字符串处理函数的实现

1. strlen函数

//返回字符串的长度
int my_strlen(const char *str)
{
    int i = 0;
    for(;*str++ != '\0';i++);
    return i;
}

2. strcpy函数

//strSrc指向的字符串拷贝到strDest
char * strcpy( char *strDest, const char *strSrc )
{
    assert((strDest != NULL) && (strSrc != NULL));
    char *address = strDest;
    while((*strDest++ = *strSrc++) != '\0');
    return address;
}

3.字符串循环右移函数

//strSrc指向的字符串循环右移n个字符后保存到strDest中并返回
char * circulate_right_move(char *strDest,const char *strSrc,const int n)
{
    assert( (strDest != NULL) && (strSrc != NULL));

    char * address = strDest;
    int len = strlen(strSrc);
    int offset = n % len;

    strSrc += (len - offset);

    for(;(*strDest = *strSrc) != '\0';strDest++,strSrc++);

    strSrc -= len;

    for(int i = 0; i < len - offset; i++)
    {
        *strDest = *strSrc;
        strDest++;
        strSrc++;
    }

    *strDest = '\0';

    return address;
}

4.字符串逆序函数

//strSrc指向的字符串逆序保存到strDest并返回
char * str_reverse( char *strDest, const char *strSrc )
{
    assert((strDest != NULL) && (strSrc != NULL));

    char *address = strDest;

    int n = strlen(strSrc);
    strSrc += (n-1);

    for(; n > 0; n--)
    {
        *strDest = *strSrc;
        strDest++;
        strSrc--;
    }

    *strDest = '\0';

    return address;
}

5.strcat函数

//strSrc字符串连接到strDest后面并返回
char * strcat( char *strDest, const char *strSrc )
{
    assert((strDest != NULL) && (strSrc != NULL));

    char *address = strDest;

    int lenDest = strlen(strDest);

    strDest += lenDest;

    while( (*strDest++ = *strSrc++) != '\0');

    *strDest = '\0';

    return address;
}

6.strtok函数

char *strtok_r(char *s, const char *delim, char **save_ptr)
{
    char *token;

    if (s == NULL)
        s = *save_ptr;

    s += strspn(s, delim);
    if (*s == '\0')
        return NULL;

    token = s;
    s = strpbrk(token, delim);

    if (s == NULL)
        *save_ptr = strchr(token, '\0');
    else {
        *s = '\0';
        *save_ptr = s + 1;
    }

    return token;
}

char *strtok(char *s, const char *delim)
{
    static char *last;

    return strtok_r(s, delim, &last);
}

7.strspn函数

//返回字符串str1中第一个不在指定字符串str2中出现的字符下标
/*
例：
char *str1 = "a b c d e f";
char *str2 = "a b c"; or char *str2 = "a b c ";
则
strspn(str1,str2) = 6
因为
str1[6] = 'd'
*/

size_t strspn(const char *str1, const char *str2)
{
    const char *p_str1;
    const char *a_str2;

    size_t count = 0;

    for (p_str1 = str1; *p_str1 != '\0'; p_str1++)
    {
        for (a_str2 = str2; *a_str2 != '\0'; a_str2++)
        {
            if (*p_str1 == *a_str2)
                break;
        }
        if (*a_str2 == '\0')
            return count;
        count++;
    }

    return count;
}