C语言习题~day22

//计数器方式
int my_strlen(const char * str)
{
 int count = 0;
 while(*str)
 {
  count++;
  str++;
 }
 return count;
}

//不能创建临时变量计数器
int my_strlen(const char * str)
{
 if(*str == '\0')
  return 0;
 else
  return 1+my_strlen(str+1);
}

//指针-指针的方式
int my_strlen(const char *s)
{
   char *p = s;
   while(*p != ‘\0’ )
       p++;
   return p-s;
}

//1.参数顺序
//2.函数的功能，停止条件
//3.assert
//4.const修饰指针
//5.函数返回值
//6.题目出自《高质量C/C++编程》书籍最后的试题部分
char* my_strcpy(char* dest, const char* src)
{ 
 char* ret = dest;
 assert(dest != NULL);
 assert(src != NULL);
 
 while((*dest++ = *src++))
 {
  ;
 }
 return ret;
}

char* my_strcat(char* dest, const char* src)
{
 char* ret = dest;
 assert(dest != NULL);
 assert(src != NULL);
 while(*dest)
 {
  dest++;
 }
 while((*dest++ = *src++))
 {
  ;
 }
 return ret;
}

int my_strcmp (const char * src, const char * dst)
{
  int ret = 0 ;
 assert(src != NULL);
  assert(dest != NULL);
  while( ! (ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst)
    ++src, ++dst;
​
  if ( ret < 0 )
    ret = -1 ;
  else if ( ret > 0 )
    ret = 1 ;
​
  return ret ;
}

char *  strstr (const char* str1, const char* str2)
{
        char* cp = (char*) str1;
        char* s1, *s2;
        if ( !*str2 )
            return((char*)str1);
        while (*cp)
        {
                s1 = cp;
                s2 = (char*) str2;
                while ( *s1 && *s2 && !(*s1-*s2) )
                        s1++, s2++;
                if (!*s2)
                        return(cp);
                cp++;
        }
        return(NULL);
}

char * mystrncpy(char * dst, const char * src, size_t n)
{
  int i;
  for (i = 0; src[i] && i < n; i++)
  {
    dst[i] = src[i];
  }
   
  if (i < n)
  {
    dst[i] = 0;
  }
  return dst;
}

char * mystrncat(char * dst, const char * src, size_t n)
{
  char * tmp = dst;
   
  while (*dst)
  {
    dst++;
  }
   
  int i;
  for (i = 0; src[i] && i < n; i++)
  {
    dst[i] = src[i];
  }
   
  dst[i] = 0;
  return tmp;
}

void * memcpy ( void * dst, const void * src, size_t count)
{
  void * ret = dst;
    assert(dst);
  assert(src);
  /*
   * copy from lower addresses to higher addresses
   */
  while (count--) {
    *(char *)dst = *(char *)src;
    dst = (char *)dst + 1;
    src = (char *)src + 1;
  }
 
  return(ret);
}

void * memmove ( void * dst, const void * src, size_t count)
{
  void * ret = dst;
​
  if (dst <= src || (char *)dst >= ((char *)src + count)) {
    /*
    * Non-Overlapping Buffers
    * copy from lower addresses to higher addresses
    */
    while (count--) {
        *(char *)dst = *(char *)src;
        dst = (char *)dst + 1;
        src = (char *)src + 1;
     }
   }
  else {
    /*
    * Overlapping Buffers
    * copy from higher addresses to lower addresses
    */
    dst = (char *)dst + count - 1;
    src = (char *)src + count - 1;
​
    while (count--) {
        *(char *)dst = *(char *)src;
        dst = (char *)dst - 1;
        src = (char *)src - 1;
     }
   }
  return(ret);
}