How to parse Variable

[felixfrtz]

Hey,

I am trying to parse variables within the calculator example. My Code is as follows:

var grammar = {
	"comment": "JSON Math Parser",
	"lex": {
		"rules": [
			["\\s+", "/* skip whitespace */"],
			["[0-9]+(?:\\.[0-9]+)?\\b", "return 'NUMBER'"],
			["\\*", "return '*'"],
			["\\/", "return '/'"],
			["-", "return '-'"],
			["\\+", "return '+'"],
			["\\^", "return '^'"],
			["!", "return '!'"],
			["%", "return '%'"],
			["\\(", "return '('"],
			["\\)", "return ')'"],
			["PI\\b", "return 'PI'"],
			["E\\b", "return 'E'"],
			["$", "return 'EOF'"]
		]
	},

	"operators": [
		["left", "+", "-"],
		["left", "*", "/"],
		["left", "^"],
		["right", "!"],
		["right", "%"],
		["left", "UMINUS"]
	],

	"bnf": {
		"expressions": [["e EOF", "return $1"]],

		"e": [
			["e + e", "$$ = $1+$3"],
			["e - e", "$$ = $1-$3"],
			["e * e", "$$ = $1*$3"],
			["e / e", "$$ = $1/$3"],
			["e ^ e", "$$ = Math.pow($1, $3)"],
			["e !", "$$ = (function(n) {if(n==0) return 1; return arguments.callee(n-1) * n})($1)"],
			["e %", "$$ = $1/100"],
			["- e", "$$ = -$2", { "prec": "UMINUS" }],
			["( e )", "$$ = $2"],
			["NUMBER", "$$ = Number(yytext)"],
			["VARIABLE", "$$ = yy.data[text]"], // this is what I've added
			["E", "$$ = Math.E"],
			["PI", "$$ = Math.PI"]
		]
	}
};

var parser = new Parser(grammar);

var parserSource = parser.generate();
parser.yy = { data: { m: 4 } };
console.log(parser.parse("m")); 

This leads to the following:

Error: Lexical error on line 1. Unrecognized text. m

I used this post to accomplish this, but it isn't working. Any idea what is wrong with the example?

[blackshadev]

Hi,

Well you don't have a lex rule for variables. The post you mentioned doesn't include this rule, neither does the calculator example.

An example would be:

["\w+", "return 'VARIABLE'"]

You need to define this rule after all your constants and before your EOF rule. It is a but more permissive than required, but it works and it is easy to read.

ps. I can really suggest using a .jison file and use the cmd like (jison grammer.jison) to generate a ,js file instead of creating your own, I find it easier to read, maintain and write. But that is just my opinion.

[AnuragVasanwala]

Hi @felixfrtz 👋🏻

It is a really old question but I came across this today and would like to share my thoughts.

As per @blackshadev suggestion, it would be good to use .jison and compile ito to generate .js. For your need, you can use official example calculator.jison and add following highlighted lines:

/* description: Parses and executes mathematical expressions. */

/* lexical grammar */
%lex
%%

\s+                       /* skip whitespace */
[0-9]+("."[0-9]+)?\b      return 'NUMBER'
"*"                       return '*'
"/"                       return '/'
"-"                       return '-'
"+"                       return '+'
"^"                       return '^'
"!"                       return '!'
"%"                       return '%'
"("                       return '('
")"                       return ')'
"PI"                      return 'PI'
"E"                       return 'E'
+[a-zA-Z_][a-zA-Z0-9_]*    return 'NAME'
+\'[^\']*\'                return 'STRING'
+\"[^\"]*\"                return 'STRING'
+"TRUE"                    return 'TRUE'
+"true"                    return 'TRUE'
+"FALSE"                   return 'FALSE'
+"false"                   return 'FALSE'
+"NULL"                    return 'NULL'
<<EOF>>                   return 'EOF'
.                         return 'INVALID'

/lex

/* operator associations and precedence */

%left '+' '-'
%left '*' '/'
%left '^'
%right '!'
%right '%'
%left UMINUS

%start expressions

%% /* language grammar */

expressions
    : e EOF
        { typeof console !== 'undefined' ? console.log($1) : print($1);
          return $1; }
    ;

e
    : e '+' e
        {$$ = $1+$3;}
    | e '-' e
        {$$ = $1-$3;}
    | e '*' e
        {$$ = $1*$3;}
    | e '/' e
        {$$ = $1/$3;}
    | e '^' e
        {$$ = Math.pow($1, $3);}
    | e '!'
        {{
          $$ = (function fact (n) { return n==0 ? 1 : fact(n-1) * n })($1);
        }}
    | e '%'
        {$$ = $1/100;}
    | '-' e %prec UMINUS
        {$$ = -$2;}
    | '(' e ')'
        {$$ = $2;}
    | NUMBER
        {$$ = Number(yytext);}
+    | NAME
+       { $$ = yy[yytext]; }
+   | STRING
+     {$$ = yytext.substring(1, yytext.length - 1);}
+   | TRUE
+     {$$ = true}
+   | FALSE
+       {$$ = false}
+   | NULL
+       {$$ = null}  
    | E
        {$$ = Math.E;}
    | PI
        {$$ = Math.PI;}
    ;