有10个学生，每个学生的数据包括学号、姓名、3门功课的成绩。从键盘输入10个学生的数据，要求按平均成绩降序显示出所有学生的数据（包括学号、姓名、3门课成绩、平均成绩），并将其写入文本文件result.txt中。用C语言编写

好的，以下是代码实现：

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

#define MAX_STUDENT_NUM 10
#define MAX_NAME_LENGTH 20

typedef struct {
    int id;
    char name[MAX_NAME_LENGTH];
    int score1;
    int score2;
    int score3;
    float avg_score;
} Student;

void input_students(Student *students, int num);
void calc_avg_score(Student *students, int num);
void sort_students(Student *students, int num);
void output_students(Student *students, int num);

int main() {
    Student students[MAX_STUDENT_NUM];
    input_students(students, MAX_STUDENT_NUM);
    calc_avg_score(students, MAX_STUDENT_NUM);
    sort_students(students, MAX_STUDENT_NUM);
    output_students(students, MAX_STUDENT_NUM);
    return 0;
}

void input_students(Student *students, int num) {
    printf("请输入%d个学生的数据：\n", num);
    for (int i = 0; i < num; i++) {
        printf("请输入第%d个学生的学号、姓名、3门课成绩，以空格分隔：", i + 1);
        scanf("%d %s %d %d %d", &students[i].id, students[i].name, &students[i].score1, &students[i].score2, &students[i].score3);
    }
}

void calc_avg_score(Student *students, int num) {
    for (int i = 0; i < num; i++) {
        students[i].avg_score = (float)(students[i].score1 + students[i].score2 + students[i].score3) / 3;
    }
}

void sort_students(Student *students, int num) {
    for (int i = 0; i < num - 1; i++) {
        for (int j = i + 1; j < num; j++) {
            if (students[i].avg_score < students[j].avg_score) {
                Student temp = students[i];
                students[i] = students[j];
                students[j] = temp;
            }
        }
    }
}

void output_students(Student *students, int num) {
    FILE *fp = fopen("result.txt", "w");
    if (fp == NULL) {
        printf("无法打开文件\n");
        exit(1);
    }
    printf("学号\t姓名\t成绩1\t成绩2\t成绩3\t平均成绩\n");
    fprintf(fp, "学号\t姓名\t成绩1\t成绩2\t成绩3\t平均成绩\n");
    for (int i = 0; i < num; i++) {
        printf("%d\t%s\t%d\t%d\t%d\t%.2f\n", students[i].id, students[i].name, students[i].score1, students[i].score2, students[i].score3, students[i].avg_score);
        fprintf(fp, "%d\t%s\t%d\t%d\t%d\t%.2f\n", students[i].id, students[i].name, students[i].score1, students[i].score2, students[i].score3, students[i].avg_score);
    }
    fclose(fp);
}

代码中使用了结构体来存储学生信息，其中 `input_students` 函数用于从键盘输入学生信息，`calc_avg_score` 函数用于计算平均成绩，`sort_students` 函数用于按平均成绩降序排序，`output_students` 函数用于显示学生信息并将其写入文件 `result.txt` 中。