lex and yacc

[Gvs]

Hi,

i'm trying to implement a few things in lex and yacc though i'm having
trouble.

What i want to do is be able to parse a term and an expression.

in EBNF my term is defined as: term ::= factor { '*' | '\' } .
factor is defined as: factor ::= ident | '(' expression ')' | number .
expression is defined as: expression ::= '+' | '-' term { '+' '-' term } .
ident is defined as: ident ::= 'a..z. A..Z' { 'a..z A..Z 0..9' } .
number is defined as: number ::= { '0..9' } .

the problem is i don't know how to include the recursion aspects of
expression, factor and ter

i have defined number and ident in lex as follows

ident [a-zA-Z][A-Za-z0-9]*
number [0-9]+

so to do factor should i do something in LEX like

factor [ident|number|expression]

or do i implement it in Yacc in a rule like

commands:
| commands command
;

command:
factor
;

factor:
IDENT
{
print("factor");
}
|
NUMBER
{
printf("factor");
}
|
LEFTPARENTH EXPRESSION RIGHTPARENTH
{
printf("factor");
}
;

If that is the case where do i define expression in the first place, it's
like one big circle.
I'm really confused .. so any help would be much appreciated.!!

Kind Regards,

Gvs

 

[Thomas Maier-Komor]

Hi,

i'm trying to implement a few things in lex and yacc though i'm having
trouble.

What i want to do is be able to parse a term and an expression.

in EBNF my term is defined as: term ::= factor { '*' | '\' } .
factor is defined as: factor ::= ident | '(' expression ')' | number .
expression is defined as: expression ::= '+' | '-' term { '+' '-' term } .
ident is defined as: ident ::= 'a..z. A..Z' { 'a..z A..Z 0..9' } .
number is defined as: number ::= { '0..9' } .

the problem is i don't know how to include the recursion aspects of
expression, factor and ter

i have defined number and ident in lex as follows

ident [a-zA-Z][A-Za-z0-9]*
number [0-9]+

so to do factor should i do something in LEX like

factor [ident|number|expression]

or do i implement it in Yacc in a rule like

commands:
| commands command
;

command:
factor
;

factor:
IDENT
{
print("factor");
}
|
NUMBER
{
printf("factor");
}
|
LEFTPARENTH EXPRESSION RIGHTPARENTH
{
printf("factor");
}
;

If that is the case where do i define expression in the first place, it's
like one big circle.
I'm really confused .. so any help would be much appreciated.!!

Kind Regards,

Gvs

you can cut the circle by making a forward declaration of a parser using
the keyword %type. Take a look at the info pages of bison. There are
some examples that are also valid for yacc...

Tom

 

[Coos Haak]

Op Wed, 11 May 2005 14:10:58 +0200 schreef Thomas Maier-Komor:

Gvs wrote: ..
you can cut the circle by making a forward declaration of a parser using
the keyword %type. Take a look at the info pages of bison. There are
some examples that are also valid for yacc...

Tom

ANSI C has no idea of beasts like Bison, Python, Yak.
Try the nearest zoo.

Thanks, Coos

 

[T.M. Sommers]

Op Wed, 11 May 2005 14:10:58 +0200 schreef Thomas Maier-Komor:


ANSI C has no idea of beasts like Bison, Python, Yak.
Try the nearest zoo.

comp.compilers is the place to go.