HIGHER ORDER FUNCTIONS IN SCALA

In this blog, I’m going to explain higher-order functions.

A higher order function takes other function as a parameter or return a function as a result.

This is possible because functions are first-class value in scala. What does that mean?

It means that functions can be passed as arguments to other functions and functions can return other function.

The map function is a classic example of a higher order function.

val list = List(1,2,3)

Let’s define a function that doubles each value that is given to it.

def doubleValue = (x: Int) => x * x
doubleValue: Int => Int

val doubledList = list.map(x => doubleValue(x))

will call List(1,2,3).map(x => doubleValue(x)) and gives us a list with values (1, 4, 9).  Function map will called on each element of the list that will be passed to doubleValue.

We can also give it an anonymous function.

List(1,2,3) map (x => x + 1)

map is a function which takes a function(x => x+1) and a list as its arguments and applies given function to all the elements of the list.

Let’s create our own higher order function:

Example 1:

def addition(f: (Int, Int) => Int,a: Int, b:Int): Int = f(a,b)

addition takes higher order function as input which inturn takes two integers as input and returns an integer.

val squareSum = (x: Int, y: Int) => (x*x + y*y)
val cubeSum = (x: Int, y: Int) => (x*x*x + y*y*y)
val intSum = (x: Int, y: Int) => (x + y)

val squaredSum = addition(squareSum, 1, 2)
val cubedSum = addition(cubeSum, 1, 2)
val normalSum = addition(intSum, 1, 2)

addition(squareSum, 1, 2) will call squareSum(1,2)
addition(cubeSum, 1, 2) will call cubeSum(1,2)
addition(intSum, 1, 2) will call intSum(1,2)

Example 2:

def applyPatternToText(text:String,f:String => String): String = {
    f(text)
}

applyPatternToText functions takes another function as a parameter.

def appendTag(data:String): String => String = {
      _ : String => s"$data"
}

appendTag returns a function itself.

val message = "scala"

println( applyPatternToText(message, appendTag(message)))

will call applyPatternToText(“scala”,appendTag(“scala”))

f:String => String will be replaced by appendTag(“scala”)

appendTag(“scala”) will print scala

Example 3:

Let’s take another example to find sum of a list and product of a list using higher order function:

  def operateList(list: List[Int], f: (Int, Int) => Int, operation: String): Int = {
    def inner(list: List[Int], result: Int): Int = {
      list match {
        case head :: tail => inner(tail, f(head, result))
        case Nil => result
      }
    }

    operation.toLowerCase match {
      case "product" => inner(list, 1)
      case "sum" => inner(list, 0)
    }
  }

operateList(List(1,2,3),(a, b) => a + b, “sum”) will call inner(List(1,2,3),0).

After match condition, next call will be inner(List(2,3),f(1,0)) which will give inner(List(2,3),1).

Next call would be inner(List(3),f(2,1)) which will give inner(List(3),3)
Next call would be inner(Nil,f(3,3)) which will give inner(Nil,6)

The final result would be 6.

Thanks for reading!