Chapter 5 - Pointers and Arrays 9

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

Solutions by Lars Wirzenius and Gregory Pietsch

 Here's Lars's solution:

/*
 * A solution to exercise 5-9 in K&R2, page 114:
 *
 * Rewrite the routines day_of_year and month_day with pointers
 * instead of indexing.
 *
 * Lars Wirzenius <liw@iki.fi>
 */

#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},
};

/* original versions, for comparison purposes */

int day_of_year(int year, int month, int day)
{
 int i, leap;
 
 leap = (year%4 == 0 && year%100 != 0) || year%400 == 0;
 for (i = 1; i < month; i++)
  day += daytab[leap][i];
 return day;
}

void month_day(int year, int yearday, int *pmonth, int *pday)
{
 int i, leap;
 
 leap = (year%4 == 0 && year%100 != 0) || year%400 == 0;
 for (i = 1; yearday > daytab[leap][i]; i++)
  yearday -= daytab[leap][i];
 *pmonth = i;
 *pday = yearday;
}

/* pointer versions */

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

 /* Set `p' to point at first month in the correct row. */
 p = &daytab[leap][1];

 /* Move `p' along the row, to each successive month. */
 for (i = 1; i < month; i++) {
  day += *p;
  ++p;
 }
 return day;
}

void month_day_pointer(int year, int yearday, int *pmonth, int *pday)
{
 int i, leap;
 char *p;
 
 leap = (year%4 == 0 && year%100 != 0) || year%400 == 0;
 p = &daytab[leap][1];
 for (i = 1; yearday > *p; i++) {
  yearday -= *p;
  ++p;
 }
 *pmonth = i;
 *pday = yearday;
}

int main(void)
{
 int year, month, day, yearday;
 
 year = 2000;
 month = 3;
 day = 1;
 printf("The date is: %d-%02d-%02d\n", year, month, day);
 printf("day_of_year: %d\n", day_of_year(year, month, day));
 printf("day_of_year_pointer: %d\n",
  day_of_year_pointer(year, month, day));

 yearday = 61; /* 2000-03-01 */
 month_day(year, yearday, &month, &day);
 printf("Yearday is %d\n", yearday);
 printf("month_day: %d %d\n", month, day);
 month_day_pointer(year, yearday, &month, &day);
 printf("month_day_pointer: %d %d\n", month, day);
 
 return 0;
}

 

 

 And here's Greg's:

/* Gregory Pietsch */

/* Given the problem, I thought that this would be a better
 * description of daytab.
 */
static int daytab[] = {
    0,
    31,
    31+28,
    31+28+31,
    31+28+31+30,
    31+28+31+30+31,
    31+28+31+30+31+30,
    31+28+31+30+31+30+31,
    31+28+31+30+31+30+31+31,
    31+28+31+30+31+30+31+31+30,
    31+28+31+30+31+30+31+31+30+31,
    31+28+31+30+31+30+31+31+30+31+30,
    0,
    31,
    31+29,
    31+29+31,
    31+29+31+30,
    31+29+31+30+31,
    31+29+31+30+31+30,
    31+29+31+30+31+30+31,
    31+29+31+30+31+30+31+31,
    31+29+31+30+31+30+31+31+30,
    31+29+31+30+31+30+31+31+30+31,
    31+29+31+30+31+30+31+31+30+31+30,
};

/* is it a leap year?  (assume it's my calendar, the Gregorian) */
int leap(int year)
{
    return ((year % 4) == 0)
            && (((year % 100) != 0)
                || (year % 400) == 0)));
}

/* day_of_year:  set day of year from month & day */
int day_of_year(int year, int month, int day)
{
    return *(daytab + ((month - 1) + (leap(year) * 12))) + day;
}

/* month_day: set month, day from day of year */
void month_day(int year, int yearday, int *pmonth, int *pday)
{
    int m, ly;

    ly = leap(year);
    if (yearday < 1 || yearday > (365 + ly))
        return; /* no real error checking */
    m = leap(year) ? 23 : 11;
    while (*(daytab + m) > yearday)
        m--;
    if (pmonth)
        *pmonth = (m % 12) + 1;
    if (pday)
        *pday = yearday - (*(daytab + m));
}

 

 

menonsahab's
/* My approach was to create an array of pointers, each element of which
points to an array of integers not necessarily all of the same length */

#include <stdio.h>

int Zer[] = {0};
int Jan[] = {31};
int Feb[] = {28, 29};
int Mar[] = {31};
int Apr[] = {30};
int May[] = {31};
int Jun[] = {30};
int Jul[] = {31};
int Aug[] = {31};
int Sep[] = {30};
int Oct[] = {31};
int Nov[] = {30};
int Dec[] = {31};

int *daytab[] = { Zer, Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec};

int day_of_year(int year, int month, int day)
{
 int i, leap;
 leap = (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
 for(i = 1; i < month; i++)
  day += *( *(daytab + i) + ((leap && i == 2) ? 1 : 0));
 return day;
}

void month_day(int year, int yearday, int *pmonth, int *pday)
{
 int i, leap;
 leap = (year % 400 == 0) || (year % 4 == 0 && year % 100 != 0);
 for(i = 1; yearday > *( *(daytab + i) + ((leap && i == 2) ? 1 : 0)); i++)
  yearday -= *( *(daytab + i) + ((leap && i == 2) ? 1 : 0)); // The cryptic expression in the RHS is nothing but daytab[i][(leap && i == 2) ? 1 : 0]
 *pmonth = i;
 *pday = yearday;
}

int main()
{
 int year, month, day, yearday, retval;

 year = 2017;
 month = 11;
 day = 22;
 yearday = 326;

 retval = day_of_year(year, month, day);
 printf("%d %d %d <---> %d %d\n", year, month, day, year, retval);

 month_day(year, yearday, &month, &day);
 printf("%d %d <---> %d %d %d\n", year, yearday, year, month, day);

 return 0;
}

/*
Program Output:
2017 11 22 <---> 2017 326
2017 326 <---> 2017 11 22
*/