Return pointer to a structure - c

Question

I have this program, where you enter two dates into two structures (same type), and then I want a function to find which date I entered is the later date. It compares only year and month. Once the later date is found, I want the function to return a pointer to the structure with the later date. I then want to print out the later date.
This is what I have so far, but I get errors and I'm not sure about the pointer syntax.
Code:
#include <stdio.h>
struct date{
int year;
int month;
int day;
};
main()
{
struct date dates[2];
int i = 0, res = 0;
for ( i = 0 ; i < 2 ; i++){
printf("Enter a year!");
scanf("%d", &dates[i].year);
printf("Enter a month!");
scanf("%d", &dates[i].month);
printf("Enter a day!");
scanf("%d", &dates[i].day);
}
res = later(&dates[1], &dates[2]);
}
struct date *later(struct date *one, struct date *two)
{
if (one->year > two->year){
return *one;
}
else if (one->year == two->year){
if(one->month > two->month){
return *one;
}
else
return *two;
}
else {
return *two;
}
}
Error messages:
q4.c:28:3: warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘int *’ [-Wformat=]
printf("%d", &res);
^
q4.c: At top level:
q4.c:32:14: error: conflicting types for ‘later’
struct date *later(struct date *one, struct date *two){
^
q4.c:26:9: note: previous implicit declaration of ‘later’ was here
res = later(&dates[1], &dates[2]);
^
q4.c:55:1: error: expected identifier or ‘(’ before ‘}’ token
}

Answer 1

There are multiple problem with your program:
There is no prototype for your function later() Solution: Add date *later(date *one, date *two); after ending declaration of struct date or move the whole function above main().
Change return type of you function to date* as you are returning a pointer to date object.
Change data type of res to date* as you want to store a pointer to date object.
Then print the object res is pointing to by printing each of its components individually as printf is not designed to print your custom data type.
This is only a suggestion but i recommend you moving declaration of int i to for(int i = 0; i < 2; i++); it is just considered better and saves memory.
If you want res to be an int you will have to return a object instead of a pointer and then typecast it to int. Guide on typecasting: http://www.cplusplus.com/doc/tutorial/typecasting/

Answer 2

There are a number of ways to handle the return as have been pointed out in the comments. You can either declare your later function as int (as you have), and use the return similar to the return from strcmp (0 - dates are equal, -1 first is earlier than last, 1 first is later than last).
You can also declare later as struct date * and return a pointer to the later date (which leaves you in a pickle as to what to return if the dates are equivalent).
Your return is more meaningful, if you return a int value similar to strcmp. An implementation that compares your struct date and returns -1, 0 or 1 based on the first being earlier, equivalent, or later could be:
int later (struct date *one, struct date *two)
{ /* the ternary operator is your friend */
if (one->year != two->year) /* years differ */
return one->year > two->year ? 1 : -1;
if (one->month != two->month) /* months differ */
return one->month > two->month ? 1 : -1;
if (one->day != two->day) /* days differ */
return one->day > two->day ? 1 : -1;
return 0; /* dates are equivalent */
}
(note: the use of the ternary operator (e.g. test ? if true : if false) to simply comparing each date component)
Also, note, in this case your are free to pass struct date instead of struct date * as the function will work fine operating on a copy of each date struct. It is up to you how you want to do it.
A small sample program testing the function (and noting a few code improvements) could be:
#include <stdio.h>
struct date {
int year;
int month;
int day;
};
int later (struct date *one, struct date *two);
void prn_result (struct date a, struct date b, char op);
/* main is a function of type 'int' and returns a value */
int main (void) {
struct date dates[2] = {{0}}; /* initialize variables */
int i = 0, res = 0;
for (i = 0; i < 2; i++) { /* user input of dates */
printf ("Enter a year: ");
scanf ("%d", &dates[i].year);
printf ("Enter a month :");
scanf ("%d", &dates[i].month);
printf ("Enter a day: ");
scanf ("%d", &dates[i].day);
putchar ('\n');
}
res = later (&dates[0], &dates[1]); /* comparison */
if (res == 0)
prn_result (dates[0], dates[1], '=');
else if (res > 0)
prn_result (dates[0], dates[1], '>');
else
prn_result (dates[0], dates[1], '<');
return 0;
}
int later (struct date *one, struct date *two)
{ /* the ternary operator is your friend */
if (one->year != two->year) /* years differ */
return one->year > two->year ? 1 : -1;
if (one->month != two->month) /* months differ */
return one->month > two->month ? 1 : -1;
if (one->day != two->day) /* days differ */
return one->day > two->day ? 1 : -1;
return 0; /* dates are equivalent */
}
void prn_result (struct date a, struct date b, char op)
{
printf ("%d/%d/%d %c %d/%d/%d\n",
a.month, a.day, a.year, op,
b.month, b.day, b.year);
}
Example Use/Output
$ ./bin/cmpyear
Enter a year: 1993
Enter a month :3
Enter a day: 13
Enter a year: 1993
Enter a month :3
Enter a day: 12
3/13/1993 > 3/12/1993
$ ./bin/cmpyear
Enter a year: 1993
Enter a month :3
Enter a day: 12
Enter a year: 1993
Enter a month :3
Enter a day: 12
3/12/1993 = 3/12/1993
$ ./bin/cmpyear
Enter a year: 1993
Enter a month :3
Enter a day: 12
Enter a year: 1993
Enter a month :3
Enter a day: 13
3/12/1993 < 3/13/1993
Look it over and let me know if you have an further questions.