Functions

Welcome to tutorial no. 6 in our Golang tutorial series.

What is a function?

A function is a block of code that performs a specific task. A function takes an input, performs some calculations on the input, and generates an output.

Function declaration

The syntax for declaring a function in go is

func functionname(parametername type) returntype {  
 //function body
}

The function declaration starts with a func keyword followed by the functionname. The parameters are specified between ( and ) followed by the returntype of the function. The syntax for specifying a parameter is, parameter name followed by the type. Any number of parameters can be specified like (parameter1 type, parameter2 type). Then there is a block of code between { and } which is the body of the function.

The parameters and return type are optional in a function. Hence the following syntax is also a valid function declaration.

func functionname() {  
}

Sample Function

Let's write a function that takes the price of a single product and the number of products as input parameters and calculates the total price by multiplying these two values and returns the output.

func calculateBill(price int, no int) int {  
    var totalPrice = price * no
    return totalPrice
}

The above function has two input parameters price and no of type int and it returns the totalPrice which is the product of price and no. The return value is also of type int.

If consecutive parameters are of the same type, we can avoid writing the type each time and it is enough to be written once at the end.ie price int, no int can be written as price, no int. The above function can hence be rewritten as,

func calculateBill(price, no int) int {  
    var totalPrice = price * no
    return totalPrice
}

Now that we have a function ready, let's call it from somewhere in the code. The syntax for calling a function is functionname(parameters). The above function can be called using the code.

calculateBill(10, 5)  

Here is the complete program which uses the above function and prints the total price.

package main

import (  
    "fmt"
)

func calculateBill(price, no int) int {  
    var totalPrice = price * no
    return totalPrice
}

func main() {  
    price, no := 90, 6
    totalPrice := calculateBill(price, no)
    fmt.Println("Total price is", totalPrice)
}

Run in playground

The above program will print

Total price is 540  

Multiple return values

It is possible to return multiple values from a function. Let's write a function rectProps which takes the length and width of a rectangle and returns both the area and perimeter of the rectangle. The area of the rectangle is the product of length and width and the perimeter is twice the sum of the length and width.

package main

import (  
    "fmt"
)

func rectProps(length, width float64)(float64, float64) {  
    var area = length * width
    var perimeter = (length + width) * 2
    return area, perimeter
}

func main() {  
     area, perimeter := rectProps(10.8, 5.6)
    fmt.Printf("Area %f Perimeter %f", area, perimeter) 
}

Run in playground

If a function returns multiple return values then they must be specified between ( and ). func rectProps(length, width float64)(float64, float64) has two float64 parameters length and width and also returns two float64 values. The above program prints

Area 60.480000 Perimeter 32.800000  

Named return values

It is possible to return named values from a function. If a return value is named, it can be considered as being declared as a variable in the first line of the function.

The above rectProps can be rewritten using named return values as

func rectProps(length, width float64)(area, perimeter float64) {  
    area = length * width
    perimeter = (length + width) * 2
    return //no explicit return value
}

area and perimeter are the named return values in the above function. Note that the return statement in the function does not explicitly return any value. Since area and perimeter are specified in the function declaration as return values, they are automatically returned from the function when a return statement is encountered.

Blank Identifier

_ is known as the blank identifier in Go. It can be used in place of any value of any type. Let's see what's the use of this blank identifier.

The rectProps function returns the area and perimeter of the rectangle. What if we only need the area and want to discard the perimeter. This is where _ is of use.

The program below uses only the area returned from the rectProps function.

package main

import (  
    "fmt"
)

func rectProps(length, width float64) (float64, float64) {  
    var area = length * width
    var perimeter = (length + width) * 2
    return area, perimeter
}
func main() {  
    area, _ := rectProps(10.8, 5.6) // perimeter is discarded
    fmt.Printf("Area %f ", area)
}

Run in playground

In line no. 13 we use only the area and the _ identifier is used to discard the perimeter.

This brings us to the end of this tutorial. Please leave your valuable comments and feedback. Thanks for reading.

Next tutorial - Packages