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;
      }
}

你好像忘了输出OPR了.应改为:
{ 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;
      }

跟你改过了，在VC++中通过，如下：
#include<cstdio>
#include<cstdlib>
void main()
{ void calc(float x,float y,char *opr);
  float a,b;
  char *opr;
  opr=(char*)malloc(sizeof(char)*2);
  printf("\nInput experssion:");
  scanf("%f",&a);
  scanf("%c",opr);
  scanf("%f",&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,*opr,y,x+y);break;
        case '-': printf("%5.2f%c%5.2f=%6.2f\n",x,*opr,y,x-y);break;
        case '*': printf("%5.2f%c%5.2f=%6.2f\n",x,*opr,y,x*y);break;
        case '/': printf("%5.2f%c%5.2f=%6.2f\n",x,*opr,y,x/y);break;
      }
}

不能加*号的!

{ 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

scanf("%f%c%f",&a,opr,&b);////OPR 取地址
改为 scanf("%f%c%d",&a,&opr,&b);