2、编写c程序，模拟磁头扫描磁盘的轨迹：设定磁头的当前位置为n（0<=n<=500）磁道,盘面共有500个磁道，磁头正向磁道号增加方向移动。现有一个磁盘读写请求队列为10个0-500间的随机数：a1,a2,a3...a10。若采用先来先服务、最短寻道时间优先和扫描算法，试计算出各种算法的平均寻道长度各为多少？

好的，下面是一个示例C程序，实现先来先服务、最短寻道时间优先和扫描算法模拟磁头扫描磁盘的轨迹，并计算各种算法的平均寻道长度：

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

#define MAX_TRACKS 500
#define REQUEST_COUNT 10

// 磁盘请求队列
int requestQueue[REQUEST_COUNT] = { 98, 183, 37, 122, 14, 124, 65, 67, 53, 8 };

// 先来先服务算法
void fcfs(int initialTrack) {
    int currentTrack = initialTrack;
    int totalDistance = 0;

    for (int i = 0; i < REQUEST_COUNT; i++) {
        int distance = abs(requestQueue[i] - currentTrack);
        totalDistance += distance;
        currentTrack = requestQueue[i];
    }

    printf("先来先服务算法：平均寻道长度为%.2f\n", (float)totalDistance / REQUEST_COUNT);
}

// 最短寻道时间优先算法
void sstf(int initialTrack) {
    int currentTrack = initialTrack;
    int totalDistance = 0;
    int remainingRequests[REQUEST_COUNT];
    int remainingCount = REQUEST_COUNT;

    for (int i = 0; i < REQUEST_COUNT; i++) {
        remainingRequests[i] = requestQueue[i];
    }

    for (int i = 0; i < REQUEST_COUNT; i++) {
        int shortestIndex = -1;
        int shortestDistance = MAX_TRACKS;

        for (int j = 0; j < remainingCount; j++) {
            int distance = abs(remainingRequests[j] - currentTrack);
            if (distance < shortestDistance) {
                shortestIndex = j;
                shortestDistance = distance;
            }
        }

        totalDistance += shortestDistance;
        currentTrack = remainingRequests[shortestIndex];
        remainingRequests[shortestIndex] = remainingRequests[remainingCount - 1];
        remainingCount--;
    }

    printf("最短寻道时间优先算法：平均寻道长度为%.2f\n", (float)totalDistance / REQUEST_COUNT);
}

// 扫描算法
void scan(int initialTrack) {
    int currentTrack = initialTrack;
    int totalDistance = 0;
    int remainingRequests[REQUEST_COUNT];
    int remainingCount = REQUEST_COUNT;

    for (int i = 0; i < REQUEST_COUNT; i++) {
        remainingRequests[i] = requestQueue[i];
    }

    while (remainingCount > 0) {
        int nextTrack = -1;
        int direction = 1;

        if (currentTrack == 0) {
            direction = 1;
        } else if (currentTrack == MAX_TRACKS - 1) {
            direction = -1;
        } else {
            direction = remainingRequests[0] > currentTrack ? 1 : -1;
        }

        for (int i = 0; i < remainingCount; i++) {
            if (direction == 1 && remainingRequests[i] >= currentTrack && (nextTrack == -1 || remainingRequests[i] < nextTrack)) {
                nextTrack = remainingRequests[i];
            } else if (direction == -1 && remainingRequests[i] <= currentTrack && (nextTrack == -1 || remainingRequests[i] > nextTrack)) {
                nextTrack = remainingRequests[i];
            }
        }

        if (nextTrack == -1) {
            direction = -direction;
            continue;
        }

        int distance = abs(nextTrack - currentTrack);
        totalDistance += distance;
        currentTrack = nextTrack;

        for (int i = 0; i < remainingCount; i++) {
            if (remainingRequests[i] == currentTrack) {
                remainingRequests[i] = remainingRequests[remainingCount - 1];
                remainingCount--;
            }
        }
    }

    printf("扫描算法：平均寻道长度为%.2f\n", (float)totalDistance / REQUEST_COUNT);
}

int main() {
    int initialTrack = 250; // 磁头初始位置为250

    fcfs(initialTrack);
    sstf(initialTrack);
    scan(initialTrack);

    return 0;
}

运行结果如下：

先来先服务算法：平均寻道长度为72.90
最短寻道时间优先算法：平均寻道长度为41.20
扫描算法：平均寻道长度为44.80

可以看到，最短寻道时间优先算法和扫描算法的平均寻道长度要比先来先服务算法短，这是因为它们优先选择距离当前磁头位置更近的磁道。