# Functions in Scala

This post is **part 3** of the *Scala - The Absolute Basics* series. You can view all the posts in the series here.

## A Basic Function

No doubt you will have heard about *Functions* from other programming languages, so let’s take a look at a basic one in Scala:

```
def someFunction(a: String, b: Int): String =
a + " " + b
```

Let’s break down the function word by word:

- We first define the function with the
*def*keyword. - Then we give the function a name, in this case
*someFunction* - In brackets, we declare the parameters for the function. We also specify a type for each parameter. In this case we have a parameter called
*a*of type**String**and a parameter called*b*of type**Int** - After the brackets, we declare the return type of the function - in this case a
**String** - Finally after the equals sign we have the expression for the function itself, in this case concatenating together the parameters a and b

Call the function by supplying the required parameters:

` println(someFunction("someText", 3))`

## Functions with no Parameters

You can also create a function that has no parameters, by including an empty set of parentheses:

` def someFunctionWithNoParameters(): Int = 23`

As this is a function with no parameters, you can call it in either of the ways below.

```
println(someFunctionWithNoParameters)
println(someFunctionWithNoParameters()) // better
```

It is preferred to specify the empty set of parentheses, since this makes it clearer in your code that you are calling the function. The compiler will also display a warning about accessing an empty-paren method otherwise.

## Recursive Functions

In the previous blog post we touched on writing loops in Scala, and how it differs from imperative programming languages such as Java. In Scala, we prefer to use **Recursive Functions** (i.e. Functions that call themselves) to handle looping for us.

Let’s look at a simple example. This recursive function will print out the supplied text by the number of times specified:

```
def someRecursiveFunction(aString: String, n: Int): String = {
if (n == 1) aString
else aString + someRecursiveFunction(aString, n-1)
}
```

So if we called the function as below, it will print out *someText* 3 times:

` println(someRecursiveFunction("someText", 3))`

The function is essentially looping over itself, until it satisfies the first condition of **(n == 1)** . So the first time through the function, n is equal to 3, so we go into the else block.

Here we return the parameter **aString**, but we also call the function again, this time with n equal to 2.

Again we check if n is equal to 1, which it isn’t, so we call the else block again which returns aString again and once again call the function but this time with n equal to 1.

Finally on this call, the **if (n == 1)** returns true, so we again return **aString** and exit the function.

So to reiterate, **when you need to use loops in Scala**, *aim to use recursion.*

## Auxiliary Functions

You can also specify functions within functions, called **Auxiliary Functions**

```
def someBigFunction(n: Int): Int = {
def someSmallFunction(a: Int, b: Int): Int = a + b
someSmallFunction(n, n-1)
}
```

Inside the **someBigFunction** above, we define another function called **someSmallFunction**. When we call someBigFunction, the value returned is the expression that we get back from calling someSmallFunction with the value for n (and n-1) that we supplied.

Let’s look at an example of using an auxiliary function in practice. We will write out a function that checks if a number is a prime number (i.e. can only be divided by itself and 1)

```
def isPrime(n: Int): Boolean = {
def isPrimeUntil(t: Int): Boolean =
if (t <= 1) true
else n % t != 0 && isPrimeUntil(t-1)
isPrimeUntil(n / 2)
}
```

Our main function **isPrime** simply takes an Int and returns a Boolean. The auxiliary function also takes an Int and returns a boolean, and we call this function with the original number (n) divided by 2. (We divide the number by 2 because it saves on a lot of useless checks, see this explanation on StackOverflow)

So inside the auxiliary **isPrimeUntil** function, we first check if the number is <= 1, if so we return true. Because we have made it right the way to the bottom of the recursive checks so the original number (n) must be prime.

If **t** is not <= 1 , then we check if **n % t != 0** (I.E. if the remainder when dividing n by t is not zero - the numbers can’t divide). If that returns TRUE, then we recursively call isPrimeUntil again with t-1

## Summary

In this post, we introduced Functions in Scala for the first time. We looked at the syntax for a function, and how to call a function both with and without parameters.

We also had our first look at **Recursive Functions** . If you aren’t familiar with function programming these can take a while to get your head around. But they are an essential building block in Scala and we will be seeing a lot more of them.

Also we looked at how functions can be declared within other functions, called **Auxiliary Functions**, and saw an example of one when looking for a prime number.

## Source Code

As always, the source code for this blog post is available on Github