C++字符串處理函數

#include<iostream>  
#include<stdlib.h> 
#include<string>
#include <assert.h>
using namespace std;


//鏈接字符串                被copy的要用const
char *my_strcat(char *d, const char *t)
{
    char *address = d;
    while (*d != '\0')  //將原字符串指針移到最後一個字符
        d++;
    while ((*d++ = *t++) != '\0')   //開始複製
        NULL;
    return address;  //返回指針
}

//字符串的複製          一樣被複制的爲了安全  要用const
char *my_strcpy(char *d,const char *t)
{
    char *str = d;
    while (*t != '\0')
    {
        *d = *t;
        *d++,*t++;
    }
         //複製到結束爲 \0
    return str;
}

//從第n個字符開始複製
char *my_strncpy(char *dest, char *t, int n)
{
    char *str = dest;      //用指針str來操做
    while (n&&*t != '\0') {
        *dest = *t;
        ++*dest, ++*t;
        n--;
    }
    return str;
}

//小寫轉大寫
char *my_strup(char *dest)
{
    char *str = dest;
    while (*dest != '\0')
    {                    //判斷是否輸的確實是小寫 
        if (*dest >= 'a' && *dest <= 'z')
            *dest -= 32;
        dest++;
    }
    return str;
}

//大寫轉小寫
char *my_strlwr(char *dest)
{
    char *str = dest;
    while (*dest != '\0')
    {           //判斷是否輸的爲大寫
        if (*dest >= 'A' && *dest <= 'Z')
            *dest += 32;
        dest++;
    }
    return str;
}

//比較字符比較大小     被比較的字符串都要用使用const
int my_strcmp(const char *s1, const char * s2)  
{ 
    while (*s1&&*s2&&*s1 == *s2)
    {
        s1++;
        s2++;
    }
    return (*s1 - *s2);
    //若是相等 返回0  大於 返回正  小於 返回負
}




int main()
{
    char a[100],b[100];
    cin >>a>> b;
    char *p1 = a, *p2 = b;

    //鏈接字符串
    //cin >> a >> b;
    //my_strcat(p1, p2);

    //把b複製到a
    //my_strcpy(p1, p2);
    
    //從第n開始複製
    //my_strncpy(p1, p2, 2);

    //cin >> b;
    //cout << my_strcmp(p1, p2);

    //小寫轉大寫 大寫轉小寫
    //cin >> a;
    //my_strup(p1);
    //cout << a<<endl;
    //my_strlwr(p1);
    //cout << a << endl;

    cout<<my_strcmp(a, b)<<endl;

    system("pause");
    return 0;
}