c++字符串的清零，赋值比较

#include <iostream>
using namespace std;
int main()
{
    char a[] = { 1,2,3,4,5,6,7,8,9,'\0'};
    char b[] = { 9,8,7,6,5,4,3,2,1,'\0'};
    cout << "把数组清零" << endl;
    memset(a, 0, sizeof(a));//把数组清零 
    for (int i= 0; i < sizeof(a)/sizeof(a[0]); i++)
    {
        cout << (int)a[i]<<" " << endl;
    }

    cout << "字符串数组的复制" << endl;
    //字符串数组的复制
    strcpy_s(a, sizeof(a) / sizeof(a[0]), b);
    for (int i = 0; i < sizeof(a) / sizeof(a[0]); i++)
    {
        cout << (int)a[i] << " " << endl;
    }
    cout << "把字符串前n个数复制到目标数组" << endl;
    //把字符串前n个数复制到目标数组
    strncpy_s(a, sizeof(a) / sizeof(a[0]), b,7);
    for (int i = 0; i < sizeof(a) / sizeof(a[0]); i++)
    {
        cout << (int)a[i] << " " << endl;
    }
    cout << "获得字符串的有效长度，返回不含0的长度。" << endl;
    //获得字符串的有效长度，返回不含0的长度。
    char c[20] = "a我";
    cout<< strlen(c)<<endl;
    //结果为3
    cout << "字符串的拼接" << endl;
    //字符串的拼接
    char d[20] = {0};
    strcat_s(d,sizeof(d)/sizeof(d[0]), a);
    strncat_s(d, sizeof(d) / sizeof(d[0]), b,5);//复制前n个
    for (int i = 0; i < sizeof(d) / sizeof(d[0]); i++)
    {
        cout << (int)d[i] << " " << endl;
    }
    //字符串的比较
    cout << "字符串的比较" << endl;
    cout << strcmp(a, b) << endl;//a<b返回-1
    cout << strncmp(a, b, 7) << endl;//a=b返回0
    cout << strncmp(d, b, 10) << endl;//d>b返回1
}

结果为：
把数组清零
0
0
0
0
0
0
0
0
0
0
字符串数组的复制
9
8
7
6
5
4
3
2
1
0
把字符串前n个数复制到目标数组
9
8
7
6
5
4
3
0
-2
-2
获得字符串的有效长度，返回不含0的长度。
3
字符串的拼接
9
8
7
6
5
4
3
9
8
7
6
5
0
-2
-2
-2
-2
-2
-2
-2
字符串的比较
-1
0
1