Question

Official problem:

Write Java method to perform recursive descent parse of the following production:

<repeat_statement> -> REPEAT <statement> UNTIL <expression> ;

This is what I’ve come up with:

void repeatStatement() {
    if(token == REPEAT) {
        token = getNextToken();
        if(parseStatement()) {
            if(token == UNTIL) {
                token = getNextToken();
                if(parseExpression()) {
                    if(token == ;) {
                        return true
                    }
                }
            }
        } return false
    }

I’m pretty confident that I have the general idea here, but I was hoping that someone could help me polish this/ confirm that I’m on the right track.

Answer 1

It looks (vaguely) like you’re trying to evaluate the repeat statement. That’s not what recursive descent parsing does. I’d expect something like this (in pseudocode):

RepeatStatement repeat_statement() throws ParseException {
    if (!consume("REPEAT")) {
        throw new ParseException("no REPEAT token");
    }
    Statement statement = statement();
    if (!consume("UNTIL")) {
        throw new ParseException("no UNTILtoken");
    }
    Expression expression = expression();
    if (!consume(";")) {
        throw new ParseException("no closing semicolon");
    }
    return new RepeatStatement(statement, expression);
}