字符串函数模拟实现（strcpy,strcat,strstr,strchr,strcmp,memcpy,memmove）

1.实现strcpy

#define _CRT_SECURE_NO_WARNINGS 1#include<stdio.h>#include<assert.h>char *my_strcpy(char *str1, const char *str2){    assert(str1);    assert(str2);    while ((*str1++) = (*str2++))        ;    return str1;}int main(){    char str1[20] = "123456";    char *str2 = "abcde";    my_strcpy(str1, str2);    printf("%s\n", str1);    return 0;}

运行结果：

2.实现strcat

#define _CRT_SECURE_NO_WARNINGS 1#include<stdio.h>#include<assert.h>void *my_strcat(char *str1, const char *str2){    assert(str1);    assert(str2);    char *dest = str1;    while (*++dest);    while (*dest++ = *str2++);    return str1;}int main(){    char str1[20] = "123456";    char *str2 = "abcdef";    my_strcat(str1, str2);    printf("%s\n", str1);    return 0;}

运行结果：

3.实现strstr

#define _CRT_SECURE_NO_WARNINGS 1#include<stdio.h>#include<assert.h>#include<string.h>char *my_strstr( char *str,  char *substr){    assert(str);    assert(substr);    char *start = str;    char *end = str + strlen(str) - strlen(substr) + 1;    while (start < end)    {        char *str_start = start;        char *sub_start = substr;        while (*sub_start)        {            if (*str_start == *sub_start)            {                str_start++, sub_start++;            }            else            {                break;            }        }        if (*sub_start == '\0')        {            return start;        }        start++;    }    return NULL;}int main(){    char dest[20] = "abcdefg123456";    char *ret = my_strstr(dest, "defg");    printf("%s\n", ret);    return 0;}

运行结果：

没有找到返回null

4.实现strchr

#define _CRT_SECURE_NO_WARNINGS 1#include<stdio.h>#include<assert.h>char *my_strchr(char *des, char ch){    assert(des);    assert(ch);    while (*des)    {        if (*des == ch)        {            return des;        }        des++;    }    return NULL;}int main(){    char des[20] = "abcdef";    char *ret=my_strchr(des, 'd');     printf("%s\n", ret);    return 0;}

运行结果：

没有找到返回null

5.实现strcmp

#define _CRT_SECURE_NO_WARNINGS 1#include<stdio.h>#include<assert.h>int my_strcmp(const char *msg1, const char *msg2){    assert(msg1);    assert(msg2);    const char *p = msg1;    const char *q = msg2;    while (*p != '\0'&&*q != '\0')    {        if (*p == *q)        {            p++, q++;        }        else if (*p > *q)        {            return 1;        }        else        {            return -1;        }    }    if (*p != '\0')    {        return 1;    }    if (*q != '\0')    {        return -1;    }    return 0;}int main(){    const char *msg1 = "abcdef";    const char *msg2 = "abcdeg";    int ret = my_strcmp(msg1, msg2);    printf("ret = %d\n", ret);    return 0;}

运行结果：

6.实现memcpy

#define _CRT_SECURE_NO_WARNINGS 1#include<stdio.h>#include<assert.h>void *my_memcpy(void *dst, const void* src, int count){    assert(dst);    assert(src);    char *dst_p = (char *)dst;    const char *src_p = (const char*)src;    while (count--)    {        *dst_p = *src_p;        dst_p++;        src_p++;    }    return dst;}int main(){    int a[] = { 1, 2, 3, 4 };    int b[4];    my_memcpy(b, a, sizeof(a));     for (int i = 0; i < 4; i++)        printf("%d ", b[i]);    printf("\n");    return 0;}

运行结果：

7.实现memmove

#define _CRT_SECURE_NO_WARNINGS 1#include<stdio.h>#include<string.h>#include<assert.h>void *my_memmove(void *dst, const void *src,int count){    assert(dst);    assert(src);    char *p = (char *)dst;    const char *q = (const char *)src;    if ((p > q) && (p < q + count))    {        p = p + count - 1;        q = q + count - 1;        while (count--)        {            *p = *q;            p--, q--;        }    }    else    {        while (count--)        {            *p = *q;            p++, q++;        }    }return dst;}int main(){    char arr[20] = "abcdefghijklmn";    my_memmove(arr+1, arr , strlen(arr));    printf("%s\n", arr);    return 0;}

运行结果：