编写一个c语言程序，用switch语句，从键盘输入一个四位整数n，输出它的各位数之和

好的，这个问题可以回答。以下是代码：

#include <stdio.h>

int main() {
    int n, sum = 0;
    printf("请输入一个四位整数：");
    scanf("%d", &n);
    while (n > 0) {
        sum += n % 10;
        n /= 10;
    }
    printf("各位数之和为：%d\n", sum);
    return 0;
}

使用 switch 语句也可以实现，以下是代码：

#include <stdio.h>

int main() {
    int n, sum = 0;
    printf("请输入一个四位整数：");
    scanf("%d", &n);
    switch (n / 1000) {
        case 1:
            sum += 1;
            break;
        case 2:
            sum += 2;
            break;
        case 3:
            sum += 3;
            break;
        case 4:
            sum += 4;
            break;
        case 5:
            sum += 5;
            break;
        case 6:
            sum += 6;
            break;
        case 7:
            sum += 7;
            break;
        case 8:
            sum += 8;
            break;
        case 9:
            sum += 9;
            break;
    }
    switch (n / 100 % 10) {
        case 1:
            sum += 1;
            break;
        case 2:
            sum += 2;
            break;
        case 3:
            sum += 3;
            break;
        case 4:
            sum += 4;
            break;
        case 5:
            sum += 5;
            break;
        case 6:
            sum += 6;
            break;
        case 7:
            sum += 7;
            break;
        case 8:
            sum += 8;
            break;
        case 9:
            sum += 9;
            break;
    }
    switch (n / 10 % 10) {
        case 1:
            sum += 1;
            break;
        case 2:
            sum += 2;
            break;
        case 3:
            sum += 3;
            break;
        case 4:
            sum += 4;
            break;
        case 5:
            sum += 5;
            break;
        case 6:
            sum += 6;
            break;
        case 7:
            sum += 7;
            break;
        case 8:
            sum += 8;
            break;
        case 9:
            sum += 9;
            break;
    }
    switch (n % 10) {
        case 1:
            sum += 1;
            break;
        case 2:
            sum += 2;
            break;
        case 3:
            sum += 3;
            break;
        case 4:
            sum += 4;
            break;
        case 5:
            sum += 5;
            break;
        case 6:
            sum += 6;
            break;
        case 7:
            sum += 7;
            break;
        case 8:
            sum += 8;
            break;
        case 9:
            sum += 9;
            break;
    }
    printf("各位数之和为：%d\n", sum);
    return 0;
}