Functions in Go

At the end of the Conditionals in Go article, we ended with a programme that prints your name, age, hobbies and a statement based on the age.

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 < 20 {
        log.Println("I am considered a teenager")
    } else if 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

}

We're now going to add some "friends" and, for each of them, print the aged based statement depending on their ages.

We've already got the logic we need from the Conditionals in Go snippet above, so all we need to do is duplicate this for our friends.

package main

import "log"

func main() {

    // Start of our code

    myName := "Simon"
    myAge := 29

    friendOneName := "David"
    friendOneAge := 17

    friendTwoName := "Bill"
    friendTwoAge := 42

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

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

    log.Println("I have two friends, and they are as follows:")

    log.Printf("%s: ", friendOneName)

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

    log.Printf("%s: ", friendTwoName)

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

    // End of our code

}

Let's break down the changes we've made:

• We changed our name and age variables to myName and myAge, so there is a clear distinction between our information and our friends’ information.
• We added two friends (friendOne and friendTwo) and added their names/ages in separate variables.
• We added a new print statement to record how many friends we have.
• We log each friends name and run an if/else if/else statement to print out the appropriate aged based statement.

This might look OK, but duplicating code should be avoided wherever possible.

In the above code, we have three if/else if/else statements with the only difference being the variable being checked (myAge, friendOneAge or friendTwoAge).

What if we added one hundred friends, using this method, and then decided we wanted to add another age bracket in-between adult and pensioner?

We'd have to go through all of the if/else if/else statements, one by one, to add the new logic.

Thankfully we can use functions to cut down on all of this duplication.

The syntax for declaring a function in Go is simple. In fact, we’ve already done it for our programme - the main function:

func main() {

}

Now let’s see a slightly more complex example; a function that takes two inputs, multiplies them, and prints the result.

func multiply(num1 int, num2 int) {
    log.Println(num1 * num2)
}

Let’s break this down, one step at a time:

• We define a new function, using the func keyword, and name it multiply.
• We specify two parameters (num1 and num2) and give them both a type of int.
• We declare our function body, which prints the result of num1 multiplied (*) by num2.

Now we can use it as many times as possible!

package main

import "log"

func multiply(num1 int, num2 int) {
    log.Println(num1 * num2)
}

func main() {

    multiply(5, 12) // => 60
    multiply(51, 9) // => 459
    multiply(12, 32) // => 384
    multiply(13, 42) // => 546
    multiply(31, 1) // => 31
    multiply(21, 3) // => 63
    multiply(30, 4) // => 120
    multiply(19, 5) // => 95
    multiply(9, 7) // => 63

}

But sometimes we don’t just want to print to the screen. What if we want to use the result of multiply and divide it by 10?

For this we can use the return keyword, which does what it says; it returns the result.

func multiply(num1 int, num2 int) int {
    return num1 * num2
}

We’ve now changed our function to return the result, instead of printing it to the screen.

You may have noticed we’ve also added an extra int in between the parameter declaration and function body. In the Go syntax, this is the return type and is required when returning a value from a function.

We specified it as int because two int’s multiplied (num1 * num2) yields an int.

Now we’re returning the result. If we want to use it, we need to assign it to a variable:

package main

import "log"

func multiply(num1 int, num2 int) int {
    return num1 * num2
}

func main() {

    res1 := multiply(5, 12)
    log.Println(res1) // => 60
    res2 := multiply(51, 9)
    log.Println(res2) // => 459
    res3 := multiply(12, 32)
    log.Println(res3) // => 384
    res4 := multiply(13, 42)
    log.Println(res4) // => 546
    res5 := multiply(31, 1)
    log.Println(res5) // => 31
    res6 := multiply(21, 3)
    log.Println(res6) // => 63
    res7 := multiply(30, 4)
    log.Println(res7) // => 120
    res8 := multiply(19, 5)
    log.Println(res8) // => 95
    res9 := multiply(9, 7)
    log.Println(res9) // => 63

}

At the moment we’re only printing the result, but we can use the result in many different ways:

package main

import "log"

func multiply(num1 int, num2 int) int {
    return num1 * num2
}

func main() {

    res1 := multiply(5, 12)
    log.Println(res1 * 10) // => 600
    res2 := multiply(51, 9)
    log.Println(res2 * res1) // => 27540
    res3 := multiply(12, 32)
    log.Println(res3 / 40) // => 9
    res4 := multiply(13, 42)
    log.Println(res4 / 10) // => 54
    res5 := multiply(31, 1)
    log.Println(res5 * 4) // => 124
    res6 := multiply(21, 3)
    log.Println(((res6 * 3) / 3) * 5) // => 315
    res7 := multiply(30, 4)
    log.Println(res7 / 2) // => 60
    res8 := multiply(19, 5)
    log.Println(res8 * 4) // => 380
    res9 := multiply(9, 7)
    log.Println(res9 / 9) // => 7

}

Now let’s use this new found wisdom to cut down on the duplication in our programme. Let’s go back and have a look at the code printing our friends:

log.Printf("%s: ", friendOneName)

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

log.Printf("%s: ", friendTwoName)

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

The code is exactly the same, other than the variable names (friendOneName , friendOneAge, friendTwoName and friendTwoAge). Let’s convert this into a function called printAgeBracket:

func printAgeBracket(name string, age int) {
    log.Printf("%s: ", name)

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

And now let’s use it in our code:

package main

import "log"

func printAgeBracket(name string, age int) {
    log.Printf("%s: ", name)

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

func main() {

    // Start of our code

    myName := "Simon"
    myAge := 29

    friendOneName := "David"
    friendOneAge := 17

    friendTwoName := "Bill"
    friendTwoAge := 42

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

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

    log.Println("I have two friends, and they are as follows:")

    printAgeBracket(friendOneName, friendOneAge)
    printAgeBracket(friendTwoName, friendTwoAge)

    // End of our code

}

Instead of all of that duplicated code, we’re calling our printAgeBracket function with the different variable names.

We can even add another three friends, without too much hassle:

package main

import "log"

func printAgeBracket(name string, age int) {
    log.Printf("%s: ", name)

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

func main() {

    // Start of our code

    myName := "Simon"
    myAge := 29

    friendOneName := "David"
    friendOneAge := 17

    friendTwoName := "Bill"
    friendTwoAge := 42

    friendThreeName := "Charlie"
    friendThreeAge := 12

    friendFourName := "Abby"
    friendFourAge := 24

    friendFiveName := "Edith"
    friendFiveAge := 74

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

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

    log.Println("I have five friends, and they are as follows:")

    printAgeBracket(friendOneName, friendOneAge)
    printAgeBracket(friendTwoName, friendTwoAge)
    printAgeBracket(friendThreeName, friendThreeAge)
    printAgeBracket(friendFourName, friendFourAge)
    printAgeBracket(friendFiveName, friendFiveAge)

    // End of our code

}

And now if we wanted to add another age bracket, all we have to do is amend our printAgeBracket function; that’s one place instead of however many friends we decided to add to our programme.

Note: Specifying our friends in this way (separate variables for age and name), is also not ideal. We’ll improve on this in a future article.

Challenge

You’ve probably already realised that we didn’t convert the code related to our name (myName) and age (myAge) to use the printAgeBracket function.

That’s because we print something different with our name compared to our friends; “Hello World. My name is” instead of just the name.

See if you can update the printAgeBracket function to accept a Boolean parameter and conditionally print either statement depending on it.

If you need another hint, or want to show us your solution, reach out to us and we’ll do what we can to help.

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