C语言的字符串函数,内存函数笔记详解

//关于strlen返回值的一个易错点
int main()
{
	const char* str1 = "abcdef";
	const char* str2 = "bbb";
	if (strlen(str2) - strlen(str1) > 0)
	{
		printf("str2>str1\n");
	}
	else
	{
		printf("srt1>str2\n");
	}
	return 0;
}

size_t my_strlen1(const char* str)
{
	assert(str);
	int count = 0;
	while (*str++)
	{
		count++;
	}
	return count;
}

size_t my_strlen2(const char* str)
{
	assert(str);
	char* start = str;
	while (*str!='\0')//注意这种写法不能写*str++;这里要先判断再++；*str++的写法在'\0'的地方也+1了
	{
		str++;
	}
	return str - start;//\0与起始位置的差就是字符数
}

size_t my_strlen3(const char* str)
{
	if ('\0' == *str)
		return 0;
	else
		return 1 + my_strlen3(str + 1);
}

char* my_strcpy(char* dest, const char* class="lazy" data-src)
{
	assert(dest && class="lazy" data-src);
	char* ret = dest;
	while (*dest++ = *class="lazy" data-src++)
	{
		;
	}
	return ret;
}
int main()
{
	char arr1[20] = "hello underworld";//注意写成数组
	char arr2[20] = "hello world";

	printf("%s\n", arr2);
	printf("%s\n", my_strcpy(arr2, arr1));

	return 0;
}

char* my_strcat(char* dest, const char* class="lazy" data-src)
{
	char* ret = dest;
	assert(dest && class="lazy" data-src);
	while (*dest)//让dest到达str1的\0位置
	{
		dest++;
	}
	while (*dest++ = *class="lazy" data-src++)//这一部分和strcpy同
	{
		;
	}
	return ret;
}
int main()
{
	char arr1[20] = "hello ";
	char arr2[20] = "underworld";

	printf("%s\n", my_strcat(arr1, arr2));

	return 0;
}

int my_strcmp(const char* str1, const char* str2)
{
	while (*str1 == *str2)
	{
		if (*str1 == '\0')//说明两个字符串同时到达结束标记
			return 0;
		str1++;
		str2++;
	}
	return *str1 - *str2;//如果不是在循环内部返回，就一定不相等，而字符相减可以反映大小
}
int main()
{
	char *str1 = "hello world";
	char *str2 = "hello underworld";

	printf("%d\n", my_strcmp(str1, str2));//w比u大

	return 0;
}

char* my_strstr(const char* str1, const char* str2)
{
	assert(str1 && str2);

	char* s1;//s1维护str1
	char* s2;//s2维护str2
	char* cp = str1;//cp用来记录比较开始的位置

	if (*str2 == '\0')//特殊情况
		return str1;

	while (*cp)
	{
		s1 = cp;
		s2 = str2;

		while (*s1 != '\0' && *s2 != '\0' && *s1 == *s2)//其实*s1='\0'且*s2!='\0'时已经没有比较下去的必要了
			//*s1==*s2就让两个维护指针分别+1；不等就让cp+1,s1和s2分别重置		
		{
			s1++;
			s2++;
		}
		if (*s2 == '\0')//*s2=='\0'说明找到了
		{
			return cp;
		}
		cp++;
	}

	return NULL;
}

void get_next(char* str, int* next)
{
	int i, k;
	i = 0;
	k = -1;
	next[0] = -1;//这个值没用；或者说是为了使i增加而j不增加
	int len = strlen(str);
	while (i < len - 1)//next数组最大下标是字符串长度减1，数组长度和字符串长度相同
	{
		if (k == -1 || *(str + i) == *(str + k))
		{
			++i;
			++k;
			next[i] = k;
		}
		else
			k = next[k];
	}

	//测试打印next
	int z;
	printf("next:");
	for (z = 0; z < len; z++)
	{
		printf("%d ", next[z]);
	}
	printf("\n");
}

int Index_KMP(char* str1, char* str2, int pos)
{
	int i = pos;
	int j = 0;
	int next[255];
	get_next(str2, next);
	int len1 = strlen(str1);
	int len2 = strlen(str2);
	int count = 0;
	while (i < len1 && j < len2)//i从0到10(len1=11)，共11次，但是考虑到走else的回溯，单字符查找一共循环22次
	{
		count++;
		if (j == -1 || *(str1 + i) == *(str2 + j))//先判断，再把下标加1
		{
			++i;
			++j;
		}
		else
		{
			j = next[j];
		}
	}
	printf("i=%d\n", i);
	printf("j=%d\n", j);
	printf("count=%d\n", count);//是存在回溯的，那么这个函数的时间复杂度还是O(m)吗？
	//if ((len2!=1) && (j >=(len2-1)))//有缺陷，对于len2=1的情况无法处理
	if (j >= (len2 - 1))//单字符查找情形，while结束时j=0，而len2-1也=0，故不能作为找到了的标志
		//对于单字符查找以外的情形，len2-1一定大于0，len2-1代表的就是目标串最后一个字符的下标，既然j
		//到达了这个位置，就说明完全匹配了
		return i - len2;//由于字符串长度与数组下标的差异造成len2=1时
	else
		return 0;
}
int main()
{
	char* str1 = "hello underworld!";
	char* str2 = "under";

	printf("%s\n", my_strstr(str1, str2));
	printf("%s\n", *(str1+Index_KMP(str1, str2, 0)));

	return 0;
}

int main()
{
	char arr1[20] = "abcdefghi";
	char arr2[] = "xxxx";

	//strncpy(arr1, arr2, 6);//从arr2拷贝6个字符给arr1?如果arr2长度不够，则补0
	//strncpy(arr1, arr2, 3);//长度不够不拷贝\0
	//strncpy(arr1, arr2, 4);
	strncpy(arr1, arr2, 5);

	printf("%s\n", arr1);
	return 0;
}

int main()
{
	char arr1[20] = "abc\0xxxxxxx";
	char arr2[] = "def";

	//strncat(arr1, arr2, 6);//在arr1后面接上arr2的六个字符？最多只接arr2这么长的字符串，包括\0
	//strncat(arr1, arr2, 3);//自己会加上\0
	strncat(arr1, arr2, 2);

	printf("%s\n", arr1);
	return 0;
}

int main()
{
	char arr1[] = "abcdew";
	char arr2[] = "abcdeqj";

	printf("%d\n",strncmp(arr1, arr2, 5));
	printf("%d\n",strncmp(arr1, arr2, 6));

	return 0;
}

int main()
{
	char arr1[] = "cjh@scu.edu";
	char arr2[100] = { 0 };//保存临时数据
	char sep[] = "@.";
	char* ret = NULL;//接收strtok的返回值
	strcpy(arr2, arr1);
	for (ret = strtok(arr2, sep); ret != NULL; ret = strtok(NULL, sep))
	{
		printf("%s\n", ret);
	}

	return 0;
}

int main()
{
	char str[] = "- This, a sample string.";
	char* pch;
	printf("Splitting string \"%s\" into tokens:\n", str);
	pch = strtok(str, ", .-");//分隔标记的位置不重要
	while (pch != NULL)
	{
		printf("%s\n", pch);
		pch = strtok(NULL, " ,.-");//注意这里有空格
	}
	return 0;
}

void* my_memcpy(void* dest, void* class="lazy" data-src, size_t count)
{
	void* ret = dest;
	assert(dest && class="lazy" data-src);

	while (count--)
	{
		*(char*)dest = *(char*)class="lazy" data-src;
		dest = (char*)dest + 1;
		class="lazy" data-src = (char*)class="lazy" data-src + 1;
	}
	//printf("%d\n", count);//count=-1
	return ret;
}
int main()
{
	int arr1[10] = { 1,2,3,4,5,6,7,8,9,10 };
	int arr2[20] = { 0 };

	my_memcpy(arr2, arr1, 10 * sizeof(int));

	int i;
	for (i = 0; i < 20; i++)
	{
		printf("%d ", arr2[i]);
	}

	return 0;

#include <stdio.h>
#include <string.h>
int main()
{
	char str[] = "memmove can be very useful......";
	printf("%c\n", *(str + 15));
	printf("%c\n", *(str + 20));

	memmove(str + 20, str + 15, 11);//注意memmove和memcpy不会遇到\0停下来，什么时候停取决于第三个参数
	puts(str);
	return 0;
}

void* my_memmove(void* dest, void* class="lazy" data-src, size_t count)//关键在于拷贝之前先判断是否会出现overlap
{
	void* ret = dest;
	if (dest <= class="lazy" data-src || (char*)dest >= ((char*)class="lazy" data-src + count))
	{
		while (count--)
		{
			*(char*)dest = *(char*)class="lazy" data-src;
			dest = (char*)dest + 1;
			class="lazy" data-src = (char*)class="lazy" data-src + 1;
		}
	}
	else
	{
		dest = (char*)dest + count - 1;
		class="lazy" data-src = (char*)class="lazy" data-src + count - 1;
		while (count--)
		{
			*(char*)dest = *(char*)class="lazy" data-src;
			dest = (char*)dest - 1;
			class="lazy" data-src = (char*)class="lazy" data-src - 1;
		}
	}
	return ret;
}
int main()
{
	int arr[10] = { 1,2,3,4,5,6,7,8,9,10 };

	my_memmove(arr + 2, arr, 4 * sizeof(int));//1 2 1 2 3 4 7 8 9 10  
	//my_memcpy(arr + 2, arr, 4 * sizeof(int));//1 2 1 2 1 2 7 8 9 10

	int i;
	for (i = 0; i < 10; i++)
	{
		printf("%d ", arr[i]);
	}
	return 0;
}

#include <stdio.h>
#include <string.h>
int main()
{
	char buffer1[] = "DWgaOtP12df0";
	char buffer2[] = "DWGAOTP12DF0";
	int n;
	n = memcmp(buffer1, buffer2, sizeof(buffer1));
	if (n > 0) printf("'%s' is greater than '%s'.\n", buffer1, buffer2);
	else if (n < 0) printf("'%s' is less than '%s'.\n", buffer1, buffer2);
	else printf("'%s' is the same as '%s'.\n", buffer1, buffer2);
	return 0;
}

#include <stdio.h>
int main ()
{
  int i=0;
  char str[]="Test String.\n";
  char c;
  while (str[i])
 {
    c=str[i];
    if (isupper(c)) 
        c=tolower(c);
    putchar (c);
    i++;
 }
  return 0; }