Flex/Bison not evaluating properly

For some reason or another, bison doesn't want to do any evaluation. Compilation of all files goes smoothly and the program runs. When I enter the expression 4+5 and press return, it creates tokens for 4 + 5 respectively. I can even put in some printf into the places where bison recognizes the attributes of each token including the plus (43).

However the program never evaluates this production expr '+' term { $$ = $1 + $3; } expr '+' term { $$ = $1 + $3; } . It's simply never called at least to my knowledge and even if it was this production assign 'n' { printf("%dn", $1); } assign 'n' { printf("%dn", $1); } never prints out the value. Upon ^D to quit, it fires void yyerror(const char *) .

Any help on this matter is much appreciated. Thanks!

//FLEX
%{
    //#include <stdio.h>
    #include "y.tab.h"
%}

%option noyywrap

letter  [A-Za-z]
digit   [0-9]
space   [ t]

var     {letter}
int     {digit}+
ws      {space}+

%%

{var}   { yylval = (int)yytext[0]; return VAR; }
{int}   { yylval = atoi(yytext); return CONST; }
{ws}    { }
.       { return (int)yytext[0]; }

%%

/* nothing */

.

//BISON
%{

//INCLUDE
//#include <ctype.h>

//DEFINE
#define YYDEBUG 1

//PROTOTYPE
void yyerror(const char *);
void print_welcome();
int get_val(int);
void set_val(int, int);

%}

%token CONST
%token VAR

%%

session
    : { print_welcome(); }
      eval
    ;

eval
    : eval line
    |
    ;

line
    : assign 'n'       { printf("%dn", $1); }
    ;

assign
    : VAR '=' expr      { set_val($1, $3); $$ = $3; }
    | expr              { $$ = $1; }
    ;

expr
    : expr '+' term     { $$ = $1 + $3; }
    | expr '-' term     { $$ = $1 - $3; }
    | term              { $$ = $1; }
    ;

term
    : term '*' factor   { $$ = $1 * $3; }
    | term '/' factor   { $$ = $1 / $3; }
    | term '%' factor   { $$ = $1 % $3; }
    | factor            { $$ = $1; }
    ;

factor
    : '(' expr ')'      { $$ = $2; }
    | CONST             { $$ = $1; }
    | VAR               { $$ = get_val($1); }
    ;

%%

void yyerror(const char * s)
{
    fprintf(stderr, "%sn", s);
}

void print_welcome()
{
    printf("Welcome to the Simple Expression Evaluator.n");
    printf("Enter one expression per line, end with ^Dnn");
}

static int val_tab[26];

int get_val(int var)
{
    return val_tab[var - 'A'];
}

void set_val(int var, int val)
{
    val_tab[var - 'A'] = val;
}

.

//MAIN

//PROTOTYPE
int yyparse();

int main()
{
    extern int yydebug;
    yydebug = 0;
    yyparse();
    return 0;
}

---

Your lex file does not have any rule which matches n , because in lex / flex , . matches any character except line-end. The default rule for lex (or flex ) echoes and otherwise ignores the matched character, so that's what happens to the n . Since the parser won't be able to accept a line unless it sees a n token, it will eventually be forced to present you with a syntax error.

So you need to change the rule

.     { return (int)yytext[0]; }

to

.|n  { return (int)yytext[0]; }

(I wouldn't have bothered with the cast to int but it's certainly not doing any harm, so I left it in.)

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