# FIRST function computation problem.

## mefrill@yandex.ru (Vladimir)22 Jun 2003 23:18:16 -0400

From comp.compilers

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 From: mefrill@yandex.ru (Vladimir) Newsgroups: comp.compilers Date: 22 Jun 2003 23:18:16 -0400 Organization: http://groups.google.com/ Keywords: parse, question Posted-Date: 22 Jun 2003 23:18:16 EDT

Hi,

I have a problem with FIRST function for 1 lookahead symbol evalation.
To lookup such the symbols for some nonterminal A I am doning follows:

1. go through all rules that left part is A. Here I do following:
a) If I have rule A --> aB, where a is terminal. I just add this
terminal to lookahead set for A.
b) If I have rule A --> e, where e is epsilon, I add e to lookahead
set for A.
c) If I have rule A --> B C ... where B - nonterminal, I at first
compute the lookahead set for B, if B's lookahead set contains
epsilon, check C and etc. I found that recursive function here is the
best. But I do not know how to work with rules like follows: A --> B,
B --> C A D... To compute lookahead set for A I have to at first
compute lookaheads for B, but if lookaheads for C contain epsilon I
need to know the same about A to understand do checking of D or
not...

Please, help to decide this problem and there may be the good FIRTS
computaion algorithm?

Thanks,