刚刚学C，采用书籍是 Ivor Horton 的《C语言入门经典》第四版。问题来自书中第三章篇尾的习题4.

原题:
习题3.4 修改本章最后的计算器例子，让用户选择输入y或Y，以执行另一个计算，输入n或N就结束程序。（注意：这需要实用goto语句，下一章将介绍一个更好的方法。）

我的代码是：

#include <stdio.h>

int main(void)
{
    //declare variables
    double operand_1 = 0.0;
    double operand_2 = 0.0;
    char operator = 0;
    char restart = 0;
   
    //input algorithm
    again: printf("Please enter your algorithm: ");
    scanf("%lf %c %lf",&operand_1,&operator,&operand_2);
   
    //calculations
    switch (operator) {
        case '+':
            printf("= %.2lf\n\n",operand_1 + operand_2);
            goto yn;
            break;
            
        case '-':
            printf("= %.2lf\n\n",operand_1 - operand_2);
            goto yn;
            break;
            
        case '*':
            printf("= %.2lf\n\n",operand_1 * operand_2);
            goto yn;
            break;
            
        case '/':
            if (operand_2 == 0) {
                printf("Can't divide by 0, please try again.\n\n");
                goto again;
            } else {
                printf("= %.2lf\n\n",operand_1 / operand_2);
                goto yn;
            }
            break;
            
        default:
            printf("Incorrect operator, please try again.\n\n");
            goto again;
            break;
    }
   
    //continue or not
    yn:
        printf("Would you like to continue? (Y/N)\n\n");
        scanf("%c",&restart);
   
        if (restart == 'y' || restart == 'Y'){
            goto again;
        }else if (restart == 'n' || restart == 'N'){
            return 0;
        }else{
            printf("invalid charactor, please enter \"Y\" or \"N\".\n\n");
            goto yn;
        }
   
    return 0;
}

Run的结果是这样：

Please enter your calculation: 1＋1
= 2.00

Would you like to continue? (Y/N)

invalid charactor, please enter "Y" or "N".

Would you like to continue? (Y/N)

进行一次计算后，显示“Would you like to continue? (Y/N)”，然后没有输入"Y" 或 "N"的机会，直接就执行后边的if-else，得到结果“invalid charactor, please enter "Y" or "N".”，再次 goto yn，才停下来让人输入Y/N。

我觉得问题可能出在：从switch方法中的语句 goto yn 之后，printf();后边的scanf();没有执行。可是。。。这是为啥啊。。。难道还有其它问题？我检查了几遍代码，以现有知识没有发现什么明显错误（小弟还没学到循环呢。。。）。

求前辈们帮忙指点。

太感谢了! 病因、病理、解药一应俱全～

我学过的内容中，scanf()没有详细的说明，只知道有这么种方法可以用。 getchar()也没出现过，更不知道回车也会进入输入缓冲区。多谢前辈介绍这些知识。