c - 为什么我的 goto 命令在编译时出现问题？

Question

我编写了一个用于数学操作的小程序，但是 goto在某些情况下命令将无法编译。请帮助我。

这是代码:

#include<stdio.h>
#define line11
#define YorN
#define End
#define Start

int main(void)
{
    float userNumber1,userNumber2;
    int mathAction;
    char tryAgain;
    Start:
    printf("Please write 2 numbers for your math action(a,b):\n");
    scanf("%f %f",&userNumber1,&userNumber2);
    Line11:
    printf("Please choose 1 of the following math actions by writing the action number :\n"
        "1)Adding a+b\n"
        "2)Subtracting a-b.\n"
        "3)Multiplying a*b.\n"
        "4)Dividing a/b.\n");
    scanf("%d",&mathAction);
    if (mathAction>4 || mathAction<1) //If the user pick something that's not 1-4 , then this if is going on.
    {
        printf("Grrrr.. pick between 1-4 ! \n");
        goto Line11;
    }
    if (mathAction==1)
    {
        printf("Adding %d + %d is %d\n",userNumber1,userNumber2,userNumber1+userNumber2);
    }
    if (mathAction==2)
    {
        printf("Subtracting %d - %d is %d\n",userNumber1,userNumber2,userNumber1-userNumber2);
    }
    if (mathAction==3)
    {
        printf("Multiplying %d * %d is %d\n",userNumber1,userNumber2,userNumber1*userNumber2);
    }
    if (mathAction==4)
    {
        if (userNumber2==0)
        {
            printf("You cant divide %d by 0 , its math error!\n");
            printf("Do you want to choose again? y/n\n");
            YorN:
            scanf("%c",&tryAgain)
            if (tryAgain !='y' || tryAgain !='n')
            {
                printf("Choose only yes or no by y for yes and n for no.");
                goto YorN;
            }
            if (tryAgain=='y')
            {
                goto Start;
            }
            if (tryAgain=='n')
            {
                goto End;
            }

        }
        printf("Dividing %d / %d is %.2f\n",userNumber1,userNumber2,userNumber1/userNumber2);
    }
    printf("Thanks for using my program !\nDo you want to try again? (Answer y/n)");
    scanf("%d",&tryAgain);
    if (tryAgain !='y' || tryAgain !='n')
    {
        printf("Choose only yes or no by y for yes and n for no.");
        goto YorN;
    }
    if (tryAgain=='y')
    {
        goto Start;
    }
    if (tryAgain=='n')
    {
        goto End;
    }
    End:
    system("PAUSE");
    return (0);
}

这是编译错误:

q5.c: In function 'main':
q5.c:12:11: error: expected expression before ':' token
      Start:
           ^
q5.c:45:12: error: expected expression before ':' token
      YorN:
          ^
q5.c:69:15: error: expected identifier or '*' before ';' token
      goto YorN;
               ^
q5.c:73:16: error: expected identifier or '*' before ';' token
      goto Start;
                ^
q5.c:77:14: error: expected identifier or '*' before ';' token
      goto End;
              ^
q5.c:79:8: error: expected identifier or '*' before ';' token
     End:
        ^

我已经尝试了一切方法来完成这项工作，请帮忙! 顺便说一句，goto之一s 实际上正在工作。

Answer 1

问题是你有

#define YorN
#define End
#define Start

在源代码的开头。这些预处理器宏将在编译为空之前进行评估，因此这段代码:

YorN:
goto YorN;

将成为

:
goto;

只需删除它们，因为拥有它们没有任何意义。