字符串函数的使用及其模拟实现

#include <stdio.h>
#include <string.h>
int main()
{
  char arr[] = "abcdefg";
  int len = strlen(arr);//strlen计算的是'\0'之前的字符个数，即a b c d e f g,共七个字符
  printf("%d ", len);
  return 0;
}

#include <stdio.h>
#include <assert.h>
int my_strlen(const char* arr)//原字符串中的内容不需要改
{
  assert(arr != NULL);//断言
  int count = 0;//计数器
  while (*arr)//arr中的元素不为'\0',进入循环
  {
    arr++;//跳到下一个元素
    count++;//计数器+1
  }
  return count;
}
int main()
{
  char arr[] = "abcdefg";
  int ret=my_strlen(arr);
  printf("%d ", ret);
  return 0;
}

#include <stdio.h>
#include <assert.h>
int my_strlen( const char* arr)
{
  assert(arr != NULL);
  char* start = arr;//start指向字符串arr的地址，也就是首字符的地址
  while (*arr)//arr中的字符为'\0'，停止循环，并且arr指向'\0'的地址
  {
    arr++;
  }
  return arr - start;//得到字符串的长度
}
int main()
{
  char arr[] = "abcdefg";
  int ret = my_strlen(arr);
  printf("%d ", ret); 
  return 0;
}

//my_strlen("abcdef")
//1+my_strlen("bcdef")
//1+1+my_strlen("cdef")
//1+1+1+my_strlen("def")
//1+1+1+1+my_strlen("ef")
//1+1+1+1+1+my_strlen("f")
//1+1+1+1+1+1+my_strlen("")
//1+1+1+1+1+1+0 = 6

#include <stdio.h>
#include <assert.h>
int my_strlen(const char* arr)
{
  assert(arr != NULL);
  if (*arr=='\0')
  {
    return 0;
  }
  return 1 + my_strlen(arr + 1);//递归
}
int main()
{
  char arr[] = "abcdef";
  int ret=my_strlen(arr);
  printf("%d ", ret);
  return 0;
}

#include <stdio.h>
int main()
{
  char a1[] = "abcdef";
  char a2[10] = "xx";
  strcpy(a2, a1);//将a1字符串中的内容拷贝到a2中去包括'\0'
  printf("%s\n", a2);
  return 0;
}

#include <stdio.h>
#include <assert.h>
void my_strcpy(char* a2, const char* a1)//a1中的内容并不希望被更改
{
  assert(a2 && a1);//断言
  while (*a1!='\0')//*a1='\0'结束循环
  {
    *a2 = *a1;//将a1中的内容拷贝到a2中
    a2++;
    a1++;
  }
  *a2 = *a1;//将a1中的'\0'拷贝到a2中
}
int main()
{
  char a1[] = "abcdef";
  char a2[20] = { 0 };
  my_strcpy(a2, a1);
  printf("%s\n", a2);
  return 0;
}

#include <stdio.h>
void my_strcpy( char* a2,const char* a1)
{
  while (*a2 = *a1)//a1中的字符一一拷贝到a2中去,也把'\0'拷贝到a2中去了，只不过这时循环结束
  {
    a2++;//遍历a2
    a1++;//遍历a1
  }
}
int main()
{
  char a1[] = "abcd";
  char a2[10] = { 0 };
  my_strcpy(a2, a1);
  printf("%s\n", a2);
  return 0;
}

#include <stdio.h>
#include <assert.h>
char* my_strcpy(char* a2,const char* a1)
{
  char* start = a2;
  assert(a2 && a1);
  while (*a2++ = *a1++)
  {
    ;
  }
  return start;
}
int main()
{
  char a1[] = "abcdef";
  char a2[10] = { 0 };
  printf("%s\n", my_strcpy(a2, a1));//返回a2的地址，便于链式访问
  return 0;
}

#include <stdio.h>
int main()
{
  char a1[20] = "hello ";
  char a2[20] = "world ";
  strcat(a1,a2);//将a2字符串的内容追加到a1中
  printf("%s\n", a1);//打印新的a1
  return 0;
}

#include <stdio.h>
#include <assert.h>
void my_strcat(char* a1, const char* a2)
{
  assert(a1 && a2);
  while (*a1 != '\0')
  {
    a1++;
  }//此时a1='\0'
  while (*a2 != '\0')
  {
    *a1++ = *a2++;//第一次循环将a1中的'\0'换成a2字符串中的首字符
  }
  *a1 = *a2;//将a2中的'\0'追加到a1中
}
int main()
{
  char a1[20] = "hello ";
  char a2[20] = "world";
  my_strcat(a1, a2);
  printf("%s\n", a1);
  return 0;
}

#include <stdio.h>
#include <assert.h>
char* my_strcat(char* a1, const char* a2)
{
  char* start = a1;//创建一个新的指针，便于储存a1首字符的地址
  assert(a1 && a2);
  while (*a1 != '\0')
  {
    a1++;
  }
  while (*a2 != '\0')
  {
    *a1++ = *a2++;
  }
  return start;//返回a1首字符的地址，便于链式访问
}
int main()
{
  char a1[20] = "hello ";
  char a2[20] = "world!!";
  printf("%s\n", my_strcat(a1, a2));//链式访问
  return 0;
}

#include <stdio.h>
int main()
{
  char a1[] = "abcdef";
  char a2[] = "axcb";
  int ret=strcmp(a1, a2);
  printf("%d ", ret);
  return 0;
}

#include <stdio.h>
int main()
{
  char a1[] = "abcdef";
  char a2[] = "aaxcb";
  int ret = strcmp(a1, a2);
  if (ret > 0)
    printf(">\n");
  else if (ret == 0)
    printf("=\n");
  else
    printf("<\n");
  return 0;
}

#include <stdio.h>
int my_strcmp(char* a1, char* a2)
{
  while (*a1 == *a2 )//当*a1=*a2时，往后遍历
  {
    if (*a1 == '\0')//此时*a1=*a2='\0'.a1=a2,返回0；
      return 0;//
    a1++;
    a2++;
  }
  if (*a1 > *a2)//当*a1!=*a2时，退出上面循环，进行判断
    return 1;
  else
    return -1;
}
int main()
{
  char a1[] = "abcdefgh";
  char a2[] = "abcdefg";
  int ret = my_strcmp(a1, a2);
  printf("%d ", ret);
  return 0;
}

#include <stdio.h>
#include <assert.h>
int my_strcmp(const char* a1,const char* a2)
{
  assert(a1 && a2);
  while (*a1 == *a2 )
  {
    if (*a1 == '\0')
      return 0;
    a1++;
    a2++;
  }
  return *a1 - *a2;
}
int main()
{
  char a1[] = "abcdefg";
  char a2[] = "abcdefg";
  int ret=my_strcmp(a1, a2);
  printf("%d ", ret);
  return 0;
}

include <stdio.h>
int my_strcmp(char* a1, char* a2)
{
  while (*a1 == *a2&&*a1!='\0')
  {
    a1++;
    a2++;
  }
  return *a1 - *a2;
}
int main()
{
  char a1[] = "abcdef";
  char a2[] = "abcdef";
  int ret = my_strcmp(a1, a2);
  printf("%d ", ret);
  return 0;
}