模拟实现strcpy,strcat,strstr函数

#define _CRT_SECURE_NO_WARNINGS 1

#include<stdio.h>
#include<assert.h>
////方法一：创建临时变量计数器
int my_strlen(char *s)
{
	int count=0;
	while (*s != '\0'){
		count++;
		s++;
	}
	return count;

}

//方法二：利用递归的方式
int my_strlen(char *str){
	if (*str == '\0')
		return 0;
	else
		return(1 + my_strlen(str + 1));
}


//方法三：利用指针减指针的方式

int my_strlen(char *s)
{
	char *p = s;
	while (p != '\0'){
		p++;
	}
	return (p - s);
}
int main(){
	char *str = "abc123";
	int ret = my_strlen(str);
	printf("%d\n ", ret);
	system("pause");
	return 0;
}

//模拟实现strcpy
char my_strcpy(char *dest, char *src)
{
	char* ret = dest;
	assert(dest != NULL);
	assert(src != NULL);
	while ((*dest++ = *src++)){
		;
	}
	return ret;
}

int main()
{
	const char* src = "abcd1234";
	char dst[20];
	my_strcpy(dst, src);
	printf("%s ", dst);
	system("pause");
	return 0;
}

//模拟实现strcat函数
char my_strcat(char* dest, const char* src)
{
	char *ret = dest;
	assert(dest);
	assert(src);
	while (*dest){
		dest++;
	}
	while ((*dest++ = *src++))
	{
		;
	}
	return ret;
}
int main()
{
	const char*src = "abcd1234";
	char dst[20] = "xyz";
	my_strcat(dst, src);
	printf("%s", dst);
	system("pause");
	return 0;
}

//模拟实现strstr函数

char my_strstr(const char*str, const char*sub_str)
{
	assert(str);
	assert(sub_str);
	char *str_begin = str;
	char *p = str_begin;
	char *q = sub_str;
	while (*str_begin){
		p = str_begin;
		q = sub_str;
		while (*p!='\0'&&*q!='\0'&&*p == *q){
			p++;
			q++;
		}
		if (*q == '\0'){
			return str_begin;
		}
		if (*p == '\0')
		{
			return NULL ;
		}
		str_begin++;
			
	}
	
}
int main()
{
	const char *str = "abc1234abcdef";
	const char*sub_str = "abcd";
	 char ret=my_strstr(str, sub_str);
	 if (ret)
	 {
		 printf("%s\n ", ret);
	 }
	 else
		 printf("没有找到！");
	 system("pause");
	 return 0;
	
}