Re: simple syntax analysis

Julia Donawald <julia.donawald@gmx.de> wrote:
> I have written the following pseudo-code to built up a parse tree of the
> following sequence: [A] [OR] [B] [AND] [C]. I read in some books about that
> theme and create the following grammar:
> expr := factor | factor OR expr | factor AND expr
> factor:= letter | (expr)


> So I want that first the both left letters are considered and after the
> result of these both in connection with the next is calculated and so on...
> so maybe I want something like: more left, first considered.
> Maybe my grammar is wrong ( I dont need any priority of the operators ), or
> the code I wrote from it. But really dont know what I have done wrong.


You probably want the operators to be left associative. This can be
done by stating the grammar as:


expr := factor | expr OR factor | expr AND factor
factor := letter | (expr)


But from this grammar one can't directly build a recursive descent
parser. One can get rid of the left recursions via well known
transformations [1]. This will lead to something like:


expr := factor expr2
expr2 := | OR factor expr2 | AND factor expr2
factor := letter | (expr)


The recursive descent parser would look like:


expr() { return expr2(factor()); }
expr2(left_tree)
{ if (next_char == OR)
        return expr2(CreateOperatorOr(left_tree, factor()));
    else if (next_char == AND)
        return expr2(CreateOperatorAnd(left_tree, factor()));
    else
        return left_tree;
}


Of course it isn't necessary to use two rules and recursion, instead
something like the follwing will work too:


expr()
{ left_tree = factor();
    while (next_char == OR || next_char == AND)
        left_tree = CreateOperator(next_char, left_tree, factor());
    return left_tree;
}


[1] for example see http://ironbark.bendigo.latrobe.edu.au/subjects/bitsys/clect/Lect07.html


--
Walter Misar misar@rbg.informatik.tu-darmstadt.de

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