练习7：输入n(4<=n<=20)个学生m(2<=m<=5)门课程的成绩，分别求每个学生的平均成绩和每门课程的平均成绩并显示，并根据输入的科目序号j（1<=j<=m）和排名i（1<=i<=n），将第j个科目的前i个成绩从高到低排序显示。以上功能均书写函数实现。

好的，我们可以创建几个C语言函数来实现这个需求。首先，我们需要定义结构体表示学生和课程信息，然后编写函数来接收输入、计算平均分以及对成绩进行排序。以下是一个可能的实现：

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

// 定义结构体表示学生和课程
typedef struct {
    int id;
    float scores[20]; // 假设最多有20门课
} Student;

typedef struct {
    char name[20];
    Student *students;
    int num_students;
} Course;

// 函数用于计算学生平均成绩
float calculate_student_average(Student *student, int m) {
    float sum = 0;
    for (int i = 0; i < m; i++) {
        sum += student->scores[i];
    }
    return sum / m;
}

// 函数用于计算课程平均成绩
float calculate_course_average(Course *course) {
    float total_sum = 0;
    for (int i = 0; i < course->num_students; i++) {
        for (int j = 0; j < 20; j++) { // 注意这里假设课程只用到了学生数组的一部分
            if (j < course->students[i].scores[course->students[i].id]) {
                total_sum += course->students[i].scores[j];
            }
        }
    }
    return total_sum / (course->num_students * 20);
}

// 对特定科目和排名排序成绩
void sort_scores_by_subject_and_rank(int j, int i, Course *course) {
    for (int k = 0; k < course->num_students; k++) {
        for (int l = 0; l < i - 1; l++) {
            if (course->students[k].scores[j - 1] < course->students[l].scores[j - 1]) {
                float temp = course->students[k].scores[j - 1];
                course->students[k].scores[j - 1] = course->students[l].scores[j - 1];
                course->students[l].scores[j - 1] = temp;
            }
        }
    }
}

int main() {
    int n, m, j, i;
    printf("Enter the number of students (4 <= n <= 20): ");
    scanf("%d", &n);

    Course course;
    course.students = malloc(n * sizeof(Student));
    course.num_students = n;

    for (int s = 0; s < n; s++) {
        printf("Enter student %d's ID: ", s + 1);
        scanf("%d", &course.students[s].id);
        
        for (int c = 0; c < m; c++) {
            printf("Enter score for subject %d: ", c + 1);
            scanf("%f", &course.students[s].scores[c]);
        }
    }

    printf("\nAverage scores by student:\n");
    for (int s = 0; s < n; s++) {
        float avg = calculate_student_average(&course.students[s], m);
        printf("Student %d: %.2f\n", s + 1, avg);
    }

    printf("\nAverage scores by course:\n");
    float avg_course = calculate_course_average(&course);
    printf("Average score across all subjects: %.2f\n", avg_course);

    printf("\nEnter subject number (1-%d) and ranking (1-%d): ", m, n);
    scanf("%d %d", &j, &i);
    sort_scores_by_subject_and_rank(j - 1, i, &course);

    printf("\nTop %d scores for subject %d:\n", i, j);
    for (int s = 0; s < i; s++) {
        printf("Student %d: %.2f\n", course.students[s].id, course.students[s].scores[j - 1]);
    }

    free(course.students);
    return 0;
}