Programar em C: Exercícios Finais: Exercício 13

 #include <stdio.h>
 #include <stdlib.h>
 int expr(void);
 int mul_exp(void);
 int unary_exp(void);
 int primary(void);
 main()
 {
  int val;
      for(;;)
        {
              printf("expression: ");
              val = expr();
              if(getchar() != '\n')
                 {
                      printf("error\n");
                      while(getchar() != '\n'); /* NULL */
                 } 
	      else
                 {
                      printf("result is %d\n", val);
              	 }
        }
    system (“pause”);
    return 0;
   }
 int expr(void)
   {
       int val, ch_in;
       val = mul_exp();
       for(;;)
	 {
           switch(ch_in = getchar())
	     {
               	default:
               	        ungetc(ch_in,stdin);
                        return(val);
              	case '+':
                        val = val + mul_exp();
                        break;
              	case '-':
                        val = val - mul_exp();
                        break;
             }
        }
   }
 int mul_exp(void)
   {
       int val, ch_in;
       val = unary_exp();
       for(;;)
	 {
             switch(ch_in = getchar())
               {
              	 default:
                         ungetc(ch_in, stdin);
                      	 return(val);
              	 case '*':
                         val = val * unary_exp();
                      	 break;
              	 case '/':
                         val = val / unary_exp();
                      	 break;
              	 case '%':
                         val = val % unary_exp();
                      	 break;
              }
       }
   }
 int unary_exp(void)
   {
       int val, ch_in;
       switch(ch_in = getchar())
	 {
    	   default:
                    ungetc(ch_in, stdin);
              	    val = primary();
              	    break;
      	   case '+':
              	    val = unary_exp();
              	    break;
      	   case '-':
              	    val = -unary_exp();
              	    break;
        }
      return(val);
   }
 int primary(void)
   {
       int val, ch_in;
       ch_in = getchar();
       if(ch_in >= '0' && ch_in <= '9')
	  {
              val = ch_in - '0';
              goto out;
      	  }
      if(ch_in == '(')
	  {
             val = expr();
             getchar();      /* skip closing ')' */
             goto out;
          }
      printf("error: primary read %d\n", ch_in);
      exit(EXIT_FAILURE);
      out:  return(val);
   }