Task: Parsing_RPN calculator algorithm
C: #include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

void die(const char *msg)
{
	fprintf(stderr, "%s", msg);
	abort();
}

#define MAX_D 256
double stack[MAX_D];
int depth;

void push(double v)
{
	if (depth >= MAX_D) die("stack overflow\n");
	stack[depth++] = v;
}

double pop()
{
	if (!depth) die("stack underflow\n");
	return stack[--depth];
}

double rpn(char *s)
{
	double a, b;
	int i;
	char *e, *w = " \t\n\r\f";

	for (s = strtok(s, w); s; s = strtok(0, w)) {
		a = strtod(s, &e);
		if (e > s)		printf(" :"), push(a);
#define binop(x) printf("%c:", *s), b = pop(), a = pop(), push(x)
		else if (*s == '+')	binop(a + b);
		else if (*s == '-')	binop(a - b);
		else if (*s == '*')	binop(a * b);
		else if (*s == '/')	binop(a / b);
		else if (*s == '^')	binop(pow(a, b));
#undef binop
		else {
			fprintf(stderr, "'%c': ", *s);
			die("unknown oeprator\n");
		}
		for (i = depth; i-- || 0 * putchar('\n'); )
			printf(" %g", stack[i]);
	}

	if (depth != 1) die("stack leftover\n");

	return pop();
}

int main(void)
{
	char s[] = " 3 4 2 * 1 5 - 2 3 ^ ^ / + ";
	printf("%g\n", rpn(s));
	return 0;
}

C#: using System;
using System.Collections.Generic;
using System.Linq;
using System.Globalization;
using System.Threading;

namespace RPNEvaluator
{
    class RPNEvaluator
    {
        static void Main(string[] args)
        {
            Thread.CurrentThread.CurrentCulture = CultureInfo.InvariantCulture;

            string rpn = "3 4 2 * 1 5 - 2 3 ^ ^ / +";
            Console.WriteLine("{0}\n", rpn);

            decimal result = CalculateRPN(rpn);
            Console.WriteLine("\nResult is {0}", result);
        }

        static decimal CalculateRPN(string rpn)
        {
            string[] rpnTokens = rpn.Split(' ');
            Stack<decimal> stack = new Stack<decimal>();
            decimal number = decimal.Zero;

            foreach (string token in rpnTokens)
            {
                if (decimal.TryParse(token, out number))
                {
                    stack.Push(number);
                }
                else
                {
                    switch (token)
                    {
                        case "^":
                        case "pow":
                            {
                                number = stack.Pop();
                                stack.Push((decimal)Math.Pow((double)stack.Pop(), (double)number));
                                break;
                            }
                        case "ln":
                            {
                                stack.Push((decimal)Math.Log((double)stack.Pop(), Math.E));
                                break;
                            }
                        case "sqrt":
                            {
                                stack.Push((decimal)Math.Sqrt((double)stack.Pop()));
                                break;
                            }
                        case "*":
                            {
                                stack.Push(stack.Pop() * stack.Pop());
                                break;
                            }
                        case "/":
                            {
                                number = stack.Pop();
                                stack.Push(stack.Pop() / number);
                                break;
                            }
                        case "+":
                            {
                                stack.Push(stack.Pop() + stack.Pop());
                                break;
                            }
                        case "-":
                            {
                                number = stack.Pop();
                                stack.Push(stack.Pop() - number);
                                break;
                            }
                        default:
                            Console.WriteLine("Error in CalculateRPN(string) Method!");
                            break;
                    }
                }
                PrintState(stack);
            }

            return stack.Pop();
        }

        static void PrintState(Stack<decimal> stack)
        {
            decimal[] arr = stack.ToArray();

            for (int i = arr.Length - 1; i >= 0; i--)
            {
                Console.Write("{0,-8:F3}", arr[i]);
            }
            
            Console.WriteLine();
        }
    }
}