总共打印三次图书目录信息

第一次，是直接按输入的顺序进行输出打印。

第二次，按字母排序，通过strcmp(）函数比较，通过新建的临时int变量，对编号进行重新赋值，再通过循环比较编号进行排序。

第三次，按书本金额排序，引入了一个临时结构，通过临时结构来对两个结构进行数据的交换，然后打印。

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

# define MAXTITL 40
# define MAXAUTL 40
# define MAXBKS  100

struct book {
    int num;    //编号
    char title[MAXTITL];
    char author[MAXAUTL];
    float value;
};

struct book library[MAXBKS];

char* s_gets(char* st, int n);

void show_list(struct book library[], int n);       //按输入顺序
void show_letter(struct book library[], int n);     //按字母顺序
void show_value(struct book library[], int n);      //按金额升序

int main()
{
    int count = 0;
    struct book * pt;
    pt = &library[0];

    printf("Please enter the book title:\n");
    printf("Press [enter] at the start of a line to quit.\n");
    while (count <= MAXBKS && s_gets(library[count].title, MAXTITL) != NULL && library[count].title[0] != '\0')
    {
        puts("Enter the author:");
        s_gets(library[count].author, MAXAUTL);
        puts("Enter the value:");
        scanf("%f",&library[count].value);
        while (getchar() != '\n')
            continue;         //清理输入行
        library[count++].num = count;
        if (count < MAXBKS)
            printf("PLease enter next title:\n");
    }

    if (count > 0)
    {
        puts("According to the entry sequence:");
        show_list(pt,count);
        puts("According to the first letter of the book_title:");
        show_letter(pt, count);
        puts("According to the value of the book:");
        show_value(pt, count);
    }
    else 
       puts("No book.");
        
    return 0;
}

void show_list(struct book library[],int n)     
{
    for (int i = 0; i < n; i++)
        printf("%d : %s by %s : $ %.2f\n",library[i].num, library[i].title, library[i].author,library[i].value);
}

void show_letter(struct book library[], int n)
{
    int temp;        //临时变量
    int k = 0;
    for (int i = 0; i < n; i++)
        for(int j = i+1;j < n ;j++)
    {
            if (strcmp(library[i].title, library[j].title) > 0)
            {
                temp = library[i].num;
                library[i].num = library[j].num;
                library[j]. num = temp;
            }
    }
    while(k < n )
    {
        for (int i = 0; i < n; i++)
        {
            if (library[i].num == k)
                printf("%d :%s by %s : %.2f\n", library[i].num, library[i].title, library[i].author, library[i].value);
        }
        k++;
    }   
}

void show_value(struct book library[], int n)
{
    struct book temp;
    for(int i = 0;i < n;i++)
        for (int j = i + 1; j < n; j++)
        {
            if (library[i].value > library[j].value)
            {
                struct book temp = library[i];
                library[i] = library[j];
                library[j] = temp;
            }
       }
    for(int k = 0;k<n;k++)
        printf("%d : %s by %s : $ %.2f\n", library[k].num, library[k].title, library[k].author, library[k].value);
}

char* s_gets(char* pt, int n)
{
    char* ret_val;
    char* find;

    ret_val = fgets(pt, n, stdin);
    if (ret_val)
    {
        find = strchr(pt, '\n');
        if (find)
            *find = '\0';
        else
            while (getchar() != '\n')
                continue;
    }
    return ret_val;
}