Recursive Descent Parser for EBNF in PHP
I am attempting to write a recursive descent parser in PHP for the following EBNF:
EXP ::= < TERM > { ( + | - ) < TERM > }
TERM ::= < FACTOR > { ( * | / ) < FACTOR > }
FACTOR ::= ( < EXP > ) | < DIGIT >
DIGIT ::= 0 | 1 | 2 | 3
I followed this guide which I saw recommended on a similar question. (I searched before I posted)
For the most part, I get how it works and I understand the grammar. I think the problem is within my syntax. I am new to PHP, so I have been referencing W3Schools. I currently am getting the following error with my code:
Warning: Wrong parameter count for exp() .... on line 101
I have tried to look up this error and have not had much luck. I read some posts about people passing in the wrong parameter typed, but I do not have any parameters set for that function. Is there something about PHP I am missing here?
Below is my code, I think the logic is correct since I based it off of the parse tree for the grammar. The $input will be coming from a form box on an HTML page. I also picked up the str_split function from a different post when I discovered that PHP4 does not have it built in.
<html>
<body>
<?php
if(!function_exists("exp")){
function exp(){
term();
while($token == "+" | $token == "-"){
if($token == "+"){
match("+");
term();
}
if($token == "-"){
match("-");
term();
}
}
}//end exp
}
if(!function_exists("term")){
function term(){
factor();
while($token == "*" | $token == "/"){
if($token == "*"){
match("*");
factor();
}
if($token == "/"){
match("/");
factor();
}
}
}//end term
}
if(!function_exists("factor")){
function factor(){
if($token == "("){
match("(");
exp();
if($token == ")")
match(")");
}
else if($token == 0|1|2|3){
if($token == 0)
match(0);
if($token == 1)
match(1);
if($token == 2)
match(2);
if($token == 3)
match(3);
}
else
error();
}//end factor
}
if(!function_exists("match")){
fun开发者_如何学JAVAction match($expected){
if($token == $expected)
nextToken();
else
error();
}//end match
}
if(!function_exists("next_Token")){
function nextToken(){
$next++;
$token = $tokenStr[$next];
if($token == "$");
legal();
}
}
if(!function_exists("error")){
function error(){
echo "Illegal token stream, try again";
}
}
if(!function_exists("legal")){
function legal(){
echo "Legal token stream, congrats!";
}
}
if(!function_exists('str_split')) {
function str_split($string, $split_length = 1) {
$array = explode("\r\n", chunk_split($string, $split_length));
array_pop($array);
return $array;
}
}
$tokenStr = str_split($input);
$next = 0;
$token = $tokenStr[0];
exp();
?>
</body>
</html>
So basically I want to know what causes that error and why and am I on the right track in terms of creating this parser.
I appreciate any comments, suggestions, criticisms, water baloons, and tomatoes. Thank you for taking the time to read my post. Have a great day/night.
exp() is a builtin PHP function. You cannot define it under that name.
You should have no reason to use the if(!function_exists(' idiom in normal PHP applications. (It's often used more as a workaround when include scripts clash or identical functions are declared at different places.)
Another syntax problem that I noticed is your use of the bitwise OR. The logical OR should be || or just or.
while($token == "*" | $token == "/"){
I'll turn my wild guess into an answer. So maybe this is, where the problem lies?
http://php.net/manual/en/function.exp.php
There is also a function named exp() in PHP already. You may prefix your function names somehow, or it's better to use classes to avoid name collisions.
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