C语言读入两组整数(每组不超过20个整数,并且同一组中的整数各不相同),编程求两组整数的交集,即在两组整数中都出现的整数,并按从大到小的顺序输出。若交集为空,则什么都不输出。

在C语言中，你可以通过以下步骤编写程序来实现这个功能：

1. 首先，你需要创建两个数组分别存储两组整数，并使用`scanf`函数读取用户输入。

#include <stdio.h>

#define MAX_NUMS_PER_SET 20

int set1[MAX_NUMS_PER_SET];
int set2[MAX_NUMS_PER_SET];

int main() {
    int num1, num2;
    for (int i = 0; i < MAX_NUMS_PER_SET; i++) {
        printf("Enter number %d of the first set (separated by space and no duplicates): ", i + 1);
        while (scanf("%d", &num1) == 1) {
            if (i == 0 || num1 != set1[i - 1]) { // Check if it's unique
                set1[i] = num1;
            } else {
                break; // Stop reading when duplicate found
            }
        }
        // Skip input for second set since we'll check later
    }
    
    // Read second set in a similar manner
    for (int i = 0; i < MAX_NUMS_PER_SET; i++) {
        scanf("%d", &num2);
        set2[i] = num2;
    }

    // Now find intersection
    int common_count = 0;
    int common[MAX_NUMS_PER_SET] = {0};
    for (int i = 0; i < MAX_NUMS_PER_SET; i++) {
        for (int j = 0; j < MAX_NUMS_PER_SET; j++) {
            if (set1[i] == set2[j]) {
                common[common_count++] = set1[i]; // Store in order, ignoring duplicates
                break;
            }
        }
    }

    // Sort the common elements from largest to smallest
    if (common_count > 0) {
        qsort(common, common_count, sizeof(int), compare_ints_descending);
        
        printf("Intersection of the two sets, in descending order:\n");
        for (int i = common_count - 1; i >= 0; i--) {
            printf("%d ", common[i]);
        }
    }

    return 0;
}

// Custom comparison function to sort integers in descending order
int compare_ints_descending(const void *a, const void *b) {
    return (*(const int*)b - *(const int*)a);
}