字符函数和字符串函数及其模拟实现

目录

0.前言

1.函数介绍及其模拟实现

1.1strlen

1.2 strcpy

 1.3 strcat

1.4 strcmp

 1.5 strstr

 1.6 memcpy

---

0.前言

C语言对字符和字符串的处理很频繁,但是C语言本身是没有字符串类型的,字符串通常存放在字符数组或者常量字符串中.

1.函数介绍及其模拟实现 1.1strlen

先来strlen在msdn上的介绍

 

 size_t strlen ( const char * str );

~  字符串已经'\0'作为结束标志,strlen函数返回的是字符串中'\0'前面出现的字符个数(不包含'\0')

~  参数指向的字符串必须要以'\0'结束

~注意函数的返回值为size_t,是无符号的

 

#include 
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;
}

strlen的模拟实现 -- 3种方法

//模拟实现strlen
//1.循环的方法
int my_strlen(const	char* arr)
{
	assert(arr);
	int ret = 0;
	while (*arr)
	{
		arr++;
		ret++;
	}
	return ret;
}

//2.递归的方法
int my_strlen(const char* arr)
{
	assert(arr);
	if (*arr)
		return my_strlen(arr + 1) + 1;
	else
		return 0;
}

//3.指针-指针的方法
int my_strlen(const char* arr)
{
	assert(arr);
	char* tmp = arr;
	while (*arr)
	{
		arr++;
	}
	return arr - tmp;
}

---

1.2 strcpy

strcpy在MSDN上的介绍

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

~ The strcpy function copies strSource, including the terminating null character, to the location specified by strDestination.

~ 源字符串必须以'\0'结束

~ 目标空间必须足够打,以确保能存放源字符串

~ 目标空间必须可变 

strcpy的模拟实现

char* my_strcpy(char* dest, const char* src)
{
	assert(dest && src);
	char* tmp = *dest;
	while (*dest++ = *src++)
	{
		;
		
	}
	return dest;
}

---

 1.3 strcat

strcat在MSDN上的介绍

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

~ The strcat function appends strSource to strDestination and terminates the resulting string with a null character. The initial character of strSource overwrites the terminating null character of strDestination.

~ 源字符串必须以'\0'结束

~ 目标空间必须足够大,能容纳下源字符串的内容

~ 目标空间必须可修改

/* strncat example */
#include 
#include 
int main ()
{
 char str1[20];
 char str2[20];
 strcpy (str1,"To be ");
 strcpy (str2,"or not to be");
 strncat (str1, str2, 6);
 puts (str1);
 return 0;
}

模拟实现strcat

char* my_strcat(char* dest,const char* src)
{
	assert(dest && src);
	char* str = dest;
	while (*dest)
	{
		dest++;
	}
	while (*dest++ = *src++)
	{
		;
	}
	return str;
}

 

---

1.4 strcmp

strcmp在MSDN上的介绍

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

 ~ The strcmp function compares string1 and string2 lexicographically and returns a value indicating their relationship

~

Value	Relationship of string1 to string2
< 0	string1 less than string2
0	string1 identical to string2
> 0	string1 greater than string2

模拟实现strcmp

int my_strcmp(const char * arr1,const char* arr2)
{
	assert(arr1 && arr2);
	while (*arr1 == *arr2)
	{
		if (*arr1 == '\0')
			return 0;
		arr1++;
		arr2++;
	}
	if (*arr1 > *arr2)	
		return 1;	
	else
		return -1;
}

---

 1.5 strstr

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

~ The strstr function returns a pointer to the first occurrence of strCharSet in string. The search does not include terminating null characters 

 

/* strstr example */
#include 
#include 
int main ()
{
  char str[] ="This is a simple string";
  char * pch;
  pch = strstr (str,"simple");
  strncpy (pch,"sample",6);
  puts (str);
  return 0;
} 

 模拟实现strstr

char* my_strstr(const char* str1, const char* str2)
{
	assert(str1 && str2);
	const char* s1 = str1;
	const char* s2 = str2;
	const char* cur = str1;
	while (*cur)
	{
		s1 = cur;
		s2 = str2;
		while ((*s1 == *s2) && *s1 && *s2)
		{
			s1++;
			s2++;
		}
		if (*s2 == 'void* my_memcpy(void* dest, const void* src, size_t num)
{
	assert(dest && src);
	void* ret = dest;
	while (num--)
	{
		*(char*)dest = *(char*)src;
		dest = (char*)dest + 1;
		src = (char*)src + 1;
	}
	return ret;
}')
		{
			return (char*)cur;
		}
		cur++;
	}
	return NULL;
}

---

 1.6 memcpy

void * memcpy ( void * destination, const void * source, size_t num );

~ 函数memcpy从source的位置开始向后复制num个字节的数据到destination的内存位置

~ 这个函数在遇到'\0'的时候并不会停下来

~ 如果source和destination有任何重叠,复制的结果都是未定义的

 

/* memcpy example */
#include 
#include 
struct {
  char name[40];
  int age;
} person, person_copy;
int main ()
{
  char myname[] = "Pierre de Fermat";
  /* using memcpy to copy string: */
  memcpy ( person.name, myname, strlen(myname)+1 );
  person.age = 46;
  /* using memcpy to copy structure: */
  memcpy ( &person_copy, &person, sizeof(person) );
  printf ("person_copy: %s, %d \n", person_copy.name, person_copy.age );
  return 0;
}

模拟实现memcpy

void* my_memove(void* dest,const void* src,size_t num)
{
	assert(dest && src);
	void* tmp = dest;
	if (dest < src)
	{
		while (num --)
		{
			*(char *)dest = *(char*)src;
			dest = (char*)dest + 1;			
			src = (char*)src + 1;
		}
	}
	else
	{
		while (num --)
		{
			*((char*)dest+num) = *((char*)src + num);		
		}
	}
	return tmp;
}

1.7 memmove

 void * memmove ( void * destination, const void * source, size_t num );

 ~ 和memcpy的差别就是memmove函数处理的源内存块和目标内存块是可以重叠的

~ 如果源空间和目标空间出现重叠,就得使用memmove函数处理

/* memcpy example */
#include 
#include 
struct {
  char name[40];
  int age;
} person, person_copy;
int main ()
{
  char myname[] = "Pierre de Fermat";
  /* using memcpy to copy string: */
  memcpy ( person.name, myname, strlen(myname)+1 );
  person.age = 46;
  /* using memcpy to copy structure: */
  memcpy ( &person_copy, &person, sizeof(person) );
  printf ("person_copy: %s, %d \n", person_copy.name, person_copy.age );
  return 0;
}

memmove的模拟实现