在C中的开关盒中检测Enter？

Question

在这方面遇到的麻烦比我应该的要多。我正在构建一个非常简单的银行菜单界面，用户可以在其中查看余额、存款、取款或退出菜单。 case 1、case2 和 case3 似乎工作正常，但无论我在 Google 上尝试什么，case 4 都无法正常工作。

在当前版本中，case 4 中的 printf() 语句出现，然后似乎没有其他任何事情发生，直到我输入其他选项之一 (1 - 3) 两次，此时循环将自动循环使用第二个输入。我尝试了getchar()，但这似乎不起作用（或者我没有正确实现它）。

为什么会出现这种行为，我该如何解决？

代码如下：

#include <stdio.h>
#include <cs50.h>

double deposit(double a, double b);
double withdraw(double a, double b);

int main(void)
{
    int resume = 1;
    int user_input = 0;
    double user_balance = 10.00;
    printf("Welcome to UBank!\n");

    while (resume)
    {
        printf("\n====================\n");
        printf("Select an operation:\n\n1. Show Balance\n2. Make a Deposit\n3. Make a Withdrawal\n4. Quit\n"
               "====================\n\n");
        scanf("%d", &user_input);

        int quit_character = 0x00;
        double deposit_amount = 0.00;
        double withdraw_amount = 0.00;

        switch (user_input)
        {
            case 1:
                printf("Balance: $%.2lf\n", user_balance);
                break;
            case 2:
                printf("How much would you like to deposit?\n");
                scanf("%lf", &deposit_amount);
                user_balance = deposit(user_balance, deposit_amount);
                break;
            case 3:
                printf("How much would you like to withdraw?\n");
                scanf("%lf", &withdraw_amount);
                user_balance = withdraw(user_balance, withdraw_amount);
                break;
            case 4:
                printf("Press Enter to finish banking or any other key to continue.\n");
                scanf("%d\n", &quit_character);
                if (quit_character == 0x0A)
                {
                    resume = 0;
                }
                break;
        }
    }
}

double deposit(double a, double b)
{
    if (b > 0 && b < 10000)
    {
        return a + b;
    }
    else
    {
        printf("Please enter a valid amount. (0.01 - 9999.99)\n");
        return a;
    }
}

double withdraw(double a, double b)
{
    if (b > 0 && a - b >= 10)
    {
        return a - b;
    }
    else if (b <= 0)
    {
        printf("Withdrawal amount must be greater than $0.00.\n");
    }
    else if (a - b < 10)
    {
        printf("Withdrawal amount invalid.  Remaining balance must be $10.00 or more.\n");
    }
    return a;
}

Answer 1

假设我想粉刷我家的外部。我买了油漆，然后回到我家（从最近的商店开车 5 小时）。当我回到家时，我意识到我忘了买一个画笔。我不想在接下来的 10 个小时里开车去拿油漆刷，但我应该怎么涂油漆呢？

我可以使用扫帚、拖把、抹布、我的手、我儿子的手等等，没有一个比刷子更可取。但后来我意识到……我有一匹马！我剪下马的主要部分并制作自己的画笔！我为这一天作画，下周当我需要另一支刷子时，我会再次给我的马剪毛。这最终比 10 小时车程便宜很多。

好吧...这是一个老生常谈的虚构故事。关键是使用 scanf() 进行用户输入就像用猪鼻子作画。这只是工作的错误工具。最好编写您自己的用户输入函数，该函数按您期望的方式工作。

考虑下面的 GetInput() 函数。我必须自己做，但它肯定比使用 scanf() 进行用户输入要好：

  #include <stdio.h>
  #include <stdlib.h>

  double deposit(double a, double b);
  double withdraw(double a, double b);

  /*-----------------------------------------------------------------------------------
  ** Get input from user, and return it as specific types.  
  **
  ** Caller may specify NULL for any (or all) input types that are not needed.
  */
  void GetInput(int *intOut, char *charOut, double *doubleOut)
    {
    // Pointer to a temporary (allocated memory) buffer to hold user input.
    char    *line_A   = NULL;

    // Number of bytes allocated to the temporary (allocated) buffer.
    size_t   lineSize = 0;

    // Holds the number of bytes entered by the user, or (-1) for error.
    ssize_t  lineLength;

    lineLength=getline(&line_A, &lineSize, stdin);
    if(-1 == lineLength)
       {
       fprintf(stderr, "getline() failed.\n"
       goto CLEANUP;
       }

    // If the caller did not specify NULL for intOut, ...
    if(intOut)
      // Convert the string to an int and pass it back to the caller.
      *intOut = atoi(line_A); 

    if(charOut)
      *charOut = *line_A;

    if(doubleOut)
      *doubleOut = atof(line_A);

  CLEANUP:
    if(line_A)
      free(line_A);

    return;
    }

  int main(void)
  {
      int resume = 1;
      int user_input = 0;
      double user_balance = 10.00;
      printf("Welcome to UBank!\n");

      while (resume)
      {
          printf("\n====================\n");
          printf("Select an operation:\n\n1. Show Balance\n2. Make a Deposit\n3. Make a Withdrawal\n4. Quit\n"
                 "====================\n\n");
          GetInput(&user_input, NULL, NULL);     // Get an integer from the user.

          char quit_character = 0x00;
          double deposit_amount = 0.00;
          double withdraw_amount = 0.00;

          switch (user_input)
          {
              case 1:
                  printf("Balance: $%.2lf\n", user_balance);
                  break;
              case 2:
                  printf("How much would you like to deposit?\n");
                  GetInput(NULL, NULL, &deposit_amount);  // Get a double from the user.
                  user_balance = deposit(user_balance, deposit_amount);
                  break;
              case 3:
                  printf("How much would you like to withdraw?\n");
                  GetInput(NULL, NULL, &withdraw_amount);  // Get a double from the user.
                  user_balance = withdraw(user_balance, withdraw_amount);
                  break;
              case 4:
                  printf("Press Enter to finish banking or any other key to continue.\n");
                  GetInput(NULL, &quit_character, NULL);  //Get a character from the user.
                  if (quit_character == 0x0A)
                  {
                      resume = 0;
                  }
                  break;
          }
      }
  }

  double deposit(double a, double b)
  {
      if (b > 0 && b < 10000)
      {
          return a + b;
      }
      else
      {
          printf("Please enter a valid amount. (0.01 - 9999.99)\n");
          return a;
      }
  }

  double withdraw(double a, double b)
  {
      if (b > 0 && a - b >= 10)
      {
          return a - b;
      }
      else if (b <= 0)
      {
          printf("Withdrawal amount must be greater than $0.00.\n");
      }
      else if (a - b < 10)
      {
          printf("Withdrawal amount invalid.  Remaining balance must be $10.00 or more.\n");
      }
      return a;
  }