/**
* Reads a factor from the current line of input and builds an expression tree that represents the
* expression.
*
* @return an ExpNode which is a pointer to the root node of the expression tree
* @throws ParseError if a syntax error is found in the input
*/
private static ExpNode factorTree() throws ParseError {
TextIO.skipBlanks();
char ch = TextIO.peek();
if (Character.isDigit(ch)) {
// The factor is a number. Return a ConstNode.
double num = TextIO.getDouble();
return new ConstNode(num);
} else if (ch == 'x' || ch == 'X') {
// The factor is the variable x.
TextIO.getAnyChar(); // Read the X.
return new VariableNode();
} else if (ch == '(') {
// The factor is an expression in parentheses.
// Return a tree representing that expression.
TextIO.getAnyChar(); // Read the "("
ExpNode exp = expressionTree();
TextIO.skipBlanks();
if (TextIO.peek() != ')') throw new ParseError("Missing right parenthesis.");
TextIO.getAnyChar(); // Read the ")"
return exp;
} else if (ch == '\n')
throw new ParseError("End-of-line encountered in the middle of an expression.");
else if (ch == ')') throw new ParseError("Extra right parenthesis.");
else if (ch == '+' || ch == '-' || ch == '*' || ch == '/')
throw new ParseError("Misplaced operator.");
else throw new ParseError("Unexpected character \"" + ch + "\" encountered.");
} // end factorTree()
