RPN Calculator (using Scala, python and Java)

1. Description

One of the earliest Programming Praxis challenges is called RPN Calculator . Our task is to write a RPN module capable of evaluating simple RPN expressions and return the right result . The exact requirment:

Implement an RPN calculator that takes an expression likeĀ 19 2.14 + 4.5 2 4.3 / - * which is usually expressed asĀ (19 + 2.14) * (4.5 - 2 / 4.3) and responds with 85.2974. The program should read expressions from standard input and print the top of the stack to standard output when a newline is encountered. The program should retain the state of the operand stack between expressions.

Programming Praxis

The natural algorithm to resolve this exercise is stack-based :

The first step is to tokenize the expression .If the expression is “19 2.14 + 4.5 2 4.3 / – *” after the tokenization the resulting structure will be an array of operands and operators, for example an array of Strings {“19”, “2.14”, “+”, “4.5”, “2”, “4.3”, “/”, “-“, “*”} .
At the second step we iterate over the array of tokens .If the token:

  • Is an operand (number, variable) – we push in onto the stack .
  • Is an operator (we apriopri know the operator takes N arguments), we pop N values from the stack, we evaluate the operator with those values as input parameters, we push the result back onto the stack .
If the RPN expression is valid, after we apply the previous steps, the stack would contain only one value, which is the result of the calculation .

2. Implementation

I will implement the solutions for this challenge using three different languages: Scala, python and Java .

2.1 Scala Solution

And if we compile and run the above code: Read More