C语言进阶⑬(字符串函数)+(指针编程题)strlen+strcpy+strcat+strstr+strtok+strerror（中）

 
#include<stdio.h>
#include<string.h>
#include<assert.h>
char* my_strcat(char* dest, const char* src)
{
    assert(dest != NULL);
    assert(src != NULL);
    char* ret = dest;//让ret指向dest的起始地址
    while (*dest)//找到'\0'  '\0'的ASCII码为0跳出循环
    {
        dest++;
    }
    while (*dest++ = *src++)//'\0'的ASCII码为0拷贝后跳出循环
    {
        ;
    }
    return ret;
}
int main()
{
    char arr1[20] = "hello ";
    char arr2[] = "world";
    //printf("%s\n", strcat(arr1, arr2));
    printf("%s\n", my_strcat(arr1, arr2));
    return 0;
}

 
#include <stdio.h>
#include <string.h>
int main()
{
    char* p1 = "abcdef";
    char* p2 = "aqwer";
    int ret = strcmp(p1, p2); // p1和p2比
    // a==a, 对比下一对，b<q，所以p2大
    printf("%d\n", ret);//-1
 
    return 0;
}

 
#include<stdio.h>
#include<string.h>
#include<assert.h>
int my_strcmp(const char* str1, const char* str2)
{
    assert(str1 != NULL);
    assert(str2 != NULL);
    while (*str1 == *str2)
    {
        if (*str1)//如果两个相等且其中一个为\0
        {
            return 0;
        }
        else
        {
            str1++;
            str2++;
        }
    }
    return *str1 - *str2;//返回>0或<0才是标准，而且可以看相差的ASCII码
}
int main()
{
    char arr1[] = "abcd";
    char arr2[] = "abcd";
    /*if (strcmp(arr1, arr2) > 0)
    {
        printf("arr1>arr2\n");
    }
    else if (strcmp(arr1, arr2) < 0)
    {
        printf("arr1<arr2\n");
    }
    else
    {
        printf("arr1=arr2\n");
    }*/
    if (my_strcmp(arr1, arr2) > 0)
    {
        printf("arr1>arr2\n");
    }
    else if (my_strcmp(arr1, arr2) < 0)
    {
        printf("arr1<arr2\n");
    }
    else
    {
        printf("arr1=arr2\n");
    }
    return 0;
}

 
#include <stdio.h>
#include <string.h>
int main()
{
    char arr1[5] = "abc";
    char arr2[] = "hello world";
    strncpy(arr1, arr2, 4); // 从arr2中拷贝4个到arr1
    printf("%s\n", arr1);// 打印了hell
    // 注意这里arr1有5个空间，如果arr1空间不够就会报错或者多打印其它东西
    return 0;
}

 
#include <stdio.h>
#include <string.h>
int main()
{
    char arr1[30] = "hello";
    char arr2[] = "world";
    strncat(arr1, arr2, 3); // 从arr2中取3个追加到arr1中
    printf("%s\n", arr1);//打印了hellowor
    return 0;
}

 
#include <stdio.h>
#include <string.h>
int main()
{
    const char* p1 = "abczdef";
    const char* p2 = "abcqwer";
    // int ret = strcmp(p1, p2);
    int ret1 = strncmp(p1, p2, 1);
    int ret2 = strncmp(p1, p2, 4);
    printf("%d %d\n", ret1, ret2);//0 1
    return 0;
}

 
#include <stdio.h>
#include <string.h>
int main()
{
    char* p1 = "abcdefgh";
    char* p2 = "def";
    char* ret = strstr(p1, p2); // 判断p2是否是p1的子串
    printf("%s\n", ret);//defgh
    return 0;
}