/// <summary>
/// Converts a given infix expression to a binary expression tree. Each token in the expression string is delimited by spaces.
/// Valid tokens are any number representable as a double, any operator in the precedence table above, and parentheses.
/// </summary>
/// <param name="s">The expression to convert.</param>
/// <returns></returns>
public static IMathNode ConvertStringToTree(String s)
{
// step one is to convert the given string to reverse polish notation
// will use Dijkstra's shunting yard algorithm
// used tutorial of algorithm from http://www.learn4master.com/algorithms/convert-infix-notation-to-reverse-polish-notation-java
// used pseudocode but not java example
String[] tokens = s.Split(' ');
Queue RPNQueue = new Queue();
Stack OperatorStack = new Stack();
foreach (string token in tokens)
{
// if the token is a number, put it into the queue as it is
double currentNumber;
if (Double.TryParse(token, out currentNumber))
{
RPNQueue.Enqueue(currentNumber);
continue;
}
// if it's an open paren, push it onto the stack to be retrieved later
if (token == "(")
{
OperatorStack.Push(token);
continue;
}
// if it's a close paren, push everything on the stack up to the open paren into the output queue
if (token == ")")
{
while (OperatorStack.Peek() as String != "(")
{
RPNQueue.Enqueue(OperatorStack.Pop());
}
OperatorStack.Pop(); // remove the "("
continue;
}
// if the token isn't a number or paren, it must be an operator
// if the stack contains an operator with higher precedence than the current one,
// put the stack operator in the queue
// repeat until stack is empty or has an op of lower precedence than the current one
// then push current op onto stack
while (OperatorStack.Count != 0 &&
(int)precedence[OperatorStack.Peek() as String] >= (int)precedence[token])
{
RPNQueue.Enqueue(OperatorStack.Pop());
}
OperatorStack.Push(token);
}
while (OperatorStack.Count != 0)
{
RPNQueue.Enqueue(OperatorStack.Pop());
}
// step 2 is to convert the rpn into a tree
// easiest way to do that that I can think of without reading more tutorials
// is to convert from postfix to prefix
// to do that, just reverse the queue
while (RPNQueue.Count != 0)
{
OperatorStack.Push(RPNQueue.Dequeue());
}
while (OperatorStack.Count != 0)
{
RPNQueue.Enqueue(OperatorStack.Pop());
}
// helper function to recursively generate a tree
IMathNode generateTree(Queue RPN)
{
if (RPN.Peek() is double)
{
return(new NumericNode((double)RPN.Dequeue()));
}
IMathNode left, right;
switch ((string)RPN.Dequeue())
{
case "+":
right = generateTree(RPN);
left = generateTree(RPN);
return(new OperatorNode(Operation.Add, left, right));
case "-":
right = generateTree(RPN);
left = generateTree(RPN);
return(new OperatorNode(Operation.Subtract, left, right));
case "*":
right = generateTree(RPN);
left = generateTree(RPN);
return(new OperatorNode(Operation.Multiply, left, right));
case "/":
right = generateTree(RPN);
left = generateTree(RPN);
return(new OperatorNode(Operation.Divide, left, right));
case "^":
right = generateTree(RPN);
left = generateTree(RPN);
return(new OperatorNode(Operation.Exponentiate, left, right));
default:
throw new InvalidOperationException("Invalid operator in expression.");
}
}
return(generateTree(RPNQueue));
}
