C语言程序中scanf读取oper时出现命令跳过问题求助

问题概述 #

编写的医院费用计算C程序在运行时，输入手术费变量oper环节出现输入被“跳过”的异常，此前还遇到char类型变量agree被错误识别为int的问题。以下是原程序代码、异常代码段及运行结果：

原程序代码 #

#include <stdio.h>

int main()
{
  int inpa, med, oper, day, total;
  char agree;
  

  printf("                     Bach Mai Hospital");
  printf("\n\nHello, please enter your fee and we will calculate\npayment based on your insurance\n");
  printf("How many days have you been in the hospital ");
  scanf("%d", &day);
  printf("How much is your medicine fee ");
  scanf("%d", &med);

  printf("Have you undergone surgery (Yes or No)");
  scanf("%s", &agree);
  switch(agree){
  case 'Y':
    printf("Enter your surgery fee ");
    scanf(" %d", &oper);
    break;
  case 'N':
    oper = 0;
  break;
  };
  printf("%s", agree);

  inpa = day * 15000;
  printf("Your total fee\n");
  printf("Inpatient fee: %-10d x 15000 = %d\n", day, inpa);
  printf("Medicine fee: %-10d\n", med);
  printf("Surgery fee: %-10d\n", oper);
  total = inpa + med + oper;
  printf("\n\nYou pay: %d\n", total);

  return 0;

}

异常代码段 #

printf("Enter your surgery fee ");
scanf(" %d", &oper);

程序执行结果 #

Bach Mai Hospital

Hello, please enter your fee and we will calculate
payment based on your insurance
How many days have you been in the hospital 8
How much is your medicine fee 90000000
Have you undergone surgery (Yes or No)Yes
Enter your surgery fee 80000000
PS D:\Desktop\Cprogram> 

问题原因分析 #

1. char变量使用%s格式符读取的内存越界问题

   • agree是单个char类型变量，仅占1字节内存，但代码中用scanf("%s", &agree);读取输入。%s用于读取字符串，会把输入的所有字符（包括后续的'e'、's'以及自动添加的字符串结束符'\0'）依次写入&agree指向的内存地址。
   • 这会直接覆盖agree后面相邻的内存区域（恰好是oper变量的内存），导致oper的初始值被破坏。后续读取oper时，内存已被篡改，进而出现输入异常甚至程序崩溃。

2. printf格式符不匹配的未定义行为

   • printf("%s", agree);中，%s要求传入字符串指针，但实际传入的是char类型的agree。程序会把agree的字符值当作内存地址去读取字符串，属于未定义行为，可能输出乱码或导致程序异常。

修复方案 #

修复后的代码 #

#include <stdio.h>

int main()
{
  int inpa, med, oper, day, total;
  char agree;
  

  printf("                     Bach Mai Hospital");
  printf("\n\nHello, please enter your fee and we will calculate\npayment based on your insurance\n");
  printf("How many days have you been in the hospital ");
  scanf("%d", &day);
  printf("How much is your medicine fee ");
  scanf("%d", &med);

  // 用%c读取单个字符，前面加空格跳过输入缓冲区的换行符
  printf("Have you undergone surgery (Yes or No)");
  scanf(" %c", &agree);
  
  switch(agree){
  case 'Y':
  case 'y': // 兼容小写输入
    printf("Enter your surgery fee ");
    scanf("%d", &oper);
    break;
  case 'N':
  case 'n': // 兼容小写输入
    oper = 0;
    break;
  default:
    printf("Invalid input, defaulting to no surgery.\n");
    oper = 0;
    break;
  };
  // 用%c打印单个字符
  printf("Your choice: %c\n", agree);

  inpa = day * 15000;
  printf("Your total fee\n");
  printf("Inpatient fee: %-10d x 15000 = %d\n", day, inpa);
  printf("Medicine fee: %-10d\n", med);
  printf("Surgery fee: %-10d\n", oper);
  total = inpa + med + oper;
  printf("\n\nYou pay: %d\n", total);

  return 0;

}

关键修复点 #

• 将scanf("%s", &agree);改为scanf(" %c", &agree);：%c专门用于读取单个字符，前面的空格会自动跳过输入缓冲区中残留的换行符，避免读取到之前输入留下的空白字符。
• 将printf("%s", agree);改为printf("%c", agree);：用%c匹配char类型变量，避免未定义行为。
• 增加大小写兼容的判断和默认分支：处理用户输入小写字母或无效字符的情况，提升程序健壮性。

内容的提问来源于stack exchange，提问作者Phan Đào Minh Quân