Exercise 5.9 - program which has day of the year and month day using pointers

Question

Rewrite the routines day_of_year and month_day with pointers instead of indexing.

#include<stdio.h>

static char daytab[2][13]={
    {0,31,28,31,30,31,30,31,31,30,31,30,31},
    {0,31,29,31,30,31,30,31,31,30,31,30,31}
};

int day_of_year(int year,int month,int day);
void month_day(int year,int yearday,int *pmonth,int *pday);

int main(void)
{
    int day,mon,dat;

    day=day_of_year(1981,10,2);
    
    printf("%d\n",day);

    month_day(1981,81,&mon,&dat);

    printf("%d,%d",mon,dat);
    
    return 0;
}

/* day_of_year: set day of year from month and day */

int day_of_year(int year,int month,int day)
{
    int i,leap;
    
    leap=year%4==0 && year%100 !=0 || year %400 == 0;

    if(month < 1 || month > 12)
        return -1;
    if(day < 1 || day > daytab[leap][month])
        return -1;

    for(i=1;i<month;i++)
        day += daytab[leap][i];

    return day;
}

/* month_day: set month,day from day of the year */

void month_day(int year,int yearday,int *pmonth,int *pday)
{
    int i,leap;
    
    if(year < 1)
    {
        *pmonth = -1;
        *pday = -1;
        return;
    }

    leap = year % 4 == 0 && year % 100 != 0 || year % 400 == 0;

    for(i=1;i<=12 && yearday > daytab[leap][i];i++)
        yearday -= daytab[leap][i];

    if(i > 12 && yearday > daytab[leap][12])
    {
        *pmonth=-1;
        *pday=-1;
    }
    else
    {
        *pmonth=i;
        *pday=yearday;
    }
}

Run this

Explanation

This program is same as the previous program Exercise 5.8 and we calculate the day of the year as before and return it to be printed in the main function.

In the month_day, we send two additional pointers int *pmonth,int *pday and after calculating the number of months and days, we return it using the pointers itself to the main function instead of printing them in the function.

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