All ANTLR grammars produce error "no viable alternative at input ''"

Question

I am needing to parse a small expression language (and, or, not, parens change precedence) so picked ANTLR for the task, I made good progress (ANTLRWorks is very nice for a newbie). I used some Getting Starting references from the antlr website and then found two blog posts that are perfect fit for what I am trying to accomplish:

http://www.codeproject.com/KB/recipes/sota_expression_evaluator.aspx http://www.alittlemadness.com/2006/06/05/antlr-by-example-part-1-the-language

The problem I am having is no matter what input I put I always get the error:

line 1:29 no viable alternative at input 'EOF'

So as part of my troubleshooting I decided to try a grammar I knew was good and generated a lexer/parser from the ECalc.g grammar found at the first link. To my surprise I got the same error when using that grammar! I am bamboozled. The only changes I made to the grammar were to make it generate Java code and took out some CSharp code in the @members section.

Here is my tester class:

public class ECalcTester {
private final static Logger logger = Logger.getLogger(ECalcTester.class);

public static void main(String[] args) {
    BasicConfigurator.configure();
    ECalcLexer lex = new ECalcLexer(new ANTLRStringStream("false || not (false and true)"));

    Token token;
    while (true) {
        token = lex.nextToken();
        if (token.getType() == Token.EOF) {
            break;
        }

        System.out.println("Token: ‘" + token.getText() + "’");
    }

    CommonTokenStream tokens = new CommonTokenStream(lex);
    lex.nextToken();

    ECalcParser parser = new ECalcParser(tokens);
    try {
        logger.debug(parser.expression().getTree());
    } catch (org.antlr.runtime.RecognitionException e) {
        logger.error("Exception ", e);
    }

}

Here is the output:

Token: ‘false’
Token: ‘ ’
Token: ‘||’
Token: ‘ ’
Token: ‘not’
Token: ‘ ’
Token: ‘(’
Token: ‘false’
Token: ‘ ’
Token: ‘and’
Token: ‘ ’
Token: ‘true’
Token: ‘)’
line 1:29 no viable alternative at input '<EOF>'
0 [main] DEBUG ECalcTester  - <unexpected: [@0,29:29='<EOF>',<-1>,1:29], resync=>

If I can figure out why this occurs in a grammar that should be good I should be able to figure out why the same thing happens in my grammar (very similar concept).

Can anyone offer any insight?

could it be that you need to reset the lexer token stream? At the moment you have moved through all the tokens and the first thing the parsers is — luketorjussen, Sep 07 '11 at 14:50
*hangs head in shame* yes, yes I needed to call lex.reset()...sigh. I am such a newb I didn't know about that method, although knowing it was a stream that I had iterated through I should have thought about needing to reset it, pretty common concept. — Michael, Sep 07 '11 at 15:00
The good news is that this fixes the problem I was having with this. However, this isn't going to fix the problem I am having with my grammar. I thought I had found the common problem but totally missed the mark....arrgghh. — Michael, Sep 07 '11 at 15:03
@Michael :-) at least it's fixed now. If you post your other problem in a new question we can take a look! — luketorjussen, Sep 07 '11 at 15:13
I have posted another question that has my real problem in it. Not sure if it is bad etiquette to link to it here, so won't. I used the same tags though. — Michael, Sep 07 '11 at 16:31

score 5 · Accepted Answer · answered Sep 07 '11 at 14:55

5

After printing out your tokens, you will be at the end of the token stream. You will need to reset the token stream by calling

lex.reset();

This will make the lexer go back to the start of the token stream so you can call your parser.

answered Sep 07 '11 at 14:55

luketorjussen

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All ANTLR grammars produce error "no viable alternative at input ''"

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