【9-3】
#include <stdio.h>
#include <stdlib.h>

void main()
{
    int num, pn = 0, nn = 0;
    FILE *fp;
    if ((fp = fopen("test.txt", "r")) == NULL)
    {
        printf("文件打不开\n");
        exit(0);
    }
    while (!feof(fp))
    {
        fscanf(fp, "%d", &num);
        if (num > 0)
            pn++;
        else if (num < 0)
            nn++;
    }
    printf("positive=%d\nnegative=%d\n", pn, nn);
}

刚才Arslanbar突然不能访问了。。。。

【9-4】
#include <stdio.h>
#include <stdlib.h>

void main()
{
    FILE *fp1, *fp2;
    char ch;
    if ((fp1 = fopen("file1.txt", "r")) == NULL || (fp2 = fopen("file2.txt", "w")) == NULL)
    {
        printf("无法打开文件\n");
        exit(0);
    }
   
    while (ch = fgetc(fp1), !feof(fp1))
    {
        putchar(ch);
        fputc(ch, fp2);
    }
    fclose(fp1);
    fclose(fp2);
}

【9-5】
#include <stdio.h>
#include <stdlib.h>

void main()
{
    FILE *fp;
    char ch;
    int a = 0, b = 0;
    if ((fp = fopen("3.c", "r")) == NULL)
    {
        printf("文件打不开\n");
        exit(0);
    }
   
    while (ch = fgetc(fp), !feof(fp))
    {
        if (ch == '{')
            a++;
        else if (ch == '}')
            b++;
    }
    if (a == b)
        printf("OK!\n");
    else
        printf("ERROR!\n");
}

【9-6】
#include <stdio.h>
#include <stdlib.h>

void main()
{
    char filename[100];
    FILE *fp;
    int count[26] = {0}; // 数组一定要初始化后再使用！
    char ch;
    int i;
    gets(filename);
    if ((fp = fopen(filename, "r")) == NULL)
    {
        printf("文件打不开\n");
        exit(0);
    }
    while (ch = fgetc(fp), !feof(fp))
    {
        if (ch >= 'A' && ch <= 'Z')
            count[ch - 'A']++;
        else if (ch >= 'a' && ch <= 'z')
            count[ch - 'a']++;
    }
    for (i = 0; i < 26; i++)
    {
        printf("%c(%c)=%4d   ", 'A' + i, 'a' + i, count[i]);
        if ((i + 1) % 5 == 0)
            putchar('\n');
    }
    putchar('\n');
    fclose(fp);
}

【9-7】
输入若干个字符串排序。。。。这个挺复杂的
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void main()
{
    FILE *fp;
    int i, j, n, min_j;
    char *temp;
    char *spaceHead;
    char **strlist;
   
    if ((fp = fopen("string.txt", "w")) == NULL)
    {
        printf("文件打不开\n");
        exit(0);
    }
   
    printf("Enter number:\n");
    scanf("%d", &n);
    strlist = (char **)calloc(n, sizeof(char *));
    spaceHead = (char *)calloc(n, 100 * sizeof(char));
    printf("Enter strings:\n");
    gets(spaceHead); // remove the \n that follows the just input number
    for (i = 0; i < n; i++)
    {
        strlist[i] = spaceHead + 100 * i;
        gets(strlist[i]);
    }
   
    for (i = 0; i < n - 1; i++)
    {
        min_j = i + 1;
        for (j = i + 2; j < n; j++)
        {
            if (strcmp(strlist[j], strlist[min_j]) < 0)
                min_j = j;
        }
        if (strcmp(strlist[i], strlist[min_j]) > 0)
        {
            temp = strlist[i];
            strlist[i] = strlist[min_j];
            strlist[min_j] = temp;
        }
    }
   
    printf("\nThe new sequence:\n");
    for (i = 0; i < n; i++)
    {
        puts(strlist[i]);
        fputs(strlist[i], fp);
    }
   
    free(spaceHead);
    free(strlist);
    fclose(fp);
}

运行结果：
Enter number:
8
Enter strings:
Blue
Pink
Yellow
Green
Red
Black
White
Purple

The new sequence:
Black
Blue
Green
Pink
Purple
Red
White
Yellow

--------------------------------
Process exited after 41.32 seconds with return value 0
Press any key to continue . . .

注意，虽然puts可以在屏幕上输出换行符，但是fputs不行。。
所以要单独加一个fputc：
fputs(strlist[i], fp);
fputc('\n', fp);

【9-8】
#include <stdio.h>
#include <stdlib.h>

#define N 10

struct student
{
    int id;
    char name[50];
    int age;
    char address[50];
} stus[N];

void save()
{
    FILE *fp;
    int i;
    if ((fp = fopen("stu.dat", "w")) == NULL)
    {
        printf("文件打不开\n");
        exit(0);
    }
    for (i = 0; i < N; i++)
        fwrite(&stus[i], sizeof(stus[i]), 1, fp);
    fclose(fp);
}

void main()
{
    int i;
    printf("Please enter data of 10 students:\n");
    for (i = 0; i < N; i++)
    {
        scanf("%d%s%d%s", &stus[i].id, stus[i].name, &stus[i].age, stus[i].address);
    }
    save();
}

【9-9】
#include <stdio.h>

struct student
{
    int id;
    char name[50];
    int age;
    char address[50];
} stu;

void main()
{
    int i;
    FILE *fp;
    if ((fp = fopen("stu.dat", "r")) == NULL)
    {
        printf("文件打不开！\n");
        exit(0);
    }
    for (i = 1; i <= 9; i += 2)
    {
        fseek(fp, (i - 1) * sizeof(struct student), SEEK_SET);
        fread(&stu, sizeof(struct student), 1, fp);
        printf("%d %10s %d %s\n", stu.id, stu.name, stu.age, stu.address);
    }
    fclose(fp);
}
运行结果：
201101       Lily 19 room01
201103      Sunny 21 room03
201105       John 19 room05
201107      Sunny 20 room06
201109        Joe 20 room07

--------------------------------
Process exited after 0.006697 seconds with return value 0
Press any key to continue . . .

201101 Lily       19 room01
201103 Sunny      21 room03
201105 John       19 room05
201107 Sunny      20 room06
201109 Joe        20 room07

--------------------------------
Process exited after 0.004761 seconds with return value 0
Press any key to continue . . .
输出的时候注意要写%-10s才对！