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!

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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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Study our 100 Scala Algorithms: 6 fully free, 100 published & 0 upcoming

Fully unit-tested, with explanations and relevant concepts; new algorithms published about once a week.

  1. Compute the length of longest valid parentheses
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  4. Remove duplicates from an unsorted List
  5. Make a queue using stacks (Lists in Scala)
  6. Find height of binary tree
  7. Single-elimination tournament tree
  8. Reverse Polish Notation calculator
  9. Quick Sort sorting algorithm in pure immutable Scala
  10. Check word in grid (depth-first search)
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  43. Count passing cars
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  46. Counting inversions of a sequence (array) using a Merge Sort
  47. Longest common prefix of strings
  48. Check if an array is a palindrome
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  52. Run-length encoding (RLE) Decoder
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  57. Find the longest palindrome within a string
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Explore the 22 most useful Scala concepts

To save you going through various tutorials, we cherry-picked the most useful Scala concepts in a consistent form.

  1. Class Inside Class
  2. Class Inside Def
  3. Collect
  4. Def Inside Def
  5. Drop, Take, dropRight, takeRight
  6. foldLeft and foldRight
  7. For-comprehension
  8. Lazy List
  9. Option Type
  10. Ordering
  11. Partial Function
  12. Pattern Matching
  13. Range
  14. scanLeft and scanRight
  15. Sliding / Sliding Window
  16. Stack Safety
  17. State machine
  18. Tail Recursion
  19. Type Class
  20. Variance
  21. View
  22. Zip

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