basic.y

%{

#include<stdio.h>
#include<stdlib.h>
#include<ctype.h>
#ifndef YYSTYPE
#define YYSTYPE int
#endif
int yylex();
extern int yyparse();
FILE* yyin;
void yyerror(const char* s);
%}

%token ADD SUB MUL DIV NUMBER

%left ADD SUB
%left MUL DIV
%right UMINUS        

%%


lines   :       lines expr ';' { printf("%d\n", $2); }
        |       lines ';'
        |
        ;

expr : expr ADD expr { $$ = $1 + $3; }
     | expr SUB expr { $$ = $1 - $3; }
     | expr MUL expr { $$ = $1 * $3; }
     | expr DIV expr { $$ = $1 / $3; }
     | '(' expr ')' { $$ = $2; }
     | SUB expr %prec UMINUS { $$ = -$2; }
     | NUMBER { $$ = $1; }
     ;

%%



int yylex()
{
    int t;
    while(1){
        t=getchar();
        if(t==' '||t=='\t'||t=='\n'){
            //do noting
        }else if(isdigit(t)){ 
            yylval = 0;
            while(isdigit(t))
            {
                yylval = yylval * 10 + (t - '0');
                t = getchar();
            }
            ungetc(t, stdin);
            return NUMBER;
        }else if(t=='+'){
            return ADD;
        }else if(t=='-'){
            return SUB;
        }else if(t=='*'){
            return MUL;
        }else if(t=='/'){
            return DIV;
        }
        else{
            return t;
        }
    }
}

int main(void)
{
    yyin=stdin;
    do{
        yyparse();
    }while(!feof(yyin));
    return 0;
}
void yyerror(const char* s){
    fprintf(stderr,"Parse error: %s\n",s);
    exit(1);
}