这个程序运行后信息窗口中会出现Null pointer assignment 而不出现结果？
main()
{ void calc(float x,float y,char opr);
  float a,b;
  char opr;
  printf("\nInput experssion:");
  scanf("%f%c%f",&a,opr,&b);
  calc(a,b,opr);
}
void calc(float x,float y,char opr)
{ switch(opr)
      { case '+': printf("%5.2f%c%5.2f=%6.2f\n",x,y,x+y);return;
    case '-': printf("%5.2f%c%5.2f=%6.2f\n",x,y,x+y);return;
    case '*': printf("%5.2f%c%5.2f=%6.2f\n",x,y,x+y);return;
    case '/': printf("%5.2f%c%5.2f=%6.2f\n",x,y,x+y);return;
      }
}

{ case '+': printf("%5.2f%c%5.2f=%6.2f\n",x,opr,y,x+y);return;
    case '-': printf("%5.2f%c%5.2f=%6.2f\n",x,opr,y,x+y);return;
    case '*': printf("%5.2f%c%5.2f=%6.2f\n",x,opr,y,x+y);return;
    case '/': printf("%5.2f%c%5.2f=%6.2f\n",x,opr,y,x+y);return;
      }
改了还是不行啊