This is an excerpt from the Scala Cookbook (partially modified for the internet). This is Recipe 9.9, “A real-world functional programming example in Scala.”

## Problem

Understanding functional programming concepts is one thing; putting them into practice in a real project is another. You’d like to see a real example of functional programming in Scala.

## Solution

To demonstrate some of the functional programming (FP) techniques that I introduced earlier in this chapter, the following example shows one way to implement *Newton’s Method*, a mathematical method that can be used to solve the roots of equations.

As you can see from the code, the method named `newtonsMethod`

takes functions as its first two parameters. It also takes two other `Double`

parameters, and returns a `Double`

. The two functions that are passed in should be the original equation (`fx`

) and the derivative of that equation (`fxPrime`

).

The method `newtonsMethodHelper`

also takes two functions as parameters, so you can see how the functions are passed from `newtonsMethod`

to `newtonsMethodHelper`

. Here is the complete source code for this example:

object NewtonsMethod { def main(args: Array[String]) { driver } /** * A "driver" function to test Newton's method. * Start with (a) the desired f(x) and f'(x) equations, * (b) an initial guess and (c) tolerance values. */ def driver { // the f(x) and f'(x) functions val fx = (x: Double) => 3*x + math.sin(x) - math.pow(math.E, x) val fxPrime = (x: Double) => 3 + math.cos(x) - math.pow(Math.E, x) val initialGuess = 0.0 val tolerance = 0.00005 // pass f(x) and f'(x) to the Newton's Method function, along with // the initial guess and tolerance val answer = newtonsMethod(fx, fxPrime, initialGuess, tolerance) println(answer) } /** * Newton's Method for solving equations. * @todo check that |f(xNext)| is greater than a second tolerance value * @todo check that f'(x) != 0 */ def newtonsMethod(fx: Double => Double, fxPrime: Double => Double, x: Double, tolerance: Double): Double = { var x1 = x var xNext = newtonsMethodHelper(fx, fxPrime, x1) while (math.abs(xNext - x1) > tolerance) { x1 = xNext println(xNext) // debugging (intermediate values) xNext = newtonsMethodHelper(fx, fxPrime, x1) } xNext } /** * This is the "x2 = x1 - f(x1)/f'(x1)" calculation */ def newtonsMethodHelper(fx: Double => Double, fxPrime: Double => Double, x: Double): Double = { x - fx(x) / fxPrime(x) } }

## Discussion

As you can see, a majority of this code involves defining functions, passing those functions to methods, and then invoking the functions from within a method.

The method name newtonsMethod will work for any two functions `fx`

and `fxPrime`

, where `fxPrime`

is the derivative of `fx`

(within the limits of the “to do” items that are not implemented).

To experiment with this example, try changing the functions `fx`

and `fxPrime`

, or implement the `@todo`

items in `newtonsMethod`

.

The algorithm shown comes from an old textbook titled *Applied Numerical Analysis*, by Gerald and Wheatley, where the approach was demonstrated in pseudocode.

## UPDATE: My functional programming book

For more information on functional programming, see my new book, Learning Functional Programming in Scala.