30-seconds-of-code/snippets/solveRPN.md

solveRPN

Solves the given mathematical expression in reverse polish notation. Throws appropriate errors if there are unrecognized symbols or the expression is wrong.

Use a dictionary, OPERATORS to specify each operator's matching mathematical operation. Use String.replace() with a regular expression to replace ^ with **, String.split() to tokenize the string and Array.filter() to remove empty tokens. Use Array.forEach() to parse each symbol, evaluate it as a numeric value or operator and solve the mathematical expression. Numeric values are converted to floating point numbers and pushed to a stack, while operators are evaluated using the OPERATORS dictionary and pop elements from the stack to apply operations.

const solveRPN = rpn => {
  const OPERATORS = {'*' : (a,b) => a * b, '+' : (a,b) => a + b, '-' : (a,b) => a - b, '/' : (a,b) => a / b, '**': (a,b) => a ** b};
  const [stack,solve] = [[],rpn.replace(/\^/g,'**').split(/\s+/g).filter(el => !/\s+/.test(el) && el !== '')];
  solve.forEach(symbol => {
    if(!isNaN(parseFloat(symbol)) && isFinite(symbol)) {stack.push(symbol)}
          else if (Object.keys(OPERATORS).includes(symbol)) {
          const [a,b] = [stack.pop(),stack.pop()];
          stack.push(OPERATORS[symbol](parseFloat(b),parseFloat(a)))
        } else { throw `${symbol} is not a recognized symbol` }
  }
)
 if(stack.length === 1)  return stack.pop()
  else  throw `${rpn} is not a proper RPN. Please check it and try again`
}

solveRPN('15 7 1 1 + - / 3 * 2 1 1 + + -'); // 5
solveRPN('3 5 6 + *'); //33
solveRPN('2 4 / 5 6 - *'); //-0.5
solveRPN('2 3 ^'); //8
solveRPN('2 3 ^'); //8