Tech Qu: Implement a post fix evaluator supporting {+,-,*,/}

Post Fix (aka Reverse Polish Notations) is what I used heavily in my student days with a HP-21 Calculator, and later HP-25. I actually have a preference for using it because of it’s superior performance and simplicity of implementation.

Code Snippet

1. public static double PostFix(IEnumerable<string> terms)
2. {
3. double first, second;
5. var work = new Stack<double>();
6. foreach (var term in terms)
7. {
8. switch (term)
9. {
10. case "+":
11. work.Push(work.Pop() + work.Pop());
12. break;
13. case "*":
14. work.Push(work.Pop() * work.Pop());
15. break;
17. case "-":
18. first = work.Pop();
19. second = work.Pop();
20. work.Push(second - first);
21. break;
22. case "/":
23. first = work.Pop();
24. second = work.Pop();
25. work.Push(second / first);
26. break;
27. default: // no error handling
28. work.Push(double.Parse(term));
29. break;
30. }
31. }
32. return work.Pop();
33. }

Unit tests are easy to write:

Code Snippet

1. char[] sep = {' ', '\t', '\r', '\n'};
2. string[] testcases = {"5 1 2 + 4 * + 3 -","10 2 /"};
3. foreach(var testcase in testcases)
4. {
5. var answer = Questions.PostFix(testcase.Split(sep, StringSplitOptions.RemoveEmptyEntries));
6. Console.WriteLine(string.Format("{0}={1}",testcase,answer));
7. }

The results are shown below.

For automated testing, just have a second array with the expected answers (or read everything in from XML).