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    <p>I've kind of chucked this c# console app together as a bit of a proof of concept. Have a feeling it could be a lot better (that switch statement in GetNode is kind of clunky (it's there coz I hit a blank trying to map a class name to an operator)). Any suggestions on how it could be improved very welcome.</p> <pre><code>using System; class Program { static void Main(string[] args) { string expression = "(((3.5 * 4.5) / (1 + 2)) + 5)"; Console.WriteLine(string.Format("{0} = {1}", expression, new Expression.ExpressionTree(expression).Value)); Console.WriteLine("\nShow's over folks, press a key to exit"); Console.ReadKey(false); } } namespace Expression { // ------------------------------------------------------- abstract class NodeBase { public abstract double Value { get; } } // ------------------------------------------------------- class ValueNode : NodeBase { public ValueNode(double value) { _double = value; } private double _double; public override double Value { get { return _double; } } } // ------------------------------------------------------- abstract class ExpressionNodeBase : NodeBase { protected NodeBase GetNode(string expression) { // Remove parenthesis expression = RemoveParenthesis(expression); // Is expression just a number? double value = 0; if (double.TryParse(expression, out value)) { return new ValueNode(value); } else { int pos = ParseExpression(expression); if (pos &gt; 0) { string leftExpression = expression.Substring(0, pos - 1).Trim(); string rightExpression = expression.Substring(pos).Trim(); switch (expression.Substring(pos - 1, 1)) { case "+": return new Add(leftExpression, rightExpression); case "-": return new Subtract(leftExpression, rightExpression); case "*": return new Multiply(leftExpression, rightExpression); case "/": return new Divide(leftExpression, rightExpression); default: throw new Exception("Unknown operator"); } } else { throw new Exception("Unable to parse expression"); } } } private string RemoveParenthesis(string expression) { if (expression.Contains("(")) { expression = expression.Trim(); int level = 0; int pos = 0; foreach (char token in expression.ToCharArray()) { pos++; switch (token) { case '(': level++; break; case ')': level--; break; } if (level == 0) { break; } } if (level == 0 &amp;&amp; pos == expression.Length) { expression = expression.Substring(1, expression.Length - 2); expression = RemoveParenthesis(expression); } } return expression; } private int ParseExpression(string expression) { int winningLevel = 0; byte winningTokenWeight = 0; int winningPos = 0; int level = 0; int pos = 0; foreach (char token in expression.ToCharArray()) { pos++; switch (token) { case '(': level++; break; case ')': level--; break; } if (level &lt;= winningLevel) { if (OperatorWeight(token) &gt; winningTokenWeight) { winningLevel = level; winningTokenWeight = OperatorWeight(token); winningPos = pos; } } } return winningPos; } private byte OperatorWeight(char value) { switch (value) { case '+': case '-': return 3; case '*': return 2; case '/': return 1; default: return 0; } } } // ------------------------------------------------------- class ExpressionTree : ExpressionNodeBase { protected NodeBase _rootNode; public ExpressionTree(string expression) { _rootNode = GetNode(expression); } public override double Value { get { return _rootNode.Value; } } } // ------------------------------------------------------- abstract class OperatorNodeBase : ExpressionNodeBase { protected NodeBase _leftNode; protected NodeBase _rightNode; public OperatorNodeBase(string leftExpression, string rightExpression) { _leftNode = GetNode(leftExpression); _rightNode = GetNode(rightExpression); } } // ------------------------------------------------------- class Add : OperatorNodeBase { public Add(string leftExpression, string rightExpression) : base(leftExpression, rightExpression) { } public override double Value { get { return _leftNode.Value + _rightNode.Value; } } } // ------------------------------------------------------- class Subtract : OperatorNodeBase { public Subtract(string leftExpression, string rightExpression) : base(leftExpression, rightExpression) { } public override double Value { get { return _leftNode.Value - _rightNode.Value; } } } // ------------------------------------------------------- class Divide : OperatorNodeBase { public Divide(string leftExpression, string rightExpression) : base(leftExpression, rightExpression) { } public override double Value { get { return _leftNode.Value / _rightNode.Value; } } } // ------------------------------------------------------- class Multiply : OperatorNodeBase { public Multiply(string leftExpression, string rightExpression) : base(leftExpression, rightExpression) { } public override double Value { get { return _leftNode.Value * _rightNode.Value; } } } } </code></pre>
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    1. POExpression Evaluation and Tree Walking using polymorphism? (ala Steve Yegge)
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    1. USWilfred Knievel
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    1. CONice to see a full implementation, but I am bit confused why you combined the parsing logic with the expression tree representation, creating a tight coupling of the parsing logic with expression tree. Also, you could generate a map (driven by xml, a database, an in-code dictionary, or a custom attribute applied to each class representing a node) between your tokens and the class they map to. The problem here is the use of reflection (via Activator or some other technique) to generate your classes.
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    2. CO@Merritt mmm good point, might have another crack at this next time I have a free lunch time. Thanks for the suggestions.
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      1. USWilfred Knievel
 

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