public float Calculate()
{
var expr = Expression;
Debug.WriteLine("CALC < " + expr);
if (string.IsNullOrEmpty(expr))
return 0F;
// define some calculation parameters
// division comes before multiplication to make it easy to do fractions
var precedences = new[] { "+", "-", "/", "*" };
Func<string, int> precendenceOf = s => Array.IndexOf(precedences, s);
// split the expression string into tokens
// pad operators with some spaces and so it's easy to tokenize
var tokens = new StringBuilder(expr)
.Replace("+", " + ")
.Replace("-", " - ")
.Replace("*", " * ")
.Replace("/", " / ")
.ToString()
.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);
// calculate the result
var ops = new StrStack();
var nums = new FloatStack();
var currentOps = (string)null;
// a modified version of:
// http://scriptasylum.com/tutorials/infix_postfix/algorithms/infix-postfix/index.htm
// however, we don't want the infix expression result,
// we just need the result of the calculation calculated right away
foreach (var token in tokens)
{
if (char.IsNumber(token[0]))
nums.Push(float.Parse(token));
else
{
if (currentOps != null && precendenceOf(currentOps) > precendenceOf(token))
nums.Push(flatten(nums, ops));
ops.Push(currentOps = token);
}
}
// calculate all remaining values
var result = flatten(nums, ops);
Debug.WriteLine("CALC > " + result);
return result;
}
// calculates the current state and return the result
private float flatten(FloatStack nums, StrStack ops)
{
var result = 0F;
while (ops.Count > 0)
{
var op = ops.Pop();
var opr2 = nums.Pop();
var opr1 = nums.Pop();
switch (op)
{
case "+": result = opr1 + opr2; break;
case "-": result = opr1 - opr2; break;
case "*": result = opr1 * opr2; break;
case "/": result = opr1 / opr2; break;
}
nums.Push(result);
}
return nums.Pop();
}
