public static bool InfixToPostfix(Expression inputExpression, out Expression postfixExpression)
{
List<Token> postfixTokens = new List<Token>();
Stack<Token> postfixStack = new Stack<Token>();
//process all tokens in input-expression, one by one
foreach (Token token in inputExpression.Tokens)
{
if (token.Type == TokenType.Constant) //handle constants
{
postfixTokens.Add(token);
}
else if (token.Type == TokenType.Variable) //handle variables
{
postfixTokens.Add(token);
}
else if (token.Type == TokenType.Operator) //handle operators
{
if (CalcUtilities.IsOpenParenthesis(token)) //handle open-parenthesis
{
postfixStack.Push(token);
}
else if (CalcUtilities.IsCloseParenthesis(token)) //handle close-parenthesis
{
//pop all operators off the stack onto the output (until left parenthesis)
while (true)
{
if (postfixStack.Count == 0)
{
postfixExpression = null; //error: mismatched parenthesis
return (false);
}
Token top = postfixStack.Pop();
if (CalcUtilities.IsOpenParenthesis(top)) break;
else postfixTokens.Add(top);
}
}
else //handle arithmetic operators
{
Operator currentOperator = AllOperators.Find(token.LinearToken);
if (postfixStack.Count > 0)
{
Token top = postfixStack.Peek();
if (CalcUtilities.IsArithmeticOperator(top))
{
Operator stackOperator = AllOperators.Find(top.LinearToken);
if ((currentOperator.Associativity == OperatorAssociativity.LeftToRight &&
currentOperator.PrecedenceLevel >= stackOperator.PrecedenceLevel) ||
(currentOperator.Associativity == OperatorAssociativity.RightToLeft &&
currentOperator.PrecedenceLevel > stackOperator.PrecedenceLevel)) //'>' operator implies less precedence
{
postfixStack.Pop();
postfixTokens.Add(top);
}
}
}
postfixStack.Push(token); //push operator to stack
}
}
}
//after reading all tokens, pop entire stack to output
while (postfixStack.Count > 0)
{
Token top = postfixStack.Pop();
if (CalcUtilities.IsOpenParenthesis(top) || CalcUtilities.IsCloseParenthesis(top))
{
postfixExpression = null; //error: mismatched parenthesis
return (false);
}
else
{
postfixTokens.Add(top);
}
}
postfixExpression = new Expression() { Tokens = postfixTokens };
return (true);
}
public static void Validate(Expression expression, ref List<Token> postfixTokens, out bool isError, out int errorTokenIndex, out string errorMessage)
{
//initialize index of each token
int tokenIndex = 0;
foreach (Token token in expression.Tokens)
{
token.Index = tokenIndex;
tokenIndex += 1;
}
Stack<Token> postfixStack = new Stack<Token>();
foreach (Token token in expression.Tokens)
{
if (token.Type == TokenType.Constant) //handle constants
{
postfixTokens.Add(token);
}
else if (token.Type == TokenType.Variable) //handle variables
{
postfixTokens.Add(token);
}
else if (token.Type == TokenType.Operator) //handle operators
{
if (CalcUtilities.IsOpenParenthesis(token)) //handle open-parenthesis
{
postfixStack.Push(token);
}
else if (CalcUtilities.IsCloseParenthesis(token)) //handle close-parenthesis
{
//pop all operators off the stack onto the output (until left parenthesis)
while (true)
{
if (postfixStack.Count == 0)
{
isError = true;
errorTokenIndex = token.Index;
errorMessage = "Mismatched parenthesis.";
return;
}
Token top = postfixStack.Pop();
if (CalcUtilities.IsOpenParenthesis(top))
{
break;
}
else
{
postfixTokens.Add(top);
}
}
}
else //handle other operators (usually arithmetic)
{
Operator operator1 = AllOperators.Find(token.LinearToken);
while (true)
{
if (postfixStack.Count == 0)
{
break;
}
Token top = postfixStack.Peek();
if (CalcUtilities.IsArithmeticOperator(top))
{
bool readyToPopOperator2 = false;
Operator operator2 = AllOperators.Find(top.LinearToken);
if (operator1.Associativity == OperatorAssociativity.LeftToRight && operator1.PrecedenceLevel >= operator2.PrecedenceLevel) //'>' operator implies less precedence
{
readyToPopOperator2 = true;
}
else if (operator1.Associativity == OperatorAssociativity.RightToLeft && operator1.PrecedenceLevel > operator2.PrecedenceLevel)
{
readyToPopOperator2 = true;
}
if (readyToPopOperator2)
{
postfixStack.Pop();
postfixTokens.Add(top);
}
else
{
break;
}
}
else
{
break;
}
}
postfixStack.Push(token);
}
}
}
//pop entire stack to output
while (postfixStack.Count > 0)
{
Token top = postfixStack.Pop();
if (CalcUtilities.IsOpenParenthesis(top) || CalcUtilities.IsCloseParenthesis(top))
{
isError = true;
errorTokenIndex = top.Index;
errorMessage = "Mismatched Parenthesis.";
return;
}
else
{
postfixTokens.Add(top);
}
}
Stack<Token> evaluateStack = new Stack<Token>();
foreach (Token token in postfixTokens)
{
if (token.Type == TokenType.Constant)
{
evaluateStack.Push(token);
}
else if (token.Type == TokenType.Variable)
{
evaluateStack.Push(token);
}
else if (token.Type == TokenType.Operator)
{
Token top;
switch (((Operator)token.TokenObject).OperandCount)
{
case 1:
if (evaluateStack.Count >= 1)
{
top = evaluateStack.Pop();
//add a dummy constant as result of arithmetic evaluation
Token dummyConstantToken = CalcUtilities.CreateNumberConstantToken("1");
evaluateStack.Push(dummyConstantToken);
}
else
{
isError = true;
errorTokenIndex = token.Index;
errorMessage = "Missing Operand for Operator '" + token.LinearToken + "'.";
return;
}
break;
case 2:
if (evaluateStack.Count >= 2)
{
Token temp = evaluateStack.Pop();
top = evaluateStack.Pop();
//add a dummy constant as result of arithmetic evaluation
Token dummyConstantToken = CalcUtilities.CreateNumberConstantToken("1");
evaluateStack.Push(dummyConstantToken);
}
else
{
isError = true;
errorTokenIndex = token.Index;
errorMessage = "Missing Operand for Operator '" + token.LinearToken + "'.";
return;
}
break;
}
}
}
if (evaluateStack.Count == 1) //there should be exactly one token left in evaluate-stack
{
//leave the last token intact (as we are not evaluating to find the result)
isError = false;
errorTokenIndex = -1;
errorMessage = string.Empty;
}
else
{
isError = true;
errorTokenIndex = expression.Tokens.Count - 1;
errorMessage = "Incomplete Expression.";
}
}
//checks whether the input expression contains only number-constants and operators.
public static bool IsArithmeticExpression(Expression expression)
{
foreach (Token token in expression.Tokens)
{
if (!(token.Type == TokenType.Constant || token.Type == TokenType.Operator))
{
return (false);
}
}
return (true);
}
