C语言常用字符串函数和内存函数的实现

Posted 2022-12-06 一只当归

• strlen函数 常用于计算字符串中的字符个数

//my_strlen的实现
int my_strlen(const char*str)
	assert(str != null);
	int len = 0;
	while (*str != '\\0')
		++len;
		++str;
	
	return len;

int main()
		//char*p1 = "hello world"; //两种字符串赋值方式 
		//printf("%d\\n", strlen(p1));
		char p2[] = "hello world";
		printf("%d\\n", my_strlen(p2));
		system("pause");
		return 0;

• strcpy函数 常用于字符串的拷贝

//my_strcpy的实现
char* my_strcpy(char*dst, const char*src)
	assert(dst != NULL);
	assert(src != NULL);
	char*ret = dst;
	while (*src != '\\0')
		*dst = *src;
		++src;
		++dst;
	
	*dst = '\\0';
	return ret;

int main()
	char p1[] = "hello";
	char p2[] = "world";
	//strcpy(p2, p1);//把p1的字符串拷贝到p2上
	//printf("%s\\n", p2);
	my_strcpy(p2, p1);
	printf("%s\\n", p2);
	system("pause");
	return 0;

• strcat函数 常用于字符串追加拷贝

//my_strcat的实现
char* my_strcat(char* dst, const char*src)
	assert(dst&&src);
	char* ret = dst;
	while (*dst != '\\0')
		++dst;
	
	while (*dst++ = *src++);
	return ret;

int main()
		char p1[11] = "hello";
		char*p2 = "world";
		my_strcat(p1, p2);
		printf("%s\\n",p1);
		system("pause");
		return 0;

• strcmp函数 常用于字符串比较大小（比较SACII码）

//my_strcmp的实现
int my_strcmp(const char* str1, const char* str2)
	assert(str1&&str2);
	int ret = 0;
	unsigned char* s1 = (unsigned char*)str1;
	unsigned char* s2 = (unsigned char*)str2;
	while (*s1 && *s2)
		if (*s1 > *s2)
		
			return 1;
		else if (*s1 < *s2)
		
			return -1;
		
		else
			++s1;
			++s2;
		
	
	if (*s1 == '\\0'&& *s2 == '\\0')
	  
		return 0;
	else if (*s1 == '\\0')
	
		return -1;
	
	else
		return 1;
	
	return ret;

int main()
	char* p1 = "hello";
	char* p2 = "world";
	printf("%d\\n", my_strcmp(p1, p2));
	system("pause");
	return 0;

• strstr函数 用于判断str2是否为str1的子串

//my_strstr的实现
const char* my_strstr(const char*src,const char*sub)
	assert(src && sub);
	const char* srci = src;
	const char* subi = sub;
	while (*srci != '\\0')
		while (*srci == *subi && *subi != '\\0')
		
			++srci;
			++subi;
		
		if (*subi == '\\0')
		
			return src;
		
		else
		
			subi = sub;
			++src;
			srci = src;
		
	
	return NULL;

int main()
	char* p1 = "abcde"; //判断是否是旋转串
	char* p2 = "cdeba";
	char p3[11];
	strcpy(p3, p1);
	strcat(p3, p2);
	if (my_strstr(p3,p2)!= NULL)
	
		printf("是旋转串\\n");
	
	else
		printf("不是旋转串\\n");
	
	char* p4 = "aaaaa";
	char* p5 = "aabb";
	if (my_strstr(p4, p5) != NULL)
	
		printf("是旋转串\\n");
	
	else
		printf("不是旋转串\\n");
	
	system("pause");
	return 0;

• memcpy函数 常用于内存拷贝

//my_memcpy的实现
void* my_memcpy(void* dst, const void* src, size_t num)  //注意是void* 因为可能是任意类型
	assert(dst && src);
	char* str_dst = (char*)dst;
	char* str_src = (char*)src;
	for (size_t i = 0; i < num; ++i)
		str_dst[i] = str_src[i];
	
	return dst;

int main()
	int a1[10] =  1, 2, 3, 4, 5 ;
	int a2[10];
	my_memcpy(a2, a1, 10 * sizeof(int));
	for (int i = 0; i < 10;i++)
	
		printf("%d ", a2[i]);
	
	printf("\\n");
	system("pause");
	return 0;

• memmove函数 常用于内存拷贝中内存重叠问题

//my_memmove的实现
void* my_memmove(void* dst,const void* src,size_t num)
	assert(dst && src);
	char* str_dst = (char*)dst;
	char* str_src = (char*)src;
	if (str_src < str_dst && dst < str_src+num) //后重叠 从前往后拷贝
	
		for (int i = num - 1; i >= 0; --i)
		
			str_dst[i] = str_src[i];
		
	
	else  //前重叠 不重叠 从前往后拷贝
		for (int i = 0; i < num; ++i)
		
			str_dst[i] = str_src[i];
		
	
	return dst;

int main()
	int a[10] =  1, 2, 3, 4, 5 ;
	my_memmove(a + 3, a, 5 * sizeof(int));
	for (size_t i = 0; i < 10;++i)
	
		printf("%d ", a[i]);
	
	printf("\\n");
	system("pause");
	return 0;