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#1 annatsos  Icon User is offline

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Flex,Bison

Posted 02 January 2007 - 07:02 AM

My problem is that i have to write a program using flex,bison and devC, and i've never used any of these tools. The teacher wants us to use only devc because it's the one she uses. The program we have to write is a calculator that supports operator presedence and use of 4 standart mathematical functions. Bacically what i need from you guys if you have any ready programs to show me how you used flex and bison to create them because i have no idea how they work. I don't want you to solve my exercise just to show me some examples if possible.

Thank You:)

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Replies To: Flex,Bison

#2 Amadeus  Icon User is offline

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Re: Flex,Bison

Posted 02 January 2007 - 09:02 AM

Did your instructor give you any indication of how she wanted you to use flex and bison? Also, which programming tool called flex is she referring to? there are a few. For bison, are you referring to the bison parser generator?
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#3 annatsos  Icon User is offline

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Re: Flex,Bison

Posted 02 January 2007 - 11:08 AM

View PostAmadeus, on 2 Jan, 2007 - 09:02 AM, said:

Did your instructor give you any indication of how she wanted you to use flex and bison? Also, which programming tool called flex is she referring to? there are a few. For bison, are you referring to the bison parser generator?


Hi.Sorry for the late reply.No the teacher herself doesn't seem to know how to use them, and besides she gave us the project just before she went on holidays and now she doesn't even read her emails.She was supposed to explain it to us during the course but she didn't because it isn't testable, but now she wants us to do this project. Anyway i just know that flex is a substitute for lex the lexical analyzer, and bison is a parser generator substitute for yacc.lex/yacc are for unix and flex/bison for windows.I've been reading the manual of bison and their is a good example of a multifunction calculator(mfcalc).If you can please take a look at it and tell me if you know how to create it using devc.

Heres the example in the manual for bison

Multi-Function Calculator: mfcalc

Now that the basics of Bison have been discussed, it is time to move on to a more advanced problem. The above calculators provided only five functions, +, -, *, / and ^. It would be nice to have a calculator that provides other mathematical functions such as sin, cos, etc.
It is easy to add new operators to the infix calculator as long as they are only single-character literals. The lexical analyzer yylex passes back all nonnumber characters as tokens, so new grammar rules suffice for adding a new operator. But we want something more flexible: built-in functions whose syntax has this form:
function_name (argument)
At the same time, we will add memory to the calculator, by allowing you to create named variables, store values in them, and use them later. Here is a sample session with the multi-function calculator:
$ mfcalc
pi = 3.141592653589
3.1415926536
sin(pi)
0.0000000000
alpha = beta1 = 2.3
2.3000000000
alpha
2.3000000000
ln(alpha)
0.8329091229
exp(ln(beta1))
2.3000000000
$
Note that multiple assignment and nested function calls are permitted.
Decl: Bison declarations for multi-function calculator.
Rules: Grammar rules for the calculator.
Symtab: Symbol table management subroutines.
________________________________________
Node:Mfcalc Decl, Next:Mfcalc Rules, Up:Multi-function Calc
Declarations for mfcalc
Here are the C and Bison declarations for the multi-function calculator.
%{
#include <math.h> /* For math functions, cos(), sin(), etc. */
#include "calc.h" /* Contains definition of `symrec' */
%}
%union {
double val; /* For returning numbers. */
symrec *tptr; /* For returning symbol-table pointers */
}

%token <val> NUM /* Simple double precision number */
%token <tptr> VAR FNCT /* Variable and Function */
%type <val> exp

%right '='
%left '-' '+'
%left '*' '/'
%left NEG /* Negation--unary minus */
%right '^' /* Exponentiation */

/* Grammar follows */

%%
The above grammar introduces only two new features of the Bison language. These features allow semantic values to have various data types (see More Than One Value Type).
The %union declaration specifies the entire list of possible types; this is instead of defining YYSTYPE. The allowable types are now double-floats (for exp and NUM) and pointers to entries in the symbol table. See The Collection of Value Types.
Since values can now have various types, it is necessary to associate a type with each grammar symbol whose semantic value is used. These symbols are NUM, VAR, FNCT, and exp. Their declarations are augmented with information about their data type (placed between angle brackets).
The Bison construct %type is used for declaring nonterminal symbols, just as %token is used for declaring token types. We have not used %type before because nonterminal symbols are normally declared implicitly by the rules that define them. But exp must be declared explicitly so we can specify its value type. See Nonterminal Symbols.
________________________________________
Node:Mfcalc Rules, Next:Mfcalc Symtab, Previous:Mfcalc Decl, Up:Multi-function Calc
Grammar Rules for mfcalc
Here are the grammar rules for the multi-function calculator. Most of them are copied directly from calc; three rules, those which mention VAR or FNCT, are new.
input: /* empty */
| input line
;

line:
'\n'
| exp '\n' { printf ("\t%.10g\n", $1); }
| error '\n' { yyerrok; }
;

exp: NUM { $$ = $1; }
| VAR { $$ = $1->value.var; }
| VAR '=' exp { $$ = $3; $1->value.var = $3; }
| FNCT '(' exp ')' { $$ = (*($1->value.fnctptr))($3); }
| exp '+' exp { $$ = $1 + $3; }
| exp '-' exp { $$ = $1 - $3; }
| exp '*' exp { $$ = $1 * $3; }
| exp '/' exp { $$ = $1 / $3; }
| '-' exp %prec NEG { $$ = -$2; }
| exp '^' exp { $$ = pow ($1, $3); }
| '(' exp ')' { $$ = $2; }
;
/* End of grammar */
%%
________________________________________
Node:Mfcalc Symtab, Previous:Mfcalc Rules, Up:Multi-function Calc
The mfcalc Symbol Table
The multi-function calculator requires a symbol table to keep track of the names and meanings of variables and functions. This doesn't affect the grammar rules (except for the actions) or the Bison declarations, but it requires some additional C functions for support.
The symbol table itself consists of a linked list of records. Its definition, which is kept in the header calc.h, is as follows. It provides for either functions or variables to be placed in the table.
/* Fonctions type. */
typedef double (*func_t) (double);

/* Data type for links in the chain of symbols. */
struct symrec
{
char *name; /* name of symbol */
int type; /* type of symbol: either VAR or FNCT */
union
{
double var; /* value of a VAR */
func_t fnctptr; /* value of a FNCT */
} value;
struct symrec *next; /* link field */
};

typedef struct symrec symrec;

/* The symbol table: a chain of `struct symrec'. */
extern symrec *sym_table;

symrec *putsym (const char *, func_t);
symrec *getsym (const char *);
The new version of main includes a call to init_table, a function that initializes the symbol table. Here it is, and init_table as well:
#include <stdio.h>

int
main (void)
{
init_table ();
return yyparse ();
}

void
yyerror (const char *s) /* Called by yyparse on error */
{
printf ("%s\n", s);
}

struct init
{
char *fname;
double (*fnct)(double);
};

struct init arith_fncts[] =
{
"sin", sin,
"cos", cos,
"atan", atan,
"ln", log,
"exp", exp,
"sqrt", sqrt,
0, 0
};

/* The symbol table: a chain of `struct symrec'. */
symrec *sym_table = (symrec *) 0;

/* Put arithmetic functions in table. */
void
init_table (void)
{
int i;
symrec *ptr;
for (i = 0; arith_fncts[i].fname != 0; i++)
{
ptr = putsym (arith_fncts[i].fname, FNCT);
ptr->value.fnctptr = arith_fncts[i].fnct;
}
}
By simply editing the initialization list and adding the necessary include files, you can add additional functions to the calculator.
Two important functions allow look-up and installation of symbols in the symbol table. The function putsym is passed a name and the type (VAR or FNCT) of the object to be installed. The object is linked to the front of the list, and a pointer to the object is returned. The function getsym is passed the name of the symbol to look up. If found, a pointer to that symbol is returned; otherwise zero is returned.
symrec *
putsym (char *sym_name, int sym_type)
{
symrec *ptr;
ptr = (symrec *) malloc (sizeof (symrec));
ptr->name = (char *) malloc (strlen (sym_name) + 1);
strcpy (ptr->name,sym_name);
ptr->type = sym_type;
ptr->value.var = 0; /* set value to 0 even if fctn. */
ptr->next = (struct symrec *)sym_table;
sym_table = ptr;
return ptr;
}

symrec *
getsym (const char *sym_name)
{
symrec *ptr;
for (ptr = sym_table; ptr != (symrec *) 0;
ptr = (symrec *)ptr->next)
if (strcmp (ptr->name,sym_name) == 0)
return ptr;
return 0;
}
The function yylex must now recognize variables, numeric values, and the single-character arithmetic operators. Strings of alphanumeric characters with a leading non-digit are recognized as either variables or functions depending on what the symbol table says about them.
The string is passed to getsym for look up in the symbol table. If the name appears in the table, a pointer to its location and its type (VAR or FNCT) is returned to yyparse. If it is not already in the table, then it is installed as a VAR using putsym. Again, a pointer and its type (which must be VAR) is returned to yyparse.
No change is needed in the handling of numeric values and arithmetic operators in yylex.
#include <ctype.h>

int
yylex (void)
{
int c;

/* Ignore whitespace, get first nonwhite character. */
while ((c = getchar ()) == ' ' || c == '\t');

if (c == EOF)
return 0;

/* Char starts a number => parse the number. */
if (c == '.' || isdigit ©)
{
ungetc (c, stdin);
scanf ("%lf", &yylval.val);
return NUM;
}

/* Char starts an identifier => read the name. */
if (isalpha ©)
{
symrec *s;
static char *symbuf = 0;
static int length = 0;
int i;

/* Initially make the buffer long enough
for a 40-character symbol name. */
if (length == 0)
length = 40, symbuf = (char *)malloc (length + 1);

i = 0;
do
{
/* If buffer is full, make it bigger. */
if (i == length)
{
length *= 2;
symbuf = (char *)realloc (symbuf, length + 1);
}
/* Add this character to the buffer. */
symbuf[i++] = c;
/* Get another character. */
c = getchar ();
}
while (c != EOF && isalnum ©);

ungetc (c, stdin);
symbuf[i] = '\0';

s = getsym (symbuf);
if (s == 0)
s = putsym (symbuf, VAR);
yylval.tptr = s;
return s->type;
}

/* Any other character is a token by itself. */
return c;
}
This program is both powerful and flexible. You may easily add new functions, and it is a simple job to modify this code to install predefined variables such as pi or e as well.
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#4 annatsos  Icon User is offline

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Re: Flex,Bison

Posted 04 January 2007 - 05:02 AM

I've solved the problem everything works perfectly.flex and bison are easier than i thought:)
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