字符串函数——strcpy 、strncpy 、strcat、 strncat、 strcmp 、strncmp、 memcpy、 memmove）

1.strcpy

#define  _CRT_SECURE_NO_WARNINGS 1#include <stdio.h>#include<windows.h>#include<assert.h>#include<string.h>char* my_strcpy(char *dest, const char *scr){ char *ret=dest;   assert(dest);   assert(scr); while(*dest++=*scr++) {  ; } return  ret; }int main(){ const char  str1[5]="abcd";  char str2[5]={0};printf("%s\n",my_strcpy(str2,str1))  ;  system("pause");  return 0;}

2.strncpy

#include <stdio.h>#include<windows.h>#include<assert.h>#include<string.h>char *my_strncpy( char *dest, const char *src, int len ){     char *ret=dest; assert(dest); assert(src); while(len&&*src) {        *dest++=*src++;  len--; } /*if(len!=0) {  while(len)  {  *dest++='\n';  len--;  } }*/   return dest;}int main(){   char string[50]="I am a student!";         const char str1[3]="he";          const char str2[3]="is";      const char str3[10]="teacher"; my_strncpy(string,str1,2); my_strncpy(string+2,str2,2); my_strncpy(string+7,str3,7);  printf("%s\n",string); system("pause");return 0;}

3.strcat

#include <stdio.h>#include<windows.h>#include<assert.h>#include<string.h>char *my_strcat( char *dest, const char *src ){    char *ret=dest; assert(dest); assert(src);    while(*dest) {  *dest++; }      while(*dest++=*src++)   {    ;   }   return ret;}int main(){    const char str1[]= "you are";       char str2[]=" a teacher";  printf("%s",my_strcat(str1,str2));   system("pause");   return 0;}

4.strncat

#include <stdio.h>#include<windows.h>#include<assert.h>#include<string.h>char *my_strncat(char *dest,const char *src,int count){ char *ret=dest; assert(dest); assert(src); while(*dest){  dest++; } while(count&&(*dest++=*src++)){  count--; } return ret;}int main(){ char str1[20]="hello"; my_strncat(str1," world",6); printf("%s\n",str1); system("pause"); return 0;}

5.strcmp

#include <stdio.h>#include<windows.h>#include<assert.h>#include<string.h>int My_strcmp( const char *str1, const char *str2){     assert(str1); assert(str2);    while(*str1!='\0'&& *str2!='\0') {      if(*str1==*str2)  {   str1++;           str2++; }  if(*str1>*str2)  {   return 1;  }  if(*str1<*str2)  {   return -1;  }   if(*str1=='\0'&&*str2!='\0')   {    return -1;   }   if(*str2=='\0'&&*str1!='\0')   {    return -1;   }   return 0; }}  int main(){   const char *str1="student!";   const char *str2="teacher!!";   if( (My_strcmp(str1, str2))==0)   {    printf("两个字符串相等\n");   }   if( (My_strcmp(str1, str2))==1)   {    printf("str1大于str2\n");}   if((My_strcmp(str1, str2))==-1)   {    printf("str1小于str2\n");   }system("pause");return 0;}  

6.strncmp

#include <stdio.h>#include<windows.h>#include<assert.h>#include<string.h>int My_strncmp( const char *str1, const char *str2, int len ){  assert(str1);  assert(str2);   while(len--)   {     while(*str1!='\0'&& *str2!='\0')  {     if(*str1==*str2)   {    str1++;            str2++;  }   if(*str1>*str2)   {    return 1;   }   if(*str1<*str2)   {    return -1;   }    if(*str1=='\0'&&*str2!='\0')    {     return -1;    }    if(*str2=='\0'&&*str1!='\0')    {     return -1;    }    return 0;  }}}int main(){ const char *str1="student!!";   const char *str2="teacher!!";   if( (My_strncmp(str1, str2,2))==0)       {        printf("两个字符串相等\n");       }           if( (My_strncmp(str1, str2,2))==1)       {        printf("str1大于str2\n");}       if((My_strncmp(str1, str2,2))==-1)       {        printf("str1小于str2\n");       }system("pause");return 0;}

7、memcpy

#include <stdio.h>#include<windows.h>#include<assert.h>#include<string.h>void *my_memcpy( void *dest, const void *src, size_t count ){  char *ret=dest;        while(count--)  {  assert(dest);       *(char*)dest = *(char *)src;    dest = (char*)dest + 1;       src = (char *)src + 1;   }    return dest;}int main(){ const char *str1="student!!"; char str2[]="abcdefg"; int num=0; printf("befor:%s\n",str1); printf("please enter:"); scanf("%d",&num);printf("%s\n",my_memcpy( str2, str1, num ));system("pause");return 0;}

8.memmove

#include <stdio.h>#include<windows.h>#include<assert.h>#include<string.h>void *my_memmove( void *dest, const void *src, size_t count ){      assert(src);   if (dest <= src || (char*)dest >= ((char*)src + count))       {             while (count--)              {                    assert(dest);                   *(char*)dest = *(char *)src;                   dest = (char*)dest + 1;                   src = (char *)src + 1;                      }       }  else   {    (char*)dest = (char*)dest + count - 1;    (char *)src = (char *)src + count - 1;    while (count--)    {     *(char*)dest = *(char *)src;     dest = (char*)dest - 1;     src = (char *)src - 1;    } }  }   int main()  {      char *src = "AABCDE";      char dest[] = "AAAAABBBB";     printf("befor:   %s\n",dest);     my_memmove(dest, src, strlen(src) + 1);    printf("enter:    %s", dest);       system("pause");       return 0;  }