Partial Functions in Scala

Introduction:
When a function is not able to produce a return for every single variable input data given to it then that function is termed as Partial function. It can determine an output for a subset of some practicable inputs only. It can only be applied partially to the stated inputs.
Some important points:

• Partial functions are beneficent in understanding many inconsistent kind of Scala functions.
• It can be interpreted by utilizing case statements.
• It is a Trait, which needs two methods namely isDefinedAt and apply to be implemented.

Example:

// Scala program of
// Partial function

// Creating object
object Case
{

    // Main method
    def main(args: Array[String])
    {

        // Creating Partial function
        // using two methods
        val r = new PartialFunction[Int, Int]
        {

            // Applying isDefinedAt method
            def isDefinedAt(q: Int) = q != 0

            // Applying apply method
            def apply(q: Int) = 12 * q

        }

        // Displays output if the
        // condition is satisfied
        println(r(10))
    }
}

Output:

120

Here, two methods are defined for applying Partial function, where isDefinedAt states the condition and apply performs the operation if the given condition is satisfied.
Methods to define Partial functions:
There are some methods to define Partial function, which includes case statements, collect method, andThen, and orElse.

• Partial function using Case statement:
  we will create a Partial function below using case statement.
  Example:

  // Scala program using
  // case statements
  
  // Creating object
  object Case
  {
  
      // Main method
      def main(args: Array[String])
      {
  
          // Creating Partial function
          val d: PartialFunction[Int, Int] =
          {
  
              // using case statement
              case x if (x % 3) == 0 => x * 3
          }
  
          // Displays output if
          // the condition is
          // satisfied
          println(d(3))
      }
  }
  Output:

  9
  

  Here, Partial function is created using case statement so, apply and isDefinedAt is not required here.

• Partial function using orElse:
  This method is helpful in chaining Partial functions together.
  Example:

  // Scala program using
  // orElse
  
  // Creating object
  object orElse
  {
  
      // Main method
      def main(args: Array[String])
      {
  
          // Creating Partial function1
          val M: PartialFunction[Int, Int] =
          {
  
              // using case statement
              case x if (x % 5) == 0 => x * 5
          }
  
          // Creating Partial function2
          val m: PartialFunction[Int, Int] =
          {
  
              // using case statement
              case y if (y % 2) == 0 => y * 2
          }
  
          // chaining two partial
          // functions using orElse
          val r = M orElse m
  
          // Displays output for
          // which the given condition
          // is satisfied
          println(r(5))
          println(r(4))
      }
  }
  Output:

  25
  8
  

  Here, orElse will return output for which the given condition is satisfied.

• Partial function using Collect method:
  Collect method requests Partial function to every single element of the collection and thus, helps in constructing a new collection.
  Example:

  // Scala program using
  // collect method
  
  // Creating object
  object Collect
  {
  
      // Main method
      def main(args: Array[String])
      {
  
          // Creating Partial function
          val M: PartialFunction[Int, Int] =
          {
  
              // using case statement
              case x if (x % 5) != 0 => x * 5
          }
  
          // Applying collect method
          val y = List(7, 15, 9) collect { M }
  
          // Displays output for which
          // the given condition
          // is satisfied
          println(y)
      }
  }
  Output:

  List(35, 45)
  

  Here, Collect will apply Partial function to all the elements of the List and will return a new List on the basis of the conditions stated.

• Partial function using andThen:
  This method appends at the end of the chains, which is utilized to continue towards additional chains of Partial functions.
  Example:

  // Scala program using
  // andThen method
  
  // Creating object
  object andThen
  {
  
      // Main method
      def main(args: Array[String])
      {
  
          // Creating Partial function
          val M: PartialFunction[Int, Int] =
          {
  
              // using case statement
              case x if (x % 4) != 0 => x * 4
          }
  
          // Creating another function
          val append = (x: Int) => x * 10
  
          // Applying andThen method
          val y = M andThen append
  
          // Displays output after
          // appending the another
          // function given
          println(y(7))
      }
  }
  Output:

  280
  

  Here, andThen will append the output of Partial function with the another function given and then will return that value.