digital clock based C

/**********************************
* Name   : timeDisplay.cpp
* Purpose: Display digital clock according to current system time.
* Author : feicaixian
***********************************/

#include <stdio.h> 
#include <time.h>
#include <stdlib.h>
void segDisp(int n);
void numToArr(int n, int *a);

/***********************
* 7段码笔画编号，编号可以随意，不同编号方案对应不同7段码
*   _0
* 5|_6| 1
* 4|_3| 2
*
***********************/
//将0~9的7段码存入数组，0表示不显示笔画，1表示显示笔画
const int segments[10][7] = {
    {1, 1, 1, 1, 1, 1, 0},//0
    {0, 1, 1, 0, 0, 0, 0},//1
    {1, 1, 0, 1, 1, 0, 1},//2
    {1, 1, 1, 1, 0, 0, 1},//3
    {0, 1, 1, 0, 0, 1, 1},//4
    {1, 0, 1, 1, 0, 1, 1},//5
    {1, 0, 1, 1, 1, 1, 1},//6
    {1, 1, 1, 0, 0, 0, 0},//7
    {1, 1, 1, 1, 1, 1, 1},//8
    {1, 1, 1, 1, 0, 1, 1} //9
};

int main()
{
    /*
    Use time() to get system time.The function prototype is:
    #include <time.h>
    time_t time(time_t *);
    
    function type is time_t, function name is time, parameter is pointer, whose type is time_t.
    time(time_t *timer) returns 
    the number of seconds 
    between January 1, 1970 and the current system time 
    to timer, which can be print as long int.
    */
    time_t t;//define a time_t type variable

    time(&t);//Pass time_t type pointer to time()

    printf("The number of seconds: %d\n",t);//print the seconds from January 1, 1970 to now.

    /*Use functions followed to convert seconds to a structure:
    struct tm *gmtime(const time_t *timer); //convert seconds to Greenwich Mean Time(GMT)
    struct tm *localtime(const time_t * timer); //convert seconds to localtime
    
    struct tm * means that the type of return value is a struct tm type pointer. 

    struct tm   
    {   
        int tm_sec;//seconds 0-61    
        int tm_min;//minutes 1-59    
        int tm_hour;//hours 0-23    
        int tm_mday;//day of the month 1-31    
        int tm_mon;//months since jan 0-11, so add 1 to get the real months.    
        int tm_year;//years from 1900, add 1900 to get the real years.    
        int tm_wday;//days since Sunday, 0-6    
        int tm_yday;//days since Jan 1, 0-365    
        int tm_isdst;//Daylight Saving time indicator    
    };  

    */
    struct tm *p = localtime(&t);//不一定要把当前系统时间传给指针，你也可以自定义各成员的值。
    int oldSeconds;

    
    
    while(1) {
        system("cls");//clear screen.

        /* 
        方法一：
        char *asctime(const struct tm *timeptr) 返回一个表示当地时间的字符串，它代表了timeptr的日期和时间。
        */
        printf("%s\n\n",asctime(p));//输出结果像这样：Sat Mar 25 06:10:10 1989

        /*
        方法二：
        char *ctime(const time_t *timer) 返回一个表示当地时间的字符串，当地时间是基于参数 timer。
        */
        printf("%s\n\n",ctime(&t));//输出结果和asctime一样

        
        /* 
        方法三：
        也可以自己输出日期和时间，自定义格式
        */
        int a[6];
        int timeToInt = p->tm_hour * 10000 + p->tm_min * 100 + p->tm_sec;
        printf("%d/%d/%d ",1900+p->tm_year,p->tm_mon + 1, p->tm_mday);//date
        numToArr(timeToInt, a);
        printf("%d%d:%d%d:%d%d\n",a[5],a[4],a[3],a[2],a[1],a[0]);
        
        //使用7段码显示时间
        
        segDisp(timeToInt);

        //不断获取当前时间，直到秒钟刷新，跳出循环，更新显示
        oldSeconds = p->tm_sec;
        do {
            time(&t);
            p = localtime(&t);
        }while (p->tm_sec == oldSeconds);
    }
    return 0;
}

void numToArr(int n, int *a)
{
    int k = 0;        //数组a[]的下标

    do {
        a[k++] = n % 10;
        n /= 10;
    }while (n != 0);    
}

void segDisp(int n)
{
    int i;            //打印7段码时形参n的各位的序号
    int k = 0;        //数组a[]的下标
    int a[6];        //存储形参的各个位的数字    
    const int num = 6;//时间显示的所需数字个数

    //将形参n的各位存入数组a[]
    numToArr(n, a);

    //-------------------------------------
    //--- 按行显示数字的7段码，共3行。
    //--- 第一行显示编号0，第二行显示编号5 6 1，第三行显示编号4 3 2.
    //-------------------------------------

    //显示编号0
    for (i = num-1; i >= 0; i--) {
        if (i == 4 | i == 2){        
        //在第二个数字和第四个数字后面要显示冒号，如12:34:43。由于使用中文下的·号，采用全角模式，占两个字符宽度，
        //所以额外增加两个空格。
            if (segments[a[i]][0])
                printf(" _    ");
            else 
                printf("      ");
        }
        else {
            if (segments[a[i]][0])
                printf(" _  ");
                //附加两个空格，显示下一个数字的笔画。
                //第一个空格是因为本数字右边还有一个笔画，第二个空格是因为两个数字之间应有间隔。
            else 
                printf("    ");//用空格代替下划线
        }
    }
    printf("\n");//在下一行打印
    
    //显示编号5 6 1
    for (i = num-1; i >= 0; i--) {
        if (segments[a[i]][5])
            printf("|");
        else 
            printf(" ");
        if (segments[a[i]][6])
            printf("_");
        else 
            printf(" ");
        if (segments[a[i]][1])
            printf("| ");
        else 
            printf("  ");
        if (i == 4 | i == 2)
            printf("·");//显示冒号
    }
    printf("\n");//在下一行打印

    //显示编号4 3 2
    for (i = num-1; i >= 0; i--) {
        if (segments[a[i]][4])
            printf("|");
        else 
            printf(" ");
        if (segments[a[i]][3])
            printf("_");
        else 
            printf(" ");
        if (segments[a[i]][2])
            printf("| ");
        else 
            printf("  ");
        if (i == 4 | i == 2)
            printf("·");
    }
}