计算器中使用逆波兰表示法

大家好，小弟这两天正在学编译原理，遇到难题，特像各位大虾求救   
  /****************************************   
  这是文法的产生式   
  使用递归下降语法分析，先消除左递归   
  Z::=AcB¦Bd   
  A::=AaB¦c   
  B::=aA¦a   
  ****************************************/   
  #include   "stdio.h"   
  char   str[]="caaca";   
  int   i=0;   
  void   Z();   
  void   A();   
  void   B();   
  void   ShowErr();   
  main()   
  {   
  Z();   
          printf("Right!\n");   
  }   
  void   Z()   
  {   
  switch(str[i])   
  {   
  case   'c':   
  A();   
  if   (str[i]=='c'){   
  i++;   
  B();   
  }else{   

  ShowErr();   
  }   
  break;   
  case   'a':   
  B();   
  if   (str[i]=='d'){   
  i++;   
  }else{   
  ShowErr();   
  }   
  break;   
  default:   
  ShowErr();   
  }   
  }   
  void   A()   
  {   
  if   (str[i]=='c'){   
  i++;   
  while(str[i]=='a'&&str[i]!='\0')   
  {   
  i++;   
  B();   
  }   
  }   
  else{   
  ShowErr();   
  }   
    
  }