模拟实现strncpy,strncat,strncmp

1.模拟实现strncpy 
<1.>strncpy相比于strcpy增加了size_t参数可以实现最多拷贝的字节数
<2.>（size_t不可以超出拷贝存放的内存大小）来保证不会超出destanaton的内存，
<3.>但是需要注意的是，如果你需要拷贝的长度小于源字符串长度，那么strncpy不会在末尾加'\0'

 #include<stdio.h>
 #include<assert.h>
 #include<string.h>
 char* Strncpy(char* destination, const char* source, size_t num)
 {
  size_t i = ;
  while (i<num)
  {
   destination[i] = source[i];
   ++i;
  }
  return destination;
 }
 int main()
 {
  char str1[] = "To be or not to be";
  char str2[];
  char str3[];
  //拷贝最大的字节
  Strncpy(str2, str1, sizeof(str2));
  //拷贝小于str1的字节
  Strncpy(str3, str2, );
  str3[] = '\0'; //需要自己加如'\0'
  puts(str1);
  puts(str2);
  puts(str3);
  return ;
 }

2.模拟实现strncat
<1.>现在destination函数中找到'\0'，然后将source函数中的num个字符拼接到destination中
<2.>注意source函数不能修改，需要加const
<3.>返回结果是指针类型

 #include<stdio.h>
 #include<assert.h>
 char* Strncat(char* destination, const char* source, size_t num)
 {
  //检查合法性
  assert(destination != NULL);
  assert(source != NULL);
  //找到destination中的'\0'
  int i = ;
  while (destination[i]!='\0')
  {
   i++;
  }
  //拼接字符串
  for (int j = ; num > ; i++, j++, num--)
  {
   destination[i] = source[j];
  }
  destination[i] = '\0';
  return destination;
 }
 int main()
 {
  char str1[] = "abcd";
  char str2[] = "efghijklmn";
  //拼接前4个字符
  Strncat(str1,str2,);
  puts(str1);
  printf("\n");
  //拼接全部字符,(因为上一个Strncat已经改变了str1的值)
  Strncat(str1, str2, sizeof(str2));
  puts(str1);
  return ;
 }

3.模拟实现strncmp
<1.>strncmp返回值与strcmp一样
<2.>如果str1与str2前num个字符一样则返回0
<3.>str1小于str2则返回一个小于0的数
<4.>str1大于str2则返回一个大于0的数

 #include <stdio.h>
 #include<assert.h>
 #include<string.h>
 int Strncmp(const char* str1, const char* str2, size_t num)
 {
  assert(str1 != NULL);
  assert(str2 != NULL);
  while (num>)
  {
   if (*str1 > *str2)
   {
    return ;
   }
   else if (*str1 < *str2)
   {
    return -;
   }
   else if (*str1 == *str2)
   {
    str1++;
    str2++;
    num--;
   }
   else
   {
    str1++;
    str2++;
    num--;
   }
  }
  return ;
 }
 int main()
 {
  char str1[] = "abcdef";
  char str2[] = "abty";
  //模拟实现Strncmp
  printf("%d\n", Strncmp(str1, str2, ));//前两个字符相等
  printf("%d\n", Strncmp(str1, str2, ));//前三个字符不相等
  printf("%d\n", Strncmp("abcde", "abc", ));//str1大于str2
  printf("%d\n", Strncmp("abc", "abcde", ));//str1小于str2
  printf("\n");
  //库函数strncmp
  printf("%d\n", strncmp(str1, str2, ));
  printf("%d\n", strncmp(str1, str2, ));
  printf("%d\n", strncmp("abcde", "abc", ));
  printf("%d\n", strncmp("abc", "abcde", ));
  return ;
 }