Figure 9.18  Functions That Convert Representations of Dates
/*
 *  Functions to change the representation of a date from a string containing
 *  day, month name and year to three integers (month day year) and vice versa
 */

#include <stdio.h>
#include <string.h>

#define STRSIZ 40
char *nums_to_string_date(char *date_string, int month, int day,
                          int year, const char *month_names[]);
int search(const char *arr[], const char *target, int n);
void string_date_to_nums(const char *date_string, int *monthp,
                         int *dayp, int *yearp, const char *month_names[]);

/*  Tests date conversion functions	*/
int
main(void)
{
      char *month_names[12] = {"January", "February", "March", "April", "May",
			       "June", "July", "August", "September",
			       "October", "November", "December"};
      int m, y, mon, day, year;
      char date_string[STRSIZ];
      for  (y = 1993;  y < 2010;  y += 10)
          for  (m = 1;  m <= 12;  ++m) {
              printf("%s", nums_to_string_date(date_string,
                                               m, 15, y, month_names));
              string_date_to_nums(date_string, &mon, &day, &year, month_names);
              printf(" = %d/%d/%d\n", mon, day, year);
          }

      return (0);
}

/*
 *  Takes integers representing a month, day and year and produces a 
 *  string representation of the same date.
 */
char *
nums_to_string_date(char       *date_string,     /* output - string 
  representation 	*/
                    int         month,           /* input  - 	*/
                    int         day,             /*   representation	*/
                    int         year,            /*   as three numbers	*/
                    const char *month_names[])   /* input  - string representations of months	*/
{
      sprintf(date_string, "%d %s %d", day, month_names[month - 1], year);
      return (date_string);
}

#define NOT_FOUND -1   /*  Value returned by search function if target 
not found	*/
/*
 *  Searches for target item in first n elements of array arr
 *  Returns index of target or NOT_FOUND
 *  Pre:  target and first n elements of array arr are defined and n>0
 */
int
search(const char *arr[],   	/*  array to search	*/
       const char *target, 	/*  value searched for	*/
       int         n)        	/*  number of array elements to search	*/
{
      int i,
          found = 0,     /*  whether or not target has been found */
          where;         /*  index where target found or NOT_FOUND*/

      /*  Compares each element to target                         */
      i = 0;
      while (!found && i < n) {
          if (strcmp(arr[i], target) == 0)
                found = 1;
          else
                ++i;
      }

      /* Returns index of element matching target or NOT_FOUND    */
      if (found)
            where = i;
      else
            where = NOT_FOUND;
      return (where);
}

/*
 *  Converts date represented as a string containing a month name to
 *  three integers representing month, day, and year
 */
void
string_date_to_nums(const char *date_string,   /* input - date to convert 	*/
                    int        *monthp,        /* output - month number	*/
                    int        *dayp,          /* output - day number	*/
                    int        *yearp,         /* output - year number	*/
                    const char *month_names[]) /* input - names used in
                                                      date string	*/
{
      char mth_nam[STRSIZ];
      int  month_index;
 
      sscanf(date_string, "%d%s%d", dayp, mth_nam, yearp);

      /*  Finds array index (range 0..11) of month name.                */
      month_index = search(month_names, mth_nam, 12);
      *monthp = month_index + 1;
}
 
15 January 1993 = 1/15/1993
15 February 1993 = 2/15/1993
. . .
15 December 2003 = 12/15/2003