int valid_program_line(char *s){

        int i=0;

   ...

        if(s[i] == '\n')
                return 0;
        else{
                while(s[i] != '\n')
                        i++;
        }
}


-----------------------


int valid_program_line(char *s){
   char * temp = s;

   if(temp == NULL || * temp < 48 || * temp > 57)
      return 0;
   else
      while(* temp >= 48 && * temp <= 57)
         temp++;

   if(temp == NULL || * temp != 46)
      return 0;
   else
      temp++;

-----------------------

   while (*s != '\n'){
      if ((*s < ' ' || *s > '~') && *s != '\n'){
         return 0;
      }
      s++;
   }


-----------------------

   while( s[i] != '\n' ) {
      if( s[i] < ' ' || s[i] > '~' )
                return 0;
      i++;
      if( i == INT_MAX )                // A little insurance never hurts.
                return 0;
   }