实验5 指针

实验任务1

task1_1.c

#include <stdio.h>
#define N 4
int main()
{
    int x[N] = {1, 9, 8, 4};
    int i;
    int *p;
    // 方式1：通过数组名和下标遍历输出数组元素
    for (i = 0; i < N; ++i)
    printf("%d", x[i]);
    printf("\n");
    // 方式2：通过指针变量遍历输出数组元素 （写法1）
    for (p = x; p < x + N; ++p)
    printf("%d", *p);
    printf("\n");
    // 方式2：通过指针变量遍历输出数组元素（写法2）
    p = x;
    for (i = 0; i < N; ++i)
    printf("%d", *(p + i));
    printf("\n");
    // 方式2：通过指针变量遍历输出数组元素（写法3）
    p = x;
    for (i = 0; i < N; ++i)
    printf("%d", p[i]);
    printf("\n");
    return 0;
}

task1_2.c

#include <stdio.h>
int main()
{
    int x[2][4] = {{1, 9, 8, 4}, {2, 0, 4, 9}};
    int i, j;
    int *p; // 指针变量，存放int类型数据的地址
    int(*q)[4]; // 指针变量，指向包含4个int型元素的一维数组
    // 使用数组名、下标访问二维数组元素
    for (i = 0; i < 2; ++i)
    {
        for (j = 0; j < 4; ++j)
        printf("%d", x[i][j]);
        printf("\n");
    }
    // 使用指针变量p间接访问二维数组元素
    for (p = &x[0][0], i = 0; p < &x[0][0] + 8; ++p, ++i)
    {
        printf("%d", *p);
        if ((i + 1) % 4 == 0)
        printf("\n");
    }
    // 使用指针变量q间接访问二维数组元素
    for (q = x; q < x + 2; ++q)
    {
        for (j = 0; j < 4; ++j)
        printf("%d", *(*q + j));
        printf("\n");
    }
    return 0;
}

实验任务2

task2_1.c

#include <stdio.h>
#include <string.h>
#define N 80
int main()
{
    char s1[] = "Learning makes me happy";
    char s2[] = "Learning makes me sleepy";
    char tmp[N];
    printf("sizeof(s1) vs. strlen(s1): \n");
    printf("sizeof(s1) = %d\n", sizeof(s1));
    printf("strlen(s1) = %d\n", strlen(s1));
    printf("\nbefore swap: \n");
    printf("s1: %s\n", s1);
    printf("s2: %s\n", s2);
    printf("\nswapping...\n");
    strcpy(tmp, s1);
    strcpy(s1, s2);
    strcpy(s2, tmp);
    printf("\nafter swap: \n");
    printf("s1: %s\n", s1);
    printf("s2: %s\n", s2);
    return 0;
}

问题1：s1大小为24，sizeof(s1)计算的是字符串大小，strlen(s1)统计的是字符串长度

问题2：不能，必须在定义时指定存储空间来存放字符串的数据。char s1[]会报错

问题3：line20-22执行后，字符数组s1和s2中的内容已经被置换

task2_2.c

#include <stdio.h>
#include <string.h>
#define N 80
int main()
{
    char *s1 = "Learning makes me happy";
    char *s2 = "Learning makes me sleepy";
    char *tmp;
    printf("sizeof(s1) vs. strlen(s1): \n");
    printf("sizeof(s1) = %d\n", sizeof(s1));
    printf("strlen(s1) = %d\n", strlen(s1));
    printf("\nbefore swap: \n");
    printf("s1: %s\n", s1);
    printf("s2: %s\n", s2);
    printf("\nswapping...\n");
    tmp = s1;
    s1 = s2;
    s2 = tmp;
    printf("\nafter swap: \n");
    printf("s1: %s\n", s1);
    printf("s2: %s\n", s2);
    return 0;
}

问题1：指针变量s1中存放的是字符串的起始地址。sizeof(s1)计算的是指针变量s1的大小。strlen(s1)统计的是指针变量s1的长度

问题2：line7代码可以替换成下面的写法。定义指针变量时其值可以是不确定的。

问题3：交换的是s1与s2的地址，字符串常量在内存存储单元没有变化。

实验任务3

#include <stdio.h>
void str_cpy(char *target, const char *source);
void str_cat(char *str1, char *str2);
int main()
{
char s1[80], s2[20] = "1984";
str_cpy(s1, s2);
puts(s1);
str_cat(s1, " Animal Farm");
puts(s1);
return 0;
}
void str_cpy(char *target, const char *source)
{
while (*target++ = *source++);
}
void str_cat(char *str1, char *str2)
{
while (*str1)
str1++;
while (*str1++ = *str2++);
}

实验任务4 

 

#include <stdio.h>
#define N 80
int func(char *);
int main()
{
    char str[80];
    while (gets(str) != NULL)
    {
        if (func(str))
        printf("yes\n");
        else
        printf("no\n");
    }
    return 0;
}
int func(char *str)
{
    char *begin, *end;
    begin = end = str;
    while (*end)
    end++;
    end--;
    while (begin < end)
    {
        if (*begin != *end)
        return 0;
        else
        {
            begin++;
            end--;
        }
    }
    return 1;
}

实验任务5 

#include <stdio.h>
#define N 80
void func(char *);
int main()
{
    char s[N];
    while (scanf("%s", s) != EOF)
    {
        func(s);
        puts(s);
    }
    return 0;
}
void func(char *str)
{
    int i;
    char *p1, *p2, *p;
    p1 = str;
    while (*p1 == '*')
    p1++;
    p2 = str;
    while (*p2)
    p2++;
    p2--;
    while (*p2 == '*')
    p2--;
    p = str;
    i = 0;
    while (p < p1)
    {
        str[i] = *p;
        p++;
        i++;
    }
    while (p <= p2)
    {
        if (*p != '*')
        {
            str[i] = *p;
            i++;
        }
        p++;
    }
    while (*p != '\0')
    {
        str[i] = *p;
        p++;
        i++;
    }
    str[i] = '\0';
}

实验任务6

task6_1.c 

#include <stdio.h>
#include <string.h>
void sort(char *name[], int n);
int main()
{
    char *course[4] = {"C Program",
    "C++ Object Oriented Program",
    "Operating System",
    "Data Structure and Algorithms"};
    int i;
    sort(course, 4);
    for (i = 0; i < 4; i++)
    printf("%s\n", course[i]);
    return 0;
}
void sort(char *name[], int n)
{
    int i, j;
    char *tmp;
    for (i = 0; i < n - 1; ++i)
    for (j = 0; j < n - 1 - i; ++j)
    if (strcmp(name[j], name[j + 1]) > 0)
    {
        tmp = name[j];
        name[j] = name[j + 1];
        name[j + 1] = tmp;
    }
}

task6_2.c 

#include <stdio.h>
#include <string.h>
void sort(char *name[], int n);
int main()
{
    char *course[4] = {"C Program",
    "C++ Object Oriented Program",
    "Operating System",
    "Data Structure and Algorithms"};
    int i;
    sort(course, 4);
    for (i = 0; i < 4; i++)
    printf("%s\n", course[i]);
    return 0;
}
void sort(char *name[], int n)
{
    int i, j, k;
    char *tmp;
    for (i = 0; i < n - 1; i++)
    {
        k = i;
        for (j = i + 1; j < n; j++)
        if (strcmp(name[j], name[k]) < 0)
        k = j;
        if (k != i)
        {
            tmp = name[i];
            name[i] = name[k];
            name[k] = tmp;
        }
    }
}

 这两种算法实现中，交换的是指针变量的值，内存中字符串的存储位置没有交换。

实验任务7

#include <stdio.h>
#include <string.h>
#define N 5
int check_id(char *str); // 函数声明
int main()
{
    char *pid[N] = {"31010120000721656X",
    "330106199609203301",
    "53010220051126571",
    "510104199211197977",
    "53010220051126133Y"};
    int i;
    for (i = 0; i < N; ++i)
    if (check_id(pid[i])) // 函数调用
    printf("%s\tTrue\n", pid[i]);
    else
    printf("%s\tFalse\n", pid[i]);
    return 0;
}
int check_id(char *str)
{
    int m=0,n,f;
    char q[]="0123456789X";
        if(strlen(str)!=18)
            return 0;
    while(*str!='\0')
    {
        f=1;
        for(n=0;n<11;n++)
            if(*str==q[n])
                f=0;
        str++;
        if(f)
            return 0;
    }
    return 1;
}

 实验任务8

#include <stdio.h>
#define N 80
void encoder(char *s); // 函数声明
void decoder(char *s); // 函数声明
int main()
{
char words[N];
printf("输入英文文本: ");
gets(words);
printf("编码后的英文文本: ");
encoder(words); // 函数调用
printf("%s\n", words);
printf("对编码后的英文文本解码: ");
decoder(words); // 函数调用
printf("%s\n", words);
return 0;
}

void encoder(char *s)
{
    while(*s)
    {
        if((*s>='a'&&*s<'z')||(*s>='A'&&*s<'Z'))
            (*s)++;
        else if(*s=='z'||*s=='Z')
            (*s)-=25;
        s++;
    }
}

void decoder(char *s)
{
    while(*s)
    {
        if((*s>'a'&&*s<='z')||(*s>'A'&&*s<='Z'))
            (*s)--;
        else if(*s=='a'||*s=='A')
            (*s)+=25;
        s++;
    }
}