c - 函数在 switch 语句中被不正确地执行/重复。其余代码不执行

Question

到目前为止，main() 执行正常，while 循环执行正常。如果我输入了错误的 PIN，程序会立即停止执行。否则，程序将继续执行其余部分。

我决定测试一个函数，transfer()，这就是我遇到问题的地方。传递函数只对 printf 语句正确执行。程序没有等待我输入要在哪个帐户上进行转账，而是跳回到 transfer() 的开头并不断重复……

transfer() 中的 switch 语句甚至没有执行!为什么？

//This program will mimick an ATM machine
//It will do deposits, withdrawals, and authentication

//Function Declarations
//float withdrawal();
//float deposit();
float transfer ();
void decision ();

float checking = 15050.00;
float savings = 8200.83;

#include <stdio.h>

int main ()
{
    int PIN;
    printf ("Welcome to Wells Fargo 24HR Teller!\n");
    printf ("Enter Your 4-Digit PIN \n");
    scanf ("%d", &PIN);

    while (PIN == 2016) {
        decision ();            //this starts the ATM choices user can select
    }
    if (PIN != 2016) {
        printf ("AUTHENTICATION FAILED");
    }
    return 0;
}

void decision ()
{

    printf ("I want to:\n");
    printf ("A) Make a Transfer\n");
    //printf("B) Make a Deposit\n");
    //printf("C) Make a Withdrawal\n");
    //printf("D) Exit\n");

    char selection;
    scanf ("%c", &selection);

    switch (selection) {
    case 'A':
        transfer ();
        //case 'B':  deposit();
        //case 'C':  withdrawal();
        //case 'D': break;
    }
}

float transfer ()
{
    float transamt;
    float chekbal;
    float savbal;
    printf ("Transfer Amount? \n");
    scanf ("%f", &transamt);
    printf ("\n Transfer Amount of %f", transamt);


    printf ("\n Which Account? \n");
    printf (" A) Checking \n");
    printf (" B) Savings \n");

    /*Right after this point, execution goes back to beginning of decision()
    program execution doesn't go through switch statement */
    char confirm;
    scanf ("%c", &confirm);

    switch (confirm) {
    case 'A':
        chekbal = (transamt + checking);
        printf ("Checking Balance is Now %f", chekbal);
        break;

    case 'B':
        savbal = (transamt + savings);
        printf ("Savings Balance is Now %f", savbal);
        break;
    }

    return 0;
}

Answer 1

问题是带有 %c 转换说明符的 scanf 不消耗空格，参见 here :

All conversion specifiers other than [, c, and n consume and discard all leading whitespace characters

输入转账金额后回车，stdin中插入一个换行符，scanf("%c", &confirm)看到后存入confirm，即confirm 现在包含换行符。要明确地看到这一点，请尝试将 case '\n': 添加到 switch(confirm) 语句；您会看到 switch 语句实际上正在执行，只是 confirm 的值与您之前的任何情况都不匹配。最好始终将 default: 标签添加到 switch 语句以捕获任何意外值。

解决方案非常简单，您只需要在格式字符串中的 %c 之前添加任意空白字符，因为(来自上面的同一来源)，

any single whitespace character in the format string consumes all available consecutive whitespace characters from the input

因此，scanf("%c", &confirm); 将在读取将分配给 确认。