# 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 Â© from www.scala-algorithms.com)

## Scala concepts & Hints

### foldLeft and foldRight

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

### 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!

### Partial Function

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

### Pattern Matching

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

### 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.