常见字符串函数的实现

strlen(),strcpy()

int my_strlen(const char *str)//字符串长度{    assert(str != NULL);           int len = 0;       while (*str ++ != '\0')    {        ++ len;    }     return len;   }char *my_strcpy(char *di,const char *si)//字符串的拷贝{    assert((di != NULL) && (si != NULL));    char * cp = di;    while(*si)    {        *di++ = *si++;    }    *di= '\0';    return cp;       }void main(){    char str[100]="";    char *dtr="hello word";    int n=my_strlen(dtr);    char *p=my_strcpy(str,dtr);    printf("字符串长度：%d\n",n);    printf("字符串拷贝后：%s\n",p);}

strncpy(),strcat(),strncat()

char *my_strncpy(char *di, const char *si, int n)//字符串的长度受限拷贝{    assert((di != NULL) && (si != NULL));    char *ncpy = di;    while(n--)    {        if(*si != '\0')        {            *di++ = *si++;        }        else        {            *di++ = '\0';        }    }    return ncpy;       }char *my_strcat(char *di, const char *si)//两个字符串的连接 {       assert((di != NULL) && (si != NULL));       char *link = di;       while (*di != '\0')    {        ++ di;    }            while ((*di ++ = *si ++) != '\0')    {    }    return link;   }char *my_strncat(char *di, const char *si, int n) // 两个字符串的受限连接{       assert((di != NULL) && (si != NULL));       char *nlink = di;       while (*di != '\0')    {        ++ di;     }      while (n-- && *si != '\0' )    {          *di ++ = *si ++;    }       *di = '\0';       return nlink;   } void main(){    char str[100]="";    char ctr[100]="ff";    char ftr[100]="ff";    char *dtr="hello word";    printf("字符串长度受限拷贝后：%s\n",my_strncpy(str,dtr,5));    printf("字符串连接后：%s\n",my_strcat(ctr,dtr));    printf("字符串受限连接后：%s\n",my_strncat(ftr,dtr,7));}

strchr(),strrchr(),strnchr()

char *my_strchr( char *di, char c)// 查找字符串中首次出现字符c的位置，找到返回指针，找不到返回NULL {       assert(di != NULL);       for (; *di !=c; ++ di)    {          if (*di == '\0')         {            return NULL;        }       }    return di;   }char *my_strrchr( char *si, char c)//查找字符串中最后一次次出现字符c的位置，找到返回指针，找不到返回NULL {    assert(si != NULL);    char *p = NULL;    while(*si != '\0')    {        if(*si == (char)c)        {            p = (char *)si;        }        si++;    }    return p;}char *my_strnchr( const char *di,char c,int n)//字符串查找给定字符，第 n 次出现的位置{    assert(di != NULL);     int count=0;    for (; *di !='\0'; ++ di)    {          if (*di==c)        {            count+=1;        }        if(count==n)        {        return (char *)di;        }    }    return NULL;}void main(){    char *str="hello a waord";    char *ctr="hello a waard";    printf("前查找字符后：%s\n",my_strchr(str,'a'));    printf("后查找字符后：%s\n",my_strrchr(str,'a'));    printf("查找第n次字符出现后：%s\n",my_strnchr(ctr,'a',3));}

strstr()

char *my_strstr(const char *di ,const char *si)//查找字符串（子串）   {       assert(di != NULL && si != NULL);       const char *s = di;       const char *t = si;       for (; *di != '\0'; ++ di)       {           for (s = di, t = si; *t != '\0' && *s == *t; ++s, ++t)        {               NULL;        }           if (*t == '\0')        {               return (char *) di;        }       }       return NULL;} void main(){    char *str="hello word";    char *ctr="word";    printf("%s\n",my_strstr(str,ctr));}

strcmp(),strncmp()

int my_strcmp(const char *di, const char *si) //字符串比较，返回负，零，正数  {       assert(di != NULL && si != NULL);       while(*di && *si && *di == *si)       {           ++ di;           ++ si;       }    return (*di - *si);   }int my_strncmp(const char *di, const char *si, int n)//受限字符串的比较  {       assert((di != NULL) && (si != NULL));       while (*di && *si && *di == *si && n --)       {           ++ di;           ++ si;       }       return(*di - *si);   }void main(){    char *str="hellobword";    char *ctr="helloaword";    printf("%d\n",my_strcmp(str,ctr));    printf("%d\n",my_strncmp(str,ctr,4));}

count_chars()

int count_chars(char const *di,char const *si)//在第一个参数中进行查找， 并返回匹配第二个参数所包含的字符的数量 {      assert((di != NULL) && (si != NULL));    int count = 0;      for (int i = 0;i < strlen(di);i++ )      {          for(int j = 0;j < strlen(si);j++)          {              if( di[i] == si[j])              {                  count++;              }          }      }      return count;  } void main(){    char *str="abcdefg";    char *ctr="acdfyuilo";    printf("相同字符数：%d\n",count_chars(str,ctr));}