如何修复银行菜单程序中switch默认错误及相关问题？

问题现象 #

1. 菜单首次运行正常，从第二次迭代开始操作会重复执行两次，且每次都会触发默认分支的Choose a valid option!提示
2. average函数只能输出整数结果，无法显示精确的浮点平均值

原代码 #

#include <stdio.h>

void askForInput(int arr5[], int dep);
void fSum(int arr[], int n);
void sortDep(int arr2[], int n1);
void average(int arr3[], int n2);
void lowestVal(int arr4[], int n3);

int d = 0;
int deposits[] = {};

int main() {
    char letter;
    do {
        printf("****BANK MENU****\n"
               "[G]et a new deposit\n"
               "[S]um of all deposits\n"
               "[D]isplay deposits\n"
               "[A]verage\n"
               "[L]owest\n"
               "[Q]uit\n"
               "Please enter a letter: ");

        scanf("%c", &letter);

        switch (letter) {
          case 'G':
            askForInput(deposits, d);
            printf("\n");
            d++;
            break;

          case 'S':
            fSum(deposits, d);
            printf("\n");
            break;

         case 'D':
           sortDep(deposits, d);
           printf("These are your deposits:\n");
           for (int k = 0; k < d; k++)
               printf("%d ",deposits[k]);
           printf("\n");
           break;

         case 'A':
           average(deposits, d);
           printf("\n");
           break;

         case 'L':
           lowestVal(deposits,d);
           printf("\n");
           break;

         case 'Q':;
         default:
           printf("Choose a valid option!\n");
       }
    } while (letter != 'Q');
    return 0;
}

void askForInput(int arr5[], int dep) {
    printf("Please enter a deposit:\n");
    scanf("%d", &(arr5[dep]));
}

void fSum(int arr[], int n) {
    int sum = 0;
    for (int x = 0; x < n; x++)
        sum = sum + arr[x];
    printf("The sum of the deposits is %d", sum);
}

void sortDep(int arr2[], int n1) {
    int i, j, temp;
    for (i = 0; i < n1 - 1; i++)
        for (j = 0; j < n1 - i - 1; j++)
            if (arr2[j] < arr2[j + 1]) {
                temp = arr2[j];
                arr2[j] = arr2[j + 1];
                arr2[j + 1] = temp;
            }
}

void average(int arr3[], int n2) {
    int sumation = 0;
    int avg;
    for (int u = 0; u < n2; u++) {
        sumation += arr3[u];
    }
    avg = sumation / n2;
    printf("The average of the deposits is %d", avg);
}

void lowestVal(int arr4[], int n3) {
    int minIndex = 0;
    for (int index = 1; index < n3; index++) {
        if (arr4[minIndex] > arr4[index])
            minIndex = index;
    }
    printf("The lowest deposit is %d", arr4[minIndex]);
}

问题根源分析 #

1. 输入缓冲区残留问题：scanf("%c", &letter)会读取输入时留下的换行符（比如用户输入G后按回车，回车的\n会留在缓冲区），第二次循环时letter被赋值为\n，触发switch的默认分支，造成重复执行的假象。
2. 数组初始化错误：int deposits[] = {};在C标准中是非法的，会创建大小为0的数组，存入数据时会发生越界，导致未定义行为。
3. average函数精度丢失：用int存储平均值，整数除法会直接丢弃小数部分，无法得到精确结果。
4. switch分支逻辑错误：case 'Q'后没有break，执行完Q分支后会继续进入default分支，导致退出时也会打印错误提示。

修复方案 #

1. 处理输入缓冲区 #

将scanf("%c", &letter);改为scanf(" %c", &letter);，前面的空格会自动跳过所有空白字符（包括换行、空格等），避免读取残留的换行符。

2. 修正数组定义 #

定义一个固定大小的数组（比如支持最多100条存款记录），避免越界：

#define MAX_DEPOSITS 100
int deposits[MAX_DEPOSITS];

同时在askForInput调用前检查d是否小于MAX_DEPOSITS，防止数组越界。

3. 修复average函数精度问题 #

将求和变量改为long避免溢出，平均值用double存储，输出时使用浮点格式：

void average(int arr3[], int n2) {
    if (n2 == 0) {
        printf("No deposits to calculate average!");
        return;
    }
    long sumation = 0;
    double avg;
    for (int u = 0; u < n2; u++) {
        sumation += arr3[u];
    }
    avg = (double)sumation / n2;
    printf("The average of the deposits is %.2f", avg);
}

4. 修正switch分支逻辑 #

给case 'Q'添加break，避免执行default分支：

case 'Q':
    break;
default:
    printf("Choose a valid option!\n");

5. 添加边界检查 #

在case 'G'中检查是否还有数组空间，同时给其他功能添加空数据判断，避免无存款时执行计算逻辑：

case 'G':
    if (d >= MAX_DEPOSITS) {
        printf("Maximum number of deposits reached!\n\n");
        break;
    }
    askForInput(deposits, d);
    printf("\n");
    d++;
    break;

修复后的完整代码 #

#include <stdio.h>

#define MAX_DEPOSITS 100

void askForInput(int arr5[], int dep);
void fSum(int arr[], int n);
void sortDep(int arr2[], int n1);
void average(int arr3[], int n2);
void lowestVal(int arr4[], int n3);

int d = 0;
int deposits[MAX_DEPOSITS];

int main() {
    char letter;
    do {
        printf("****BANK MENU****\n"
               "[G]et a new deposit\n"
               "[S]um of all deposits\n"
               "[D]isplay deposits\n"
               "[A]verage\n"
               "[L]owest\n"
               "[Q]uit\n"
               "Please enter a letter: ");

        // 跳过空白字符，避免读取换行符
        scanf(" %c", &letter);

        switch (letter) {
          case 'G':
            if (d >= MAX_DEPOSITS) {
                printf("Maximum number of deposits reached!\n\n");
                break;
            }
            askForInput(deposits, d);
            printf("\n");
            d++;
            break;

          case 'S':
            if (d == 0) {
                printf("No deposits to calculate sum!\n");
            } else {
                fSum(deposits, d);
            }
            printf("\n");
            break;

         case 'D':
            if (d == 0) {
                printf("No deposits to display!\n");
            } else {
                sortDep(deposits, d);
                printf("These are your deposits:\n");
                for (int k = 0; k < d; k++)
                    printf("%d ", deposits[k]);
            }
            printf("\n");
            break;

         case 'A':
            average(deposits, d);
            printf("\n");
            break;

         case 'L':
            if (d == 0) {
                printf("No deposits to find lowest value!\n");
            } else {
                lowestVal(deposits, d);
            }
            printf("\n");
            break;

         case 'Q':
            break;
         default:
           printf("Choose a valid option!\n");
       }
    } while (letter != 'Q');
    return 0;
}

void askForInput(int arr5[], int dep) {
    printf("Please enter a deposit:\n");
    scanf("%d", &(arr5[dep]));
}

void fSum(int arr[], int n) {
    long sum = 0;
    for (int x = 0; x < n; x++)
        sum = sum + arr[x];
    printf("The sum of the deposits is %ld", sum);
}

void sortDep(int arr2[], int n1) {
    int i, j, temp;
    for (i = 0; i < n1 - 1; i++)
        for (j = 0; j < n1 - i - 1; j++)
            if (arr2[j] < arr2[j + 1]) {
                temp = arr2[j];
                arr2[j] = arr2[j + 1];
                arr2[j + 1] = temp;
            }
}

void average(int arr3[], int n2) {
    if (n2 == 0) {
        printf("No deposits to calculate average!");
        return;
    }
    long sumation = 0;
    double avg;
    for (int u = 0; u < n2; u++) {
        sumation += arr3[u];
    }
    avg = (double)sumation / n2;
    printf("The average of the deposits is %.2f", avg);
}

void lowestVal(int arr4[], int n3) {
    int minIndex = 0;
    for (int index = 1; index < n3; index++) {
        if (arr4[minIndex] > arr4[index])
            minIndex = index;
    }
    printf("The lowest deposit is %d", arr4[minIndex]);
}

内容的提问来源于stack exchange，提问作者RedPumpkin