public Evaluator(String expression)
{
Stack<Expression> expressionStack = new Stack<Expression>();
foreach (String token in expression.Split(new string[] {" "}, StringSplitOptions.None))
{
if (token.Equals("+")) {
Expression subExpression = new Plus(expressionStack.Pop(), expressionStack.Pop());
expressionStack.Push( subExpression );
}
else if (token.Equals("-"))
{
// it's necessary remove first the right operand from the stack
Expression right = expressionStack.Pop();
// ..and after the left one
Expression left = expressionStack.Pop();
Expression subExpression = new Minus(left, right);
expressionStack.Push(subExpression);
}
else
expressionStack.Push(new Variable(token));
}
syntaxTree = expressionStack.Pop();
}
public Minus(Expression left, Expression right)
{
leftOperand = left;
rightOperand = right;
}
