第09课 · 指针（下）

char string[] = "I love China!";   /* 长度自动定为 13 个字符 + 1 个 '\0' = 14 */
printf("%s\n", string);            /* 数组名 + %s，输出整串 */
printf("%c\n", string[7]);         /* 数组名 + 下标，输出第 8 个字符 'C' */

char *string = "I love China!";    /* 把字符串第 1 个字符的地址赋给指针 */
printf("%s\n", string);            /* 从 string 指向处开始输出，直到 '\0' */

char *string;                      /* 定义一个 char * 型变量 */
string = "I love China!";          /* 把首字符地址赋给它 */

string = "I am a student.";        /* 便签改写成新字符串首字符的地址 */

for (i = 0; *(a + i) != '\0'; i++)
    *(b + i) = *(a + i);        /* 把 a[i] 复制到 b[i] */
*(b + i) = '\0';                /* 别忘了给 b 补句号 */

for (; *p1 != '\0'; p1++, p2++)
    *p2 = *p1;                  /* p1 指到哪，就把字符搬到 p2 指的地方 */
*p2 = '\0';

void copy_string(char *from, char *to)
{
    for (; *from != '\0'; from++, to++)
        *to = *from;
    *to = '\0';                 /* 复制完补句号 */
}

while ((*to = *from) != '\0') { to++; from++; }   /* 赋值和判断合在一起 */
while ((*to++ = *from++) != '\0');                /* 再合并：先赋值，再各自 ++ */
while (*from)                                      /* '\0' 的 ASCII 码是 0，*from 非 0 就是真 */
    *to++ = *from++;

char *a;
a = "I love China!";      /* 合法：把首字符地址赋给指针 */
char str[14];
str[0] = 'I';             /* 合法：给某一个元素赋值 */
str = "I love China!";    /* 非法：数组名是地址常量 */

char *a = "I love China!";      /* 等价于 char *a; a = "I love China!"; */
char str[14] = "I love China!"; /* 把字符逐个复制进数组 */
char str[14];
str = "I love China!";          /* 错误：不能用赋值语句整体赋值 */

char *a;
scanf("%s", a);      /* 错误！野指针 */
char *a, str[10];
a = str;             /* 正确：先让它指向确定的房间 */
scanf("%s", a);

char *a = "I love China!";
a = a + 7;           /* 便签改指第 8 个字符 */
printf("%s\n", a);   /* 输出 China! */

char a[2][6] = {"House", "House"};   /* 5 个字符 + '\0'，第二维至少 6 */
char *b = "House";
a[0][2] = 'u';       /* 合法：改数组元素 */
b[2] = 'u';          /* 非法：字符串常量不能改变 */

char *format;
format = "a=%d, b=%f\n";
printf(format, a, b);     /* 等价于 printf("a=%d, b=%f\n", a, b); */

int (*p)(int, int);     /* p 指向"参数为两个 int、返回 int"的函数 */

int (*p)(int, int);     /* 定义指向函数的指针变量 */
p = max;                /* 把 max 的入口地址赋给 p */
c = (*p)(a, b);         /* 通过 p 调用，等价于 c = max(a, b); */

类型名 (*指针变量名)(函数参数表列);

int fun(int x, int y, int (*p)(int, int))
{
    int result;
    result = (*p)(x, y);      /* 调谁？看实参传的是哪个函数名 */
    printf("%d\n", result);
    return 0;
}

类型名 *函数名(参数表列);      /* 一般形式 */
int *a(int x, int y);          /* a 是函数，调用后得到一个 int * 型的值 */

float *search(float (*pointer)[4], int n)
{
    float *pt;
    pt = *(pointer + n);      /* 先移行，再取该行第 0 个元素地址 */
    return pt;                /* 返回 &score[n][0] */
}

float *search(float (*pointer)[4])
{
    int i = 0;
    float *pt = NULL;                              /* NULL 表示"没找到" */
    for (i = 0; i < 4; i++)
        if (*(*pointer + i) < 60) pt = *pointer;   /* 发现不及格，指向本行第 0 个元素 */
    return pt;
}

int *p[4];        /* p 是数组，有 4 个元素，每个元素都是 int * */

void sort(char *name[], int n)
{
    char *temp;
    int i, j, k;
    for (i = 0; i < n - 1; i++) {
        k = i;                                        /* 先假定第 i 个最小 */
        for (j = i + 1; j < n; j++)
            if (strcmp(name[k], name[j]) > 0) k = j;  /* 谁小就把谁的下标记在 k */
        if (k != i) {                                 /* 换名片 */
            temp = name[i];
            name[i] = name[k];
            name[k] = temp;
        }
    }
}

if (*name[k] > *name[j]) k = j;     /* 错！只比较了两个字符串的第 1 个字符 */

char **p;          /* p 指向一个 char * 型数据 */

char *name[] = {"Follow me", "BASIC", "Great Wall", "FORTRAN", "Computer design"};
char **p;
for (i = 0; i < 5; i++) {
    p = name + i;        /* p 指向第 i 张名片 */
    printf("%s\n", *p);  /* *p 就是 name[i]，一个字符串地址 */
}

int a[5] = {1, 3, 5, 7, 9};
int *num[5] = {&a[0], &a[1], &a[2], &a[3], &a[4]};
int **p;
p = num;                 /* p 指向 num[0] */
printf("%d\n", **p);     /* **p = *(num[0]) = a[0] = 1 */

while (--argc > 0)
    printf("%s%c", *++argv, (argc > 1) ? ' ' : '\n');

void *malloc(unsigned int size);
void *calloc(unsigned n, unsigned size);
void *realloc(void *p, unsigned int size);
void free(void *p);

p = malloc(100);          /* 开辟 100 字节，返回首字节地址 */

p = calloc(50, 4);        /* 50 个元素，每个 4 字节，共 200 字节，且全部清零 */

p = realloc(p, 50);       /* 把 p 原来那块空间改成 50 字节 */

free(p);                  /* 释放 p 指向的动态空间 */

int a = 3;
int *p1 = &a;
void *p3;
p3 = (void *)p1;          /* int * 转成 void * */
p1 = (int *)p3;           /* void * 转回 int * */

#include <stdio.h>

int main(void)
{
    char string[] = "I love China!";      /* 长度自动定为 14 */
    printf("%s\n", string);               /* 输出整个字符串 */
    printf("%c\n", string[7]);            /* 输出下标为 7 的字符 */
    return 0;
}

I love China!
C

#include <stdio.h>

int main(void)
{
    char a[] = "I am a boy.", b[20], *p1, *p2;
    p1 = a;                                      /* p1 指向 a 的首字符 */
    p2 = b;                                      /* p2 指向 b 的首元素 */
    for (; *p1 != '\0'; p1++, p2++)              /* 只要没到 '\0' 就继续 */
        *p2 = *p1;                               /* 把字符抄到 p2 指的位置 */
    *p2 = '\0';                                  /* 补结束标志 */
    printf("string a is: %s\n", a);
    printf("string b is: %s\n", b);
    return 0;
}

string a is: I am a boy.
string b is: I am a boy.

#include <stdio.h>

void copy_string(char *from, char *to)
{
    for (; *from != '\0'; from++, to++)
        *to = *from;
    *to = '\0';
}

int main(void)
{
    char a[] = "I am a teacher.";
    char b[] = "You are a student.";
    char *p = b;
    printf("string a = %s\nstring b = %s\n", a, b);
    copy_string(a, p);                     /* 实参：数组名、指针，传的都是地址 */
    printf("string a = %s\nstring b = %s\n", a, b);
    return 0;
}

string a = I am a teacher.
string b = You are a student.
string a = I am a teacher.
string b = I am a teacher.

#include <stdio.h>

int max(int x, int y)
{
    return x > y ? x : y;
}

int main(void)
{
    int (*p)(int, int);        /* 指向函数的指针变量 */
    int a = 34, b = 89, c;
    p = max;                   /* 只写函数名，不写括号和实参 */
    c = (*p)(a, b);            /* 等价于 c = max(a, b); */
    printf("a=%d, b=%d, max=%d\n", a, b, c);
    return 0;
}

a=34, b=89, max=89

#include <stdio.h>

float *search(float (*pointer)[4], int n)
{
    float *pt;
    pt = *(pointer + n);                    /* 先移行，再取该行首元素地址 */
    return pt;                              /* 返回 &score[n][0] */
}

int main(void)
{
    float score[][4] = {{60, 70, 80, 90},
                        {56, 89, 67, 88},
                        {34, 78, 90, 66}};
    float *p;
    int i, k;
    printf("enter the number of student: ");
    scanf("%d", &k);
    p = search(score, k);
    for (i = 0; i < 4; i++)
        printf("%5.2f\t", *(p + i));
    printf("\n");
    return 0;
}

enter the number of student: 1
56.00   89.00   67.00   88.00

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

void sort(char *name[], int n)
{
    char *temp;
    int i, j, k;
    for (i = 0; i < n - 1; i++) {
        k = i;                                        /* 先假定第 i 个最小 */
        for (j = i + 1; j < n; j++)
            if (strcmp(name[k], name[j]) > 0) k = j;  /* 谁小就记谁的下标 */
        if (k != i) {                                 /* 换名片 */
            temp = name[i];
            name[i] = name[k];
            name[k] = temp;
        }
    }
}

void print(char *name[], int n)
{
    int i;
    for (i = 0; i < n; i++)
        printf("%s\n", name[i]);
}

int main(void)
{
    char *name[] = {"Follow me", "BASIC", "Great Wall", "FORTRAN", "Computer design"};
    sort(name, 5);
    print(name, 5);
    return 0;
}

BASIC
Computer design
FORTRAN
Follow me
Great Wall

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

void check(int *p, int n)
{
    int i;
    printf("They are fail: ");
    for (i = 0; i < n; i++)
        if (p[i] < 60) printf("%d ", p[i]);
    printf("\n");
}

int main(void)
{
    int *p1, i;
    p1 = (int *)malloc(5 * sizeof(int));      /* 开 5 个 int 的空间 */
    if (p1 == NULL) {                         /* 必须检查是否成功 */
        printf("malloc failed\n");
        return 1;
    }
    for (i = 0; i < 5; i++)
        scanf("%d", p1 + i);                  /* 按地址法输入 */
    check(p1, 5);
    free(p1);                                 /* 退房 */
    p1 = NULL;                                /* 防止野指针 */
    return 0;
}

67 98 59 78 57
They are fail: 59 57

#include <stdio.h>

void swap(int *p1, int *p2)
{
    int t = *p1; *p1 = *p2; *p2 = t;
}

int main(void)
{
    int a, b, c;
    scanf("%d%d%d", &a, &b, &c);
    if (a > b) swap(&a, &b);
    if (a > c) swap(&a, &c);
    if (b > c) swap(&b, &c);
    printf("%d %d %d\n", a, b, c);
    return 0;
}

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

void swap(char **p1, char **p2)
{
    char *t = *p1; *p1 = *p2; *p2 = t;
}

int main(void)
{
    char s1[50], s2[50], s3[50];
    char *p[3];
    scanf("%49s%49s%49s", s1, s2, s3);
    p[0] = s1; p[1] = s2; p[2] = s3;
    if (strcmp(p[0], p[1]) > 0) swap(&p[0], &p[1]);
    if (strcmp(p[0], p[2]) > 0) swap(&p[0], &p[2]);
    if (strcmp(p[1], p[2]) > 0) swap(&p[1], &p[2]);
    printf("%s\n%s\n%s\n", p[0], p[1], p[2]);
    return 0;
}

#include <stdio.h>

void input(int *p, int n)
{
    int i;
    for (i = 0; i < n; i++) scanf("%d", p + i);
}

void process(int *p, int n)
{
    int *min = p, *max, i, t;
    for (i = 1; i < n; i++)
        if (*(p + i) < *min) min = p + i;
    t = *p; *p = *min; *min = t;                 /* 最小数与第一个对换 */

    max = p;                                     /* 重新找最大 */
    for (i = 1; i < n; i++)
        if (*(p + i) > *max) max = p + i;
    t = *(p + n - 1); *(p + n - 1) = *max; *max = t;
}

void output(int *p, int n)
{
    int i;
    for (i = 0; i < n; i++) printf("%d ", *(p + i));
    printf("\n");
}

int main(void)
{
    int a[10];
    input(a, 10);
    process(a, 10);
    output(a, 10);
    return 0;
}

#include <stdio.h>

void move(int *p, int n, int m)
{
    int i, k, t;
    m = m % n;                        /* m 可能大于 n */
    for (k = 0; k < m; k++) {
        t = *(p + n - 1);             /* 保存最后一个 */
        for (i = n - 1; i > 0; i--)
            *(p + i) = *(p + i - 1);  /* 从后往前依次后移 */
        *p = t;
    }
}

int main(void)
{
    int a[100], n, m, i;
    scanf("%d%d", &n, &m);
    for (i = 0; i < n; i++) scanf("%d", a + i);
    move(a, n, m);
    for (i = 0; i < n; i++) printf("%d ", *(a + i));
    printf("\n");
    return 0;
}

#include <stdio.h>

int main(void)
{
    int a[100], n, i, count = 0, left;
    int *p = a;
    scanf("%d", &n);
    for (i = 0; i < n; i++) *(p + i) = 1;    /* 1 表示还在圈里 */
    left = n;
    i = 0;
    while (left > 1) {
        if (*(p + i)) {                      /* 还在圈里的人才报数 */
            count++;
            if (count == 3) {
                *(p + i) = 0;                /* 报到 3，退出 */
                left--;
                count = 0;
            }
        }
        i = (i + 1) % n;                     /* 走到末尾绕回开头 */
    }
    for (i = 0; i < n; i++)
        if (*(p + i)) printf("最后留下的是原来第%d号\n", i + 1);
    return 0;
}

#include <stdio.h>

int my_strlen(char *s)
{
    char *p = s;
    while (*p != '\0') p++;
    return (int)(p - s);              /* 两指针相减 = 元素个数 */
}

int main(void)
{
    char str[100];
    scanf("%99s", str);
    printf("%d\n", my_strlen(str));
    return 0;
}

#include <stdio.h>

void copy_from(char *to, char *from, int m)
{
    from = from + m - 1;              /* 指向第 m 个字符（下标 m-1） */
    while (*from != '\0') {
        *to = *from;
        to++; from++;
    }
    *to = '\0';
}

int main(void)
{
    char a[100], b[100];
    int m;
    scanf("%99s%d", a, &m);
    copy_from(b, a, m);
    printf("%s\n", b);
    return 0;
}

#include <stdio.h>

void count(char *s, int *upper, int *lower, int *space, int *digit, int *other)
{
    *upper = *lower = *space = *digit = *other = 0;
    while (*s != '\0') {
        if (*s >= 'A' && *s <= 'Z') (*upper)++;
        else if (*s >= 'a' && *s <= 'z') (*lower)++;
        else if (*s == ' ') (*space)++;
        else if (*s >= '0' && *s <= '9') (*digit)++;
        else (*other)++;
        s++;
    }
}

int main(void)
{
    char str[200];
    int up, low, sp, dig, oth;
    scanf("%199[^\n]", str);          /* 读一整行 */
    count(str, &up, &low, &sp, &dig, &oth);
    printf("大写:%d 小写:%d 空格:%d 数字:%d 其他:%d\n", up, low, sp, dig, oth);
    return 0;
}

#include <stdio.h>

void transpose(int (*p)[3])
{
    int i, j, t;
    for (i = 0; i < 3; i++)
        for (j = i + 1; j < 3; j++) {   /* 只处理 j > i */
            t = p[i][j]; p[i][j] = p[j][i]; p[j][i] = t;
        }
}

int main(void)
{
    int a[3][3], i, j;
    for (i = 0; i < 3; i++)
        for (j = 0; j < 3; j++) scanf("%d", &a[i][j]);
    transpose(a);
    for (i = 0; i < 3; i++) {
        for (j = 0; j < 3; j++) printf("%d ", a[i][j]);
        printf("\n");
    }
    return 0;
}

#include <stdio.h>

void change(int a[5][5])
{
    int i, j, k, t;
    int ci[4] = {0, 0, 4, 4};         /* 4 个角的行号 */
    int cj[4] = {0, 4, 0, 4};         /* 4 个角的列号 */
    int locked[5][5] = {0};           /* 放好的格子不再参与 */
    int mi = 0, mj = 0;

    for (i = 0; i < 5; i++)           /* 找最大 */
        for (j = 0; j < 5; j++)
            if (a[i][j] > a[mi][mj]) { mi = i; mj = j; }
    t = a[2][2]; a[2][2] = a[mi][mj]; a[mi][mj] = t;
    locked[2][2] = 1;

    for (k = 0; k < 4; k++) {         /* 依次处理 4 个角 */
        int pi = -1, pj = -1;
        for (i = 0; i < 5; i++)
            for (j = 0; j < 5; j++) {
                if (locked[i][j]) continue;
                if (pi == -1 || a[i][j] < a[pi][pj]) { pi = i; pj = j; }
            }
        t = a[ci[k]][cj[k]]; a[ci[k]][cj[k]] = a[pi][pj]; a[pi][pj] = t;
        locked[ci[k]][cj[k]] = 1;     /* 这个角已放好，锁定 */
    }
}

int main(void)
{
    int a[5][5], i, j;
    for (i = 0; i < 5; i++)
        for (j = 0; j < 5; j++) scanf("%d", &a[i][j]);
    change(a);
    for (i = 0; i < 5; i++) {
        for (j = 0; j < 5; j++) printf("%4d", a[i][j]);
        printf("\n");
    }
    return 0;
}

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

void sort(char (*p)[20], int n)
{
    int i, j;
    char temp[20];
    for (i = 0; i < n - 1; i++)
        for (j = i + 1; j < n; j++)
            if (strcmp(p[i], p[j]) > 0) {
                strcpy(temp, p[i]);
                strcpy(p[i], p[j]);
                strcpy(p[j], temp);
            }
}

int main(void)
{
    char s[10][20];
    int i;
    for (i = 0; i < 10; i++) scanf("%19s", s[i]);
    sort(s, 10);
    for (i = 0; i < 10; i++) printf("%s\n", s[i]);
    return 0;
}

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

void sort(char *p[], int n)
{
    int i, j;
    char *t;
    for (i = 0; i < n - 1; i++)
        for (j = i + 1; j < n; j++)
            if (strcmp(p[i], p[j]) > 0) {
                t = p[i]; p[i] = p[j]; p[j] = t;
            }
}

int main(void)
{
    char s[10][100];
    char *p[10];
    int i;
    for (i = 0; i < 10; i++) {
        scanf("%99s", s[i]);
        p[i] = s[i];                  /* 名片指向第 i 个字符串 */
    }
    sort(p, 10);
    for (i = 0; i < 10; i++) printf("%s\n", p[i]);
    return 0;
}

#include <stdio.h>
#include <math.h>

double integral(double a, double b, double (*fun)(double))
{
    int n = 10000, i;
    double h = (b - a) / n;             /* 小矩形宽 */
    double s = 0.0;
    for (i = 0; i < n; i++)
        s += fun(a + (i + 0.5) * h) * h;  /* 中点矩形法：高 × 宽 */
    return s;
}

double f1(double x) { return sin(x); }
double f2(double x) { return cos(x); }
double f3(double x) { return exp(x); }

int main(void)
{
    printf("sin: %f\n", integral(0.0, 1.0, f1));
    printf("cos: %f\n", integral(0.0, 1.0, f2));
    printf("exp: %f\n", integral(0.0, 1.0, f3));
    return 0;
}

#include <stdio.h>

void reverse(int *p, int n)
{
    int *q = p + n - 1, t;        /* q 指向最后一个 */
    while (p < q) {
        t = *p; *p = *q; *q = t;
        p++; q--;
    }
}

int main(void)
{
    int a[100], n, i;
    scanf("%d", &n);
    for (i = 0; i < n; i++) scanf("%d", a + i);
    reverse(a, n);
    for (i = 0; i < n; i++) printf("%d ", *(a + i));
    printf("\n");
    return 0;
}

#include <stdio.h>
#define STU 4
#define COUR 5

void average1(float (*p)[COUR])
{
    int i;
    float sum = 0;
    for (i = 0; i < STU; i++) sum += p[i][0];
    printf("第1门课平均分: %.2f\n", sum / STU);
}

void fail2(float (*p)[COUR])
{
    int i, j, cnt;
    for (i = 0; i < STU; i++) {
        cnt = 0;
        for (j = 0; j < COUR; j++)
            if (p[i][j] < 60) cnt++;
        if (cnt >= 2) {
            float sum = 0;
            printf("学号 %d 不及格 %d 门: ", i + 1, cnt);
            for (j = 0; j < COUR; j++) {
                printf("%.2f ", p[i][j]);
                sum += p[i][j];
            }
            printf("平均 %.2f\n", sum / COUR);
        }
    }
}

void excellent(float (*p)[COUR])
{
    int i, j, all85;
    for (i = 0; i < STU; i++) {
        float sum = 0;
        all85 = 1;
        for (j = 0; j < COUR; j++) {
            sum += p[i][j];
            if (p[i][j] <= 85) all85 = 0;
        }
        if (sum / COUR > 90 || all85)
            printf("学号 %d 平均 %.2f%s\n", i + 1, sum / COUR,
                   all85 ? " 全部85以上" : "");
    }
}

int main(void)
{
    float score[STU][COUR] = {
        {85, 90, 92, 88, 95},
        {50, 55, 70, 60, 65},
        {95, 96, 92, 98, 90},
        {70, 80, 60, 90, 85}
    };
    average1(score);
    fail2(score);
    excellent(score);
    return 0;
}

#include <stdio.h>

int main(void)
{
    char str[200];
    int a[100], n = 0, num = 0, in_num = 0, i;
    char *p;
    scanf("%199[^\n]", str);
    for (p = str; ; p++) {
        if (*p >= '0' && *p <= '9') {
            num = num * 10 + (*p - '0');   /* 拼到末尾 */
            in_num = 1;
        } else {
            if (in_num) {                  /* 一段数字刚结束 */
                a[n++] = num;
                num = 0;
                in_num = 0;
            }
            if (*p == '\0') break;
        }
    }
    printf("共%d个整数: ", n);
    for (i = 0; i < n; i++) printf("%d ", a[i]);
    printf("\n");
    return 0;
}

#include <stdio.h>

int my_strcmp(char *p1, char *p2)
{
    while (*p1 != '\0' && *p1 == *p2) {
        p1++; p2++;
    }
    return *p1 - *p2;                 /* 第一个不同字符的差 */
}

int main(void)
{
    char s1[100], s2[100];
    scanf("%99s%99s", s1, s2);
    printf("%d\n", my_strcmp(s1, s2));
    return 0;
}

#include <stdio.h>

int main(void)
{
    char *month[] = {"January", "February", "March", "April", "May", "June",
                     "July", "August", "September", "October", "November", "December"};
    int n;
    scanf("%d", &n);
    if (n >= 1 && n <= 12)
        printf("%s\n", month[n - 1]);   /* 月份从 1 起，下标从 0 起 */
    else
        printf("输入错误\n");
    return 0;
}

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

void *new(int n)
{
    return malloc((unsigned)n);   /* 借用标准库的 malloc */
}

void my_free(void *p)             /* 不能叫 free，会和标准库重名 */
{
    free(p);
}

int main(void)
{
    char *p = (char *)new(20);
    if (p == NULL) return 1;
    scanf("%19s", p);
    printf("%s\n", p);
    my_free(p);
    p = NULL;
    return 0;
}

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

void sort(char **p, int n)
{
    int i, j;
    char *t;
    for (i = 0; i < n - 1; i++)
        for (j = i + 1; j < n; j++)
            if (strcmp(*(p + i), *(p + j)) > 0) {
                t = *(p + i);
                *(p + i) = *(p + j);
                *(p + j) = t;
            }
}

int main(void)
{
    char *name[] = {"Follow me", "BASIC", "Great Wall", "FORTRAN", "Computer design"};
    int i;
    sort(name, 5);
    for (i = 0; i < 5; i++) printf("%s\n", *(name + i));
    return 0;
}

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

void sort(int **p, int n)
{
    int i, j, *t;
    for (i = 0; i < n - 1; i++)
        for (j = i + 1; j < n; j++)
            if (**(p + i) > **(p + j)) {   /* 比较最终指向的整数 */
                t = *(p + i);
                *(p + i) = *(p + j);
                *(p + j) = t;              /* 交换指针 */
            }
}

int main(void)
{
    int n, i;
    int *a, **p;
    scanf("%d", &n);
    a = (int *)malloc(n * sizeof(int));        /* 存数据 */
    p = (int **)malloc(n * sizeof(int *));     /* 存指针 */
    if (a == NULL || p == NULL) return 1;
    for (i = 0; i < n; i++) {
        scanf("%d", a + i);
        *(p + i) = a + i;                      /* 第 i 个指针指向第 i 个数 */
    }
    sort(p, n);
    for (i = 0; i < n; i++) printf("%d ", **(p + i));
    printf("\n");
    free(p);
    free(a);
    return 0;
}

#include <stdio.h>
int main(void)
{
    char s[] = "abcdef";
    char *p = s;
    printf("%c\n", *(p + 2));
    printf("%s\n", p + 3);
    printf("%c\n", p[5]);
    p += 2;
    printf("%s\n", p);
    return 0;
}

#include <stdio.h>
int add(int x, int y) { return x + y; }
int mul(int x, int y) { return x * y; }
int main(void)
{
    int (*p)(int, int) = add;
    printf("%d\n", (*p)(3, 4));
    p = mul;
    printf("%d\n", p(3, 4));
    return 0;
}

#include <stdio.h>
int main(void)
{
    char *name[] = {"Follow me", "BASIC", "Great Wall"};
    char **p = name;
    printf("%s\n", *(p + 1));
    printf("%c\n", **(p + 2));
    printf("%s\n", *p + 7);
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
int main(void)
{
    int *p = (int *)malloc(3 * sizeof(int));
    int *q;
    if (p == NULL) return 1;
    p[0] = 10; p[1] = 20; p[2] = 30;
    q = p;
    q++;
    printf("%d %d %d\n", *p, *q, *(p + 2));
    printf("%d\n", (int)(q - p));
    free(p);
    p = NULL;
    return 0;
}

// ❌ 错误示例（故意）
#include <stdio.h>
int main(void)
{
    char *p;
    scanf("%s", p);
    printf("%s\n", p);
    return 0;
}

// ❌ 错误示例（故意）
#include <stdio.h>
int main(void)
{
    char str[20];
    str = "hello";
    printf("%s\n", str);
    return 0;
}

// ❌ 错误示例（故意）
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
    int *p = (int *)malloc(5 * sizeof(int));
    int i;
    if (p == NULL) return 1;
    for (i = 0; i < 5; i++) p[i] = i;
    free(p);
    printf("%d\n", p[0]);
    return 0;
}