c - Why is the result of the bison is not displayed? -


i tested example of teamwork flex , bison, result of calculation not displayed in console. test.l:

%{ #include "parser.tab.h" %} %option yylineno %option noyywrap %% [/][/].*\n      ; // comment [0-9]+          { yylval = atoi(yytext);                   return num;                 } [ \t\r\n]      ; // whitespace .              { return *yytext; }  %%

parser.y:

%{ #include <stdio.h> void yyerror(char *s) {   fprintf (stderr, "%s\n", s); } %} %token num %start evaluate %% evaluate: expr          {printf("=%d\n", $$);} ;  expr: expr '+' term { $$ = $1 + $3; }     | expr '-' term { $$ = $1 - $3; }     | term ;  term: term '*' num  { $$ = $1 * $3; }     | term '/' num  { $$ = $1 / $3; }     | num ;  %% int main() {   return yyparse(); }

but if add getchar(), after enter additional character same calculation result displayed. why not change(evaluate: expr{printf("=%d\n", $$); getchar();} ;) , not see result? sorry english.

you parsing input coming stdin, "stream". before stream terminated, parser cannot know complete parse tree. example, if enter expression 1+1, complete input 1+11, 1+1-1 or 1+11*4 - different expressions lead different parse trees result.

you can create terminated input doing 1 of following:

  • pressing ctrl+d after typing in input (on unix shells)
  • piping input: echo "1+1" | ./parser
  • reading input file inputfile.txt containing input 1+1:
    ./parser < inputfile.txt

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