    // End of our code

}

Now if we run our programme with an age of 10, we get our message printed!

$ go run conditionals.go
2019/06/04 17:36:14 I am 10 years young

Problem 2

Regardless of what value is given to age, we're always checking two things:

• Is the value less than or equal to 10?
• Is the value greater than 10?

In this situation age can only match one of these expressions, so we're asking the computer to do extra work. This might not seem like a big deal now, but if we grew our application to have thousands or millions of unnecessary checks we'd see a big performance hit.

Instead of two if statements, we can use an if/else statement. The syntax for declaring an if/else statement is very simple:

if expression {
    // Run this code if expression evaluates to true
} else {
    // Run this code if expression evaluates to false
}

The age variable is only ever going to be less than or equal to 10, or greater than 10, as we've previously stated. So we only need to check for one of those conditions, everything else fits in the else statement:

package main

import "log"

func main() {

    // Start of our code

    age := 10

    if age <= 10 {
        log.Printf("I am %d years young", age)
    } else {
        log.Printf("I am %d years old", age)
    }

    // End of our code

}

Take some time to have a play with the code - we'll still be here when you're ready to continue.

Ready?

Excellent. Let's take this to the next level and make our programme more "age aware". This time we're going to check for four possible outcomes:

• If the age variable is less than 13, we will print "I am considered a child"
• If the age variable is between 13 and 19, we will print "I am considered a teenager"
• If the age variable is between 19 and 69, we will print 'I am considered an adult'.
• If the age variable is 70 or older, we will print "I am considered a pensioner".

We already know having four separate if statements isn't a good idea, but an if/else statement can only handle two outcomes so what are we to do now? We can use the if/else if/else statement!

The syntax for declaring an if/else if/else statement is very simple:

if expression {
    // Run this code if expression evaluates to true
} else if anotherExpression {
    // Run this code if anotherExpression evaluates to true
} else if yetAnotherExpression {
    // Run this code if yetAnotherExpression evaluates to true
} else {
    // Run this code if none of the above expressions have evaluated to true
}

The difference between else and else if is the latter allows us to specify another expression.

If the first expression hasn't evaluated to true, then the next expression is checked. This continues until an expression does evaluate to true, an else statement or the conditional statement ends.

If an expression does evaluate to true, it skips the rest of the else if and else statements.

We can easily use this logic in our programme, without needing to learn many new concepts:

package main

import "log"

func main() {

    // Start of our code

    age := 70

    if age < 13 {
        log.Println("I am considered a child")
    } else if age >= 13 && age < 20 {
        log.Println("I am considered a teenager")
    } else if age >= 20 && age < 70 {
        log.Println("I am considered an adult")
    } else {
        log.Println("I am considered a pensioner")
    }

    // End of our code

}

Let's run through these expressions, one by one:

• if age < 13 - we check if age is less than ( < ) 13. If it is, we print out the "child" statement and don't have to perform any more checks.
• else if age >= 13 && age < 20 - we check if age is greater than or equal to ( >= ) 13 and ( && ) age is less than 20. This gives us a range of 13 - 19. If the expression evaluates to true we print out the "teenager" statement and don't have to perform any more checks.
• else if age >= 20 && age < 70 - we check if age is greater than or equal to ( >= ) 20 and ( && ) age is less than 70. This gives us a range of 20 - 69. If the expression evaluates to true we print out the "adult" statement, and don't have to perform any more checks.
• else - if age is any other value (i.e. 70 or above) we print out the "pensioner" statement.

Now let's add back the code from the beginning of this article:

package main

import "log"

func main() {

    // Start of our code

    name := "Simon"
    age := 29
    hobbies := [4]string{"example1", "mountain biking", "example2", "example3"}

    log.Printf("Hello World. My name is %s.", name)

    if age < 13 {
        log.Println("I am considered a child")
    } else if age >= 13 && age < 20 {
        log.Println("I am considered a teenager")
    } else if age >= 20 && age < 70 {
        log.Println("I am considered an adult")
    } else {
        log.Println("I am considered a pensioner")
    }

    log.Printf("I have %d hobbies, and they are: ", len(hobbies))

    for i := 0; i < len(hobbies); i++ {
        log.Println(hobbies[i])
    }

    // End of our code

}

That's it - we've now added an if/else if/else statement to our Go programme! In a future article we'll look at how to achieve this with a different conditional statement called a switch statement.

Before you go, see if you can complete the following challenge:

• It is possible to optimise the last if/else if/else statement further, see if you can figure out how. Hint: we're still asking the computer to do more work than is necessary.

If you need another hint, or want to show us your solution, get in touch on Social Media (Twitter, Facebook, LinkedIn, Instagram) or join our Slack group.