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));
}