Mega Code Archive
This module contains the recursive descent parser that recognizes variables
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/*
C#: The Complete Reference
by Herbert Schildt
Publisher: Osborne/McGraw-Hill (March 8, 2002)
ISBN: 0072134852
*/
/*
This module contains the recursive descent
parser that recognizes variables.
*/
using System;
// Exception class for parser errors.
class ParserException : ApplicationException {
public ParserException(string str) : base(str) { }
public override string ToString() {
return Message;
}
}
class Parser {
// Enumerate token types.
enum Types { NONE, DELIMITER, VARIABLE, NUMBER };
// Enumerate error types.
enum Errors { SYNTAX, UNBALPARENS, NOEXP, DIVBYZERO };
string exp; // refers to expression string
int expIdx; // current index into the expression
string token; // holds current token
Types tokType; // holds token's type
// Array for variables.
double[] vars = new double[26];
public Parser() {
// Initialize the variables to zero.
for(int i=0; i < vars.Length; i++)
vars[i] = 0.0;
}
// Parser entry point.
public double Evaluate(string expstr)
{
double result;
exp = expstr;
expIdx = 0;
try {
GetToken();
if(token == "") {
SyntaxErr(Errors.NOEXP); // no expression present
return 0.0;
}
EvalExp1(out result); // now, call EvalExp1() to start
if(token != "") // last token must be null
SyntaxErr(Errors.SYNTAX);
return result;
} catch (ParserException exc) {
// Add other error handling here, as desired.
Console.WriteLine(exc);
return 0.0;
}
}
// Process an assignment.
void EvalExp1(out double result)
{
int varIdx;
Types ttokType;
string temptoken;
if(tokType == Types.VARIABLE) {
// save old token
temptoken = String.Copy(token);
ttokType = tokType;
// Compute the index of the variable.
varIdx = Char.ToUpper(token[0]) - 'A';
GetToken();
if(token != "=") {
PutBack(); // return current token
// restore old token -- not an assignment
token = String.Copy(temptoken);
tokType = ttokType;
}
else {
GetToken(); // get next part of exp
EvalExp2(out result);
vars[varIdx] = result;
return;
}
}
EvalExp2(out result);
}
// Add or subtract two terms.
void EvalExp2(out double result)
{
string op;
double partialResult;
EvalExp3(out result);
while((op = token) == "+" || op == "-") {
GetToken();
EvalExp3(out partialResult);
switch(op) {
case "-":
result = result - partialResult;
break;
case "+":
result = result + partialResult;
break;
}
}
}
// Multiply or divide two factors.
void EvalExp3(out double result)
{
string op;
double partialResult = 0.0;
EvalExp4(out result);
while((op = token) == "*" ||
op == "/" || op == "%") {
GetToken();
EvalExp4(out partialResult);
switch(op) {
case "*":
result = result * partialResult;
break;
case "/":
if(partialResult == 0.0)
SyntaxErr(Errors.DIVBYZERO);
result = result / partialResult;
break;
case "%":
if(partialResult == 0.0)
SyntaxErr(Errors.DIVBYZERO);
result = (int) result % (int) partialResult;
break;
}
}
}
// Process an exponent.
void EvalExp4(out double result)
{
double partialResult, ex;
int t;
EvalExp5(out result);
if(token == "^") {
GetToken();
EvalExp4(out partialResult);
ex = result;
if(partialResult == 0.0) {
result = 1.0;
return;
}
for(t=(int)partialResult-1; t > 0; t--)
result = result * (double)ex;
}
}
// Evaluate a unary + or -.
void EvalExp5(out double result)
{
string op;
op = "";
if((tokType == Types.DELIMITER) &&
token == "+" || token == "-") {
op = token;
GetToken();
}
EvalExp6(out result);
if(op == "-") result = -result;
}
// Process a parenthesized expression.
void EvalExp6(out double result)
{
if((token == "(")) {
GetToken();
EvalExp2(out result);
if(token != ")")
SyntaxErr(Errors.UNBALPARENS);
GetToken();
}
else Atom(out result);
}
// Get the value of a number or variable.
void Atom(out double result)
{
switch(tokType) {
case Types.NUMBER:
try {
result = Double.Parse(token);
} catch (FormatException) {
result = 0.0;
SyntaxErr(Errors.SYNTAX);
}
GetToken();
return;
case Types.VARIABLE:
result = FindVar(token);
GetToken();
return;
default:
result = 0.0;
SyntaxErr(Errors.SYNTAX);
break;
}
}
// Return the value of a variable.
double FindVar(string vname)
{
if(!Char.IsLetter(vname[0])){
SyntaxErr(Errors.SYNTAX);
return 0.0;
}
return vars[Char.ToUpper(vname[0])-'A'];
}
// Return a token to the input stream.
void PutBack()
{
for(int i=0; i < token.Length; i++) expIdx--;
}
// Handle a syntax error.
void SyntaxErr(Errors error)
{
string[] err = {
"Syntax Error",
"Unbalanced Parentheses",
"No Expression Present",
"Division by Zero"
};
throw new ParserException(err[(int)error]);
}
// Obtain the next token.
void GetToken()
{
tokType = Types.NONE;
token = "";
if(expIdx == exp.Length) return; // at end of expression
// skip over white space
while(expIdx < exp.Length &&
Char.IsWhiteSpace(exp[expIdx])) ++expIdx;
// trailing whitespace ends expression
if(expIdx == exp.Length) return;
if(IsDelim(exp[expIdx])) { // is operator
token += exp[expIdx];
expIdx++;
tokType = Types.DELIMITER;
}
else if(Char.IsLetter(exp[expIdx])) { // is variable
while(!IsDelim(exp[expIdx])) {
token += exp[expIdx];
expIdx++;
if(expIdx >= exp.Length) break;
}
tokType = Types.VARIABLE;
}
else if(Char.IsDigit(exp[expIdx])) { // is number
while(!IsDelim(exp[expIdx])) {
token += exp[expIdx];
expIdx++;
if(expIdx >= exp.Length) break;
}
tokType = Types.NUMBER;
}
}
// Return true if c is a delimiter.
bool IsDelim(char c)
{
if((" +-/*%^=()".IndexOf(c) != -1))
return true;
return false;
}
}
// Demonstrate the parser.
public class ParserDemo1 {
public static void Main()
{
string expr;
Parser p = new Parser();
Console.WriteLine("Enter an empty expression to stop.");
for(;;) {
Console.Write("Enter expression: ");
expr = Console.ReadLine();
if(expr == "") break;
Console.WriteLine("Result: " + p.Evaluate(expr));
}
}
}
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