[数据结构与算法]c语言字符串函数

一.字符串检验

? 1.strlen:返回给定空终止字符串的长度

size_t my_strlen(const char* str);

2. strcmp:以字典序比较两个空终止字节字符串。结果的符号是被比较的字符串中首对不同字符的值间的差的符号。

int my_strcmp(const char* lhs, const char* rhs);

#include<stdio.h>
#include<assert.h>
#define N 30

int my_strcmp(const char* arr, const char* arr1)
{
	assert(arr != NULL && arr1 != NULL);
	while (*arr != '\0' && *arr1 != '\0')
	{
		if (*arr == *arr1)
		{
			arr++;
			arr1++;
		}
		if (*arr < *arr1)
		{
			return -1;
		}
		else
		{
			return 1;
		}
	}
	return 0;
}

int main()
{
	char arr[N] = { "xuyeyyds" };
	char arr1[N] = { "xuyeyyus" };
	int l = my_strcmp(arr, arr1);
	printf("%d\n", l);
	return 0;
}

?3.strncmp:比较两个可能空终止字节字符串。结果的符号是被比较的字符串中首对不同字符（都转译成unsigned char）的值间的差的符号

4.strchr:查找字符的首次出现

#include<stdio.h>
#include<assert.h>
#define N 30
char* my_strchr(const char* arr, char ch)
{
	if (arr == NULL)
	{
		return NULL;
	}
	char* ip = (char*)arr;
	while (*ip != '\0' && *ip != ch)
	{
		ip++;
	}
	return ip;
}


int main()
{
	char arr[N] = { "fkdgnfd" };
	char arr1[N] = { "fkdgasd" };
	char* p = NULL;
	p=my_strchr(arr, 'g');
	printf("%s\n",p);
	return 0;
}

5.strrchr:查找字符的最后一次出现

#include<stdio.h>
#include<assert.h>
#define N 30
char* my_strrchr(const char* arr, char ch)
{
	if (arr == NULL)
	{
		return NULL;
	}
	char* ip = (char*)arr;
	while (*ip != '\0')
	{
		ip++;
	}
	while (*(ip-1) != '\0')
	{
		if (*ip == ch)
		{
			return ip;
		}
		ip--;
	}
}

int main()
{
	char arr[N] = { "fkdgngd" };
	char* p = NULL;
	p = my_strrchr(arr, 'g');
	printf("%s\n", p);
	return 0;
}

6.strstr:查找字串字符的首次出现

#include<stdio.h>
#include<assert.h>
#define N 30
int my_strlen(char* arr)
{
	assert(arr != NULL);
	int count = 0;
	while (*arr != '\0')
	{
		arr++;
		count++;
	}
	return count;
}

char* my_strrchr(const char* arr, const char* arr1)
{
	if (NULL == arr || NULL == arr1)return NULL;
	int n = my_strlen(arr);
	int m = my_strlen(arr1);
	for (int i = 0; i < n - m + 1; i++)
	{
		int k = i;
		int j = 0;
		while (j < m && arr[k] == arr1[j])
		{
			j++;
			k++;
		}
		if (j == m)
		{
			return (char*)arr + i;
		}
	}
	return NULL;
}

int main()
{
	char arr[N] = { "fkdgwfkeongd" };
	char arr1[N] = { "wfkeo" };
	char* p = NULL;
	p = my_strrchr(arr,arr1);
	printf("%s\n", p);
	return 0;
}

二.字符串操作

1.strcpy:复制一个字符串给另一个

#include<stdio.h>
#include<assert.h>
#define N 30

char* my_strcpy(char* dest, char* src)
{
	assert(dest != NULL && src != NULL);
	char* cp = dest;
	while (*src != '\0' )
	{
		*cp = *src;
		src++;
		cp++;
	}
	*cp = '\0';
	return dest;
}


int main()
{
	char arr[N] = { "fkdgwfkeongd" };
	char arr1[N] = { "wfkeo" };
	my_strcpy(arr, arr1);
	printf("%s\n", arr);
	return 0;
}

?2.strncpy:从一个字符串复制一定数量的字符到另一个

#include<stdio.h>
#include<assert.h>
#define N 30

char* my_strncpy(char* dest, const char* src, size_t count)
{
	assert(dest != NULL && src != NULL);
	char* cp = dest;
	while (count--)
	{
		*cp = *src;
		cp++;
		src++;
	}
	return dest;
}
int main()
{
	char arr[N] = { "jkdgj" };
	char arr1[N] = { "wfkeo" };
	my_strncpy(arr, arr1,sizeof(arr1));
	printf("%s\n", arr);
	return 0;
}

?3.strcat:连接两个字符串

#include<stdio.h>
#include<assert.h>
#define N 30

char* my_strcat(char* dest, char* src)
{
	assert(dest != NULL && src != NULL);
	while (*dest != '\0')
	{
		dest++;
	}
	while (*src != '\0')
	{
		*dest = *src;
		dest++;
		src++;
	}
	*dest = '\0';
	return dest;
}

int main()
{
	char arr[N] = { "jkdgj" };
	char arr1[N] = { "wfkeo" };
	my_strcat(arr, arr1);
	printf("%s\n", arr);
	return 0;
}

4.strncat:连接两个字符串的一定数量字符

#include<stdio.h>
#include<assert.h>
#define N 30

char* my_strncat(char* dest, const char* src,size_t count)
{
	assert(dest != NULL && src != NULL);
	char* cp = dest;
	while (*cp != '\0')
	{
		cp++;
	} 
	int ret = strlen(src);
	while (ret--)
	{
		*cp = *src;
		cp++;
		src++;
		if (strlen(dest) > count)
		{
			return NULL;
		}
	}
	return dest;
}

int main()
{
	char arr[N] = { "jkdgfghfyugoji" };
	char arr1[N] = { "ppppp" };
	char*p=my_strncat(arr, arr1, sizeof(arr)/sizeof(arr[0]));
	printf("%s\n", p);
	return 0;
}

三.内存操作函数&泛型函数

1.memset：以字符填充缓冲区

void *memset(void *dest,int ch,size_t count);

复制值ch(如同以（unsgined char)ch转换到unsgined char 后)到dest所指对象的首count个字节

#include<stdio.h>
#include<assert.h>
#define N 30

void* my_memset(void* dest, unsigned char val, size_t count)
{
	if (NULL == dest)return dest;
	char* cp = (char*)dest;
	while (count--)
	{
		*cp = val;
		cp = cp + 1;
	}
	return dest;
}

int main()
{
	char arr[N] = {"dsjhfhujsdh"};
	my_memset(arr, 'a', sizeof(arr));
	for (int i = 0; i < N; i++)
	{
		printf("%d\n", arr[i]);
	}
	return 0;
}

?2.memcmp比较两块缓冲区

int memcmp(const void*dest,const void*src,size_t count);

比较dest,src所指对象的首个count字节。比较按字典序进行。

结果的符号是在被比较对象中相异的首个字节的值（都被转译成unsigned char）的差

若在dest和src所指向的任意对象结尾出现后访问，则行为未定义

#include<stdio.h>
#include<assert.h>
#define N 30

int my_memcmp(const char* dest, const char* src, size_t count)
{
	assert(dest != NULL && src != NULL);
	const char* cp = (const char *)dest;
	const char* ip = (const char *)src;
	while (count--)
	{
		if (*dest < *src)
		{
			return -1;
		}
		else if (*dest > *src)
		{
			return 1;
		}
		else
		{
			cp++;
			ip++;
		}
	}
	return 0;
}

int main()
{
	char arr[N] = {"dsjhfhujsdh"};
	char arr1[N] = { "sdfjhjwe" };
	int x=my_memcmp(arr, arr1, sizeof(int)*6);
	printf("%d\n", x);
	return 0;
}

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