2021-10-21

• • 模拟实现strlen
  • 模拟实现strcpy
  • 模拟实现strncpy
  • 模拟实现strcat
  • 模拟实现strncat
  • 模拟实现strcmp
  • 模拟实现strncmp
  • 模拟实现strstr
  • 模拟实现strchr

函数原型:

size_t strlen(const char* str);

#define _CRT_SECURE_NO_WARNINGS 1
#include 
#include 
unsigned my_strlen(const char* str)
{
	unsigned count = 0;
	assert(str);//断言一下,保证指针的有效性,也可以写成assert(str!=NULL);
	while (*str != '')
	{
		count++;
		str++;
	}
	return count;
}
int main()
{	
	char str[30] = { 0 };
	gets(str);//输入一行字符串
	unsigned len = my_strlen(str);
	printf("%dn", len);
	return 0;
}

函数原型:

char* strcpy(char* destination,const char* source);

#include 
#include 
char* my_strcpy(char* dest, const char* sour)
{
	char* tmp = dest;
	assert(dest && sour);//也可以写成assert(dest!=NULL && sour!=NULL);
	while (*sour != '')  //也可以写成while(*dest++=*sour++){;}
	{
		*dest++= *sour++;//先解引用再++,也可写成*dest=*sour;dest++;sour++;
	}
	*dest = *sour;
	return tmp;
}
int main()
{
	char str1[30] = "Life depends not on luck";
	const char* str2 = "good";
	char* ret = my_strcpy(str1, str2);
	printf("str1=%sn", str1);//good
	return 0;
}

函数原型:

char* strncpy(char* destination,const char* source,size_t n);

#define _CRT_SECURE_NO_WARNINGS 1
#include 
#include 
char* my_strcpy(char* dest, const char* sour,unsigned n)
{
	char* tmp = dest;
	assert(dest && sour);
	while (n)  
	{
		*dest++ = *sour++;
		n--;
	}
	return tmp;
}
int main()
{
	char str1[30] = "Life depends not on luck";
	const char* str2 = "good";
	unsigned n=0;
	scanf("%u", &n);
	char* ret = my_strncpy(str1, str2,n);
	printf("str1=%sn", str1);
	return 0;
}
输入:3   则输出:str1=gooe depends not on luck
输入:4   则输出:str1=good depends not on luck
输入:5   则输出:str1=good
输入:8   则输出:str1=good

↑想知道为什么请看这

函数原型:

char* strcat(char* destination,const char* source);

#include 
#include 
int main()
{
	char* my_strcat(char*, const char*);
	char str1[20] = "Life";
	const char* str2 = "Good";
	char* ret = my_strcat(str1, str2);
	printf("%sn", str1);
	return 0;
}
char* my_strcat(char* dest, const char* sour)
{
	char* tmp = dest;
	assert(dest && sour);
	//找到目的字符串的''
	while (*dest)//这里不可以写成while(*dest++){;}  因为当*dest=''时,条件判断虽然为假,但是dest还是会++,导致最后str1中的状态为LifeGood
	{
		dest++;
	}
	while (*dest++ = *sour++)//追加
	//注意:当*sour=''时,即*dest='',条件为假,此时dest和sour还是会++
	{
		;
	}
	return tmp;
}

函数原型:

char* strcat(char* destination,const char* source,size_t n);

#define _CRT_SECURE_NO_WARNINGS 1
#include 
#include 
int main()
{
	char* my_strncat(char*, const char*,unsigned);
	char str1[20] = "Life";
	const char* str2 = "Good";
	unsigned n = 0;
	scanf("%u",&n);
	char* ret = my_strncat(str1, str2,n);
	printf("%sn", str1);
	return 0;
}
char* my_strncat(char* dest, const char* sour,unsigned n)
{
	char* tmp = dest;
	assert(dest && sour);
	//找到目的字符串的''
	while (*dest)
	{
		dest++;
	}
	while (n)//追加
	{
		*dest++=*sour++;
		n--;
	}
	return tmp;//返回目标空间的地址
}

函数原型:

int strcmp(const char* str1,const char* str2);

#include 
#include 
int main()
{
	int my_strcmp(const char*, const char*);
	const char* str1 = "helloworld";
	const char* str2 = "helloboy";
	int ret = my_strcmp(str1, str2);
	printf("%dn", ret);//1
	return 0;
}
int my_strcmp(const char* str1, const char* str2)
{
	assert(str1 && str2);
	while (*str1 == *str2)
	{
		if (*str1 == '')
		{
			return 0;//等于
		}
		str1++;
		str2++;
	}
	if (*str1 > *str2)
		return 1;//大于
	else
		return -1;//小于
}
若在gcc编译器上面if-else语句可替换成:
return (*str1-*str2);

函数原型:

int strcmp(const char* str1,const char* str2,size_t n);

#define _CRT_SECURE_NO_WARNINGS 1
#include 
#include 
int main()
{
	int my_strcmp(const char*, const char*, unsigned);
	const char* str1 = "helloworld";
	const char* str2 = "helloboy";
	unsigned n = 0;
	scanf("%u", &n);
	int ret = my_strcmp(str1, str2, n);
	printf("%dn", ret);//1
	return 0;
}
int my_strcmp(const char* str1, const char* str2, unsigned n)
{
	assert(str1 && str2);
	while (n)
	{
		if (*str1 == *str2)
		{
			str1++;
			str2++;
		}
		else if (*str1 > *str2)
			return 1;//大于
		else
			return -1;//小于
		n--;
	}
	return 0;

}

函数原型:

char* strstr(const char* str1,const char* str2);

#include 
#include 
int main()
{
	char* my_strstr(const char*, const char*);
	char* str1 = "hellowordword";//"hello"	"abbbc"
	char* str2 = "word";		 //"hello"	"bbc"
	char* ret = my_strstr(str1, str2);
	printf("%sn", ret);//wordword
	return 0;
}
char* my_strstr(const char* str1, const char* str2)
{
	assert(str1 && str2);
	char* s1 = (char*)str1;
	char* s2 = (char*)str2;
	char* tmp =(char*)str1;//用tmp来存放str2在str1中第一次出现的地址
	if (*str2 == '')
	{
		return (char*)str1;
	}
	while (*tmp!='')
	{
		s1 = tmp;
		s2 = str2;
		while((*s1 == *s2) && (*s1!='') && (*s2!=''))
		{
			s1++;
			s2++;
		}
		if (*s2 == '')
			return tmp;
		tmp++;
	}
	return NULL;
}

函数原型:

char* strchr(const char* str,char c);

#define _CRT_SECURE_NO_WARNINGS 1
#include 
char* my_strchr(const char* str, char c)
{
	while (*str != '')
	{
		if (*str == c)
			return (char*)str;
		str++;
	}
	return NULL;
}
int main()
{
	char* str = "How Do You Do?";
	char c;
	scanf("%c", &c);
	char* ret = my_strchr(str,c);
	if (ret != NULL)
	{
		printf("%sn", ret);
	}
	else
		printf("NULLn");
	return 0;
}