# Scala algorithm: Reverse Polish Notation calculator

Published

## Algorithm goal

Reverse Polish notation, which looks like '1 2 +' to do '1 + 2' enables you to write arithmetic without having to use parentheses.

Create an evaluator for RPN.

## Test cases in Scala

``````assert(compute("1 2 +") == 3)
assert(compute("10 4 /") == 2.5)
assert(compute("-10 2 /") == -5)
assert(compute("1 2 + 5 / 10 *") == 6.0)
``````

## Algorithm in Scala

31 lines of Scala (compatible versions 2.13 & 3.0), showing how concise Scala can be!

## Explanation

We split the expression into constituent parts. We use a foldLeft and foldRight to effectively do a while-loop while keeping a stateful variable, which is the stack. The end of the stack is eventually the result. (this is © from www.scala-algorithms.com)

## Scala concepts & Hints

1. ### foldLeft and foldRight

A 'fold' allows you to perform the equivalent of a for-loop, but with a lot less code.

``````def foldMutable[I, O](initialState: O)(items: List[I])(f: (O, I) => O): O =
items.foldLeft(initialState)(f)
``````
2. ### Option Type

The 'Option' type is used to describe a computation that either has a result or does not. In Scala, you can 'chain' Option processing, combine with lists and other data structures. For example, you can also turn a pattern-match into a function that return an Option, and vice-versa!

``````assert(Option(1).flatMap(x => Option(x + 2)) == Option(3))

assert(Option(1).flatMap(x => None) == None)
``````
3. ### Partial Function

A Partial Function in Scala is similar to function type `A => Option[B]` (Option Type).

``````def getNaming(num: Int): Option[String] =
PartialFunction.condOpt(num) { case 1 => "One" }

assert(getNaming(1) == Some("One"))

assert(getNaming(2) == None)
``````
4. ### Pattern Matching

Pattern matching in Scala lets you quickly identify what you are looking for in a data, and also extract it.

``````assert("Hello World".collect {
case character if Character.isUpperCase(character) => character.toLower
} == "hw")
``````
5. ### Stack Safety

Stack safety is present where a function cannot crash due to overflowing the limit of number of recursive calls.

This function will work for n = 5, but will not work for n = 2000 (crash with java.lang.StackOverflowError) - however there is a way to fix it :-)

In Scala Algorithms, we try to write the algorithms in a stack-safe way, where possible, so that when you use the algorithms, they will not crash on large inputs. However, stack-safe implementations are often more complex, and in some cases, overly complex, for the task at hand.

``````def sum(from: Int, until: Int): Int =
if (from == until) until else from + sum(from + 1, until)

def thisWillSucceed: Int = sum(1, 5)

def thisWillFail: Int = sum(1, 300)
``````

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