/// <summary>
/// Initializes a new instance of the <see cref="ExpressionTree"/> class.
/// </summary>
/// <param name="expression">The arithmetic expression to be generated.</param>
public ExpressionTree(string expression)
{
bool isSingleNodeTree = true;
foreach (char op in this.Operators)
{
if (expression.Contains(op))
{
isSingleNodeTree = false;
break;
}
}
if (isSingleNodeTree)
{
try
{
this.singleNodeDouble = Convert.ToDouble(expression);
}
catch (Exception)
{
this.singleNodeString = expression;
}
return;
}
// Generate the postfix notation for the expression.
string[] postfixArray = this.ConvertToPostfix(expression);
// Declare local variables.
Node left, right;
OperatorNode center;
Stack <Node> nodeStack = new Stack <Node>();
ExpressionTreeFactory etf = new ExpressionTreeFactory();
// Iterate through each string in the postfix expression.
for (int i = 0; i < postfixArray.Length; i++)
{
string s = postfixArray[i];
// Check for an empty string.
if (s == string.Empty)
{
continue;
}
// Operator found!
if (s.Length == 1 && this.Operators.Contains(s[0]))
{
// Create a new node with the top two nodes of the stack
// as children and push it onto the top of the stack.
center = (OperatorNode)etf.CreateNode(s);
right = nodeStack.Pop();
left = nodeStack.Pop();
center.Left = left;
center.Right = right;
nodeStack.Push(center);
}
else
{
nodeStack.Push(etf.CreateNode(s)); // Push a non-operator node onto the stack.
}
}
// Set the new head of the expression tree.
this.head = (OperatorNode)nodeStack.Peek();
nodeStack.Pop();
}
/// <summary>
/// Edsger Dijkstra's Shunting Yard algorithm to convert an infix expression into postfix expression.
/// </summary>
/// <param name="infixExpression"> Infix expression. </param>
/// <returns> Postfix expression. </returns>
private Queue <Node> ShuntingYardAlgorithm(string infixExpression)
{
Queue <Node> postfixExpression = new Queue <Node>();
Stack <OperatorNode> operatorStack = new Stack <OperatorNode>();
for (int index = 0; index < infixExpression.Length;)
{
// Need to create substring until the first operator in the expression is reached.
string substring = string.Empty;
int substringIndex;
for (substringIndex = index; substringIndex < infixExpression.Length && !ExpressionTreeFactory.MatchesCharacter(infixExpression[substringIndex]); substringIndex++)
{
substring += infixExpression[substringIndex];
}
// If the substring is a number then turn that substring into one.
if (double.TryParse(substring, out double numSubstring))
{
postfixExpression.Enqueue(new ConstantNode()
{
Value = numSubstring
});
index = substringIndex;
}
else
{
if (ExpressionTreeFactory.MatchesOperator(infixExpression[index]))
{
OperatorNode op = ExpressionTreeFactory.CreateOperatorNode(infixExpression[index]);
try
{
// If there are operators on the operatorStack with higher precedence than op then they need to be removed from the stack and added to the postfixExpression first.
while (operatorStack.Peek().Precedence >= op.Precedence && operatorStack.Peek().Operator != '(')
{
postfixExpression.Enqueue(operatorStack.Pop());
}
}
catch
{
// Possible for there to be an empty operatorStack. In that case, nothing happens.
}
operatorStack.Push(op);
}
else if (infixExpression[index] == '(')
{
operatorStack.Push(new OperatorNode('(')
{
Precedence = 4
});
}
else if (infixExpression[index] == ')')
{
try
{
while (operatorStack.Peek().Operator != '(')
{
postfixExpression.Enqueue(operatorStack.Pop());
}
}
catch
{
throw new ArgumentException("Missing a left parenthesis");
}
operatorStack.Pop();
}
else
{
// If the substring cannot be parsed as a double (constant) or parentheses then it's a variable.
postfixExpression.Enqueue(new VariableNode()
{
Name = substring
});
if (!this.Variables.ContainsKey(substring))
{
this.SetVariable(substring, 0);
}
index = substringIndex - 1;
}
index++;
}
}
// End of the expression, popping all operators onto the queue
while (operatorStack.Count != 0)
{
if (operatorStack.Peek().Operator != '(')
{
postfixExpression.Enqueue(operatorStack.Pop());
}
else
{
throw new ArgumentException("Missing a right parenthesis");
}
}
return(postfixExpression);
}
