Shunting Yard in C#

• Download source code - 22.8 KB

Introduction

The Shunting yard algorithm takes an expression, e.g., 3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3 and evaluates it in RPN.

Background

I was looking for a Shunting yard algorithm in C# for a web development project to lexical analyse XHTML code. As I was unable to find anything I spent some time writing code to solve my problem. I think others can use it as well.

An good article on the Shunting yard algorithm can be found at Wikipedia http://en.wikipedia.org/wiki/Shunting-yard_algorithm .

Using the code

To use the code we have to make a new class inheriting from my class ShuntingYardBase<Result,Input>, where Result is the type returned from the code, and Input is the type of source, normally String, but can be any type including your own list of classes.

Then we have to implement some virtual functions:

This is from my demo code - a simple math evaluator.

public override double Evaluate(double result1, char opr, double result2)
{
    if (!Oprs.ContainsKey(opr))
        throw new Exception("Wrong operator!!");
    switch (opr)
    {
        case '+':
            return (double)result1 + result2;
        case '-':
            return (double)result1 - result2;
        case '*':
            return (double)result1 * result2;
        case '/':
            return (double)result1 / result2;
        case '^':
            return Math.Pow(result1, result2);
    }
    return base.Evaluate(result1, opr, result2);
}

After implementing all the virtual functions (see the code example), use the object:

ShuntingYardSimpleMath SY = new ShuntingYardSimpleMath();
String s = "3 + 4 * 2 / ( 1 - 5 ) ^ 2 ^ 3";
Console.WriteLine("input: {0}", s); Console.WriteLine();
List<String> ss = s.Split(' ').ToList();
SY.DebugRPNSteps += new ShuntingYardBase<double, string>.DebugRPNDelegate(SY_DebugRPNSteps);
SY.DebugResSteps += new ShuntingYardBase<double, string>.DebugResDelegate(SY_DebugResSteps);
Double res = SY.Execute(ss, null);
bool ok = res == 3.0001220703125;
Console.WriteLine("input: {0} = {1} {2}", s, res, (ok ? "Ok" : "Error"));
Console.ReadKey();

Points of Interest

It is interesting that this code also can be used to lexical analyze source code, C# , HTML, etc.