Ok I have a PHP RDR for the language:
EXP ::= EXP + TERM | EXP - TERM | TERM
TERM ::= TERM * FACTOR | TERM / FACTOR | FACTOR
FACTOR ::= ( EXP ) | DIGIT
DIGIT ::= 0 | 1 | 2 | 3
That should be able to accept the input 1+2*2$. However, when the input is entered and the form is accessed there is no output. Same with (1+2)-2$. I have an echo at the beginning of the PHP form making sure that it is accessed. Also, I can get grammar's like 1$ to work, just not when there is an multiple operators such as more than one of +,-,*,/, also doesn't work with (, or ). I believe that the problem occurs in the exper, term or factor function. I just can't get a grasp on it. The HTML page accepts the user input, and here is the HTML code:
<!DOCTYPE html>
<html>
<head>
<title>PHP Recursive Descent Parser</title>
</head>
<body>
<center><form action="rdrTest2.php" method="post">
<input type="text" name="userInput" />
<input type="submit" name="sub" value="submit" />
</form></center>
</body>
</html>
Here is the PHP code:
<?php
echo "PHP Form Accessed.<br>";
$input = $_POST["userInput"];
$parser = new RDR4($input);
abstract class Grammar{
// User input - string
protected $inputString;
//Pointer pointing to current position in input string
protected $pointerInString;
// boolean variable which will return true or false based on parsing result
protected $resultString;
//end of string variable '$ - in this case'.
protected $endOfString;
/**
* Recursive Descent Parser
* This function will get overridden by child classes
*/
abstract protected function exper();
function __construct($input, $delimiter = '$') {
$this->inputString = $input; // user input string taken from input page
$this->pointerInString = 0; // initial pointer value will be 0 - pointer pointing to first character in input string
$this->resultString = true; // it will be set to false if program can not match string to the expected at any point in time while execution
$this->endOfString = $delimiter;
$this->exper(); // starting point for each parsing
if(!$this->endOfInput())
$this->resultString = false; // this means the string contains some unparsable character
}
/*
* True if expression is resultString else False
*/
function isresultString() {
return $this->resultString;
}
/*
*/
protected function endOfInput() {
// check for end of the string
$isDone = ($this->pointerInString >= strlen($this->inputString)) || (strlen($this->inputString) == 0);
if($this->pointerInString == (strlen($this->inputString) - 1)) {
if($this->inputString[$this->pointerInString] == $this->endOfString) {
$isDone = true;
}
echo "Grammar is valid.";
return $isDone;
}
}
/*
* match function basically matches character with current pointer character
* if matches, it will advance pointer to next character and return true.
*/
protected function match($myToken) {
if(($this->pointerInString < strlen($this->inputString)) &&
($this->inputString[$this->pointerInString] == $myToken))
{
$this->pointerInString += 1;
return true;
}
else
return false;
}
}
/**
* Grammar for RDR4 is:
* EXPER ::= EXPER + TERM | EXPER - TERM | TERM
* TERM ::= TERM * FACTOR | EXPER / FACTOR | FACTOR
* DIGIT ::= 0 | 1 | 2 | 3
* Assume the input ends with '$'.
*/
class RDR4 extends Grammar {
function exper() {
$this->term();
if($this->endOfInput())
{
$this->resultString = false;
echo "Grammar is invalid.";
}
else
{
if($this->resultString)
{
if(($this->inputString[$this->pointerInString] == '+') || ($this->inputString[$this->pointerInString] == '-'))
{
$this->match($this->inputString[$this->pointerInString]);
}
//echo "About to call term in exper.";
$this->term();
}
}
}
function term() {
if($this->endOfInput())
{
$this->resultString = false;
echo "Grammar is invalid.";
}
else
{
if($this->resultString)
{
if(($this->inputString[$this->pointerInString] == '*') || ($this->inputString[$this->pointerInString] == '/'))
{
$this->match($this->inputString[$this->pointerInString]);
}
//echo "About to call factor in term.";
$this->factor();
}
}
}
function factor() {
if($this->endOfInput()) {
$this->resultString = false;
echo "Grammar is invalid.";
}
if($this->resultString)
{
if($this->inputString[$this->pointerInString] == '0' || '1' || '2' || '3')
{
$this->digit(); //$this->match($this->inputString[$this->pointerInString]);
}
elseif($this->inputString[$this->pointerInString] == '(')
{
$this->match($this->inputString[$this->pointerInString]);
$this->exper();
if($this->endOfInput()) {
$this->resultString = false;
echo "Grammar is invalid.";
}
if($this->resultString)
{
$this->match(')');
}
}
else {
$this->resultString = false;
echo "Grammar is invalid.";
}
}
}
function digit() {
$digitArray = array('0', '1', '2', '3');
if($this->endOfInput()) {
$this->resultString = false;
echo "Grammar is invalid.";
}
elseif(array_search($this->inputString[$this->pointerInString], $digitArray) !== False)
{
$this->resultString = $this->resultString && $this->match($this->inputString[$this->pointerInString]);
}
else{
$this->resultString = false;
echo "Grammar is invalid.";
}
}
}
