c语言＜string.h＞库函数的模拟实现

1.strstr

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

用途：看str1中是否有str2，若有则返回str1中相同的起始地址，若无则返回空指针

模拟实现1：

​
​
char* my_strstr(const char*str1,const char*str2)
{
	char* p = str1;
	char* s1 = NULL;
	char* s2 = NULL;
	while (*p)
	{
		s1 = p;
		s2 = str2;
		while (*s1 && *s2 && (*s1 == *s2))
		{
			s1++;
			s2++;
		}
		if (*s2 == 0)
		{
			return p;
		}
		p++;
	}
	
	return NULL;
}

​

​

模拟实现2：

char* my_strstr(char* arr, char* arr2)
{
	char* ret2 = arr2;
	while(*(arr++)!=*(arr2)) {}
	arr--;
	
	if (*arr == '\0')
	{
		return NULL;
	}
	char* ret = arr;
	while (*(arr++) == *(arr2++) && *(arr2)&&*(arr))
	{}
	if (*(arr2) == '\0')
	{
		return ret;
	}
	
	return my_strstr(ret+1,ret2);
}

2.strtok

char * strtok ( char * str, const char * delimiters );

第一个str为目标字符数组，第二个delimiters为需要分割的字符的数组

用途:从str中分割delimiters

模拟实现：

char* my_strtok(char*arr,const char *arr2)
{
	static char*p;
	char* tmp;
	if (arr != NULL)
	{
		tmp = arr;
	}
	else
	{
		tmp = p+1;
	}
	char* start = tmp;
	
	for (; *tmp; tmp++) 
	{
		char* tmp2=arr2;
		for (; *tmp2; tmp2++)
		{
			if (*tmp == *tmp2)
			{
				p = tmp;
				*tmp = '\0';
				return start;
			}
		}
	}
	if (*start != '\0')
	{
		p = tmp;
		return start;
	}
	return NULL;
}


int main()
{
	char arr[] = "hello*vantasy%you^are&good";
	char arr2[] = "*%^&";
	char temp[30] = {0};
	strcpy(temp, arr);
	char* temp2 = NULL;
	for (temp2=my_strtok(temp, arr2); temp2 != NULL; )
	{
		
		printf("%s\n", temp2);
		temp2 = my_strtok(NULL, arr2);
	}

	return 0;
}

3.strcmp(strncmp)

strcmp

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

用途：依序比较str1和str2字符数组的大小

strncmp：

int strncmp ( const char * str1, const char * str2, size_t num )

用途：依序比较str1和str2的前num个字符数组的大小

模拟实现：

int my_strncmp(const char*str,const char*str2,size_t n)
{
	int count = 1;
	while (*str&&*str2&&*str==*str2&&count<n)
	{
        str++; str2++;
		count++;
	}

	if (*str > *str2)
	{
		return 1;
	}
	else if (*str < *str2)
	{
		return -1;
	}
	else
	{
		return 0;
	}
}
int main()
{
	char arr[] = "abcde";
	char arr2[] = "abcfe";
	printf("%d", my_strncmp(arr, arr2, 4));

	return 0;
}

4.strcat(strncat)

strcat

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

用途：在destination的字符串后面加上source的字符串

模拟实现1：

#include<assert.h>
char* my_strcat(char* arr, const char* arr2)
{
	assert(arr && arr2);
	char* ret = arr;
	while (*arr++) {}
	arr--;
	while(*arr2)
	{
		*arr = *arr2;
		arr++;
		arr2++;
	}
	return ret;
}
int main()
{
    char arr[20] = "hello";
	char arr2[] = "Mr_vantasy";
	printf("%s", my_strcat(arr, arr2));

	return 0;
}

模拟实现2：

int main()
{
	char arr[20] ="hello";
	char arr2[] = "world";
	sprintf(arr+strlen(arr), arr2);
	printf(arr);
	return 0;
}

strncat

char * strncat ( char * destination, const char * source, size_t num );

用途：在destination的字符串后面加上source的字符串的前n位

模拟实现：

#include<assert.h>
char* my_strncat(char* arr, const char* arr2,size_t num)
{
	assert(arr && arr2);
	int count=0;
	char* ret = arr;
	while (*arr++) {}
	arr--;
	while(*arr2&&count<num)
	{
		*arr = *arr2;
		arr++;
		arr2++;
		count++;
	}
	return ret;
}
int main()
{
    char arr[20] = "hello";
	char arr2[] = "Mr_vantasy";
	printf("%s", my_strncat(arr, arr2,3));

	return 0;
}

5.strcpy(strncpy)

strcpy：

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

用途：复制source的字符串到destination并覆盖

模拟实现：

#include<assert.h>
char* my_strncpy(char* str, char* str2)
{
	assert(str && str2);
	char* ret = str;
	while (*str2)
	{
		*str = *str2;
		str++;
		str2++;
	}
	return ret;
}
int main()
{
	char arr[20] = "hello__";
	char arr2[] = "vantasy";
	printf("%s", my_strncpy(arr, arr2));

	return 0;
}

strncpy：

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

用途：复制source的n个字符到destination并覆盖

模拟实现：

#include<assert.h>
char* my_strncpy(char* str, char* str2,size_t num)
{
	assert(str && str2);
	char* ret = str;
	int count = 0;
	while (*str2&&count<num)
	{
		*str = *str2;
		str++;
		str2++;
		count++;
	}
	return ret;
}
int main()
{
	char arr[20] = "hello__";
	char arr2[] = "vantasy";
	printf("%s", my_strncpy(arr, arr2,3));

	return 0;
}

后记：也许会有人说库里的函数直接用就行，为什么还要去模拟实现一下。但是模拟实现是帮我们更好的去理解这些库函数，更甚可以做到以后自己编写自己的库，学习前人的智慧并去模仿也是会让我们更上一层楼的（新人博主用词不准，或代码有误请多多指教…..φ(´U｀●)╭(●｀∀´●)╯╰(●’◡’●)╮ (●’◡’●)ﾉ ）