Functions to extract the year, the month and the day of a date integer (like 20151126), without using strings.

extract_from_date.c

/* Functions to extract the year, the month and the day of a date integer (like 20151126), without using strings.
 *
 * by Unlimiter
 */

#include <stdio.h>
#include <math.h>

// returns the number of digits in an integer
int get_int_len(int n) {
	return ceil(log10(n));
}

// returns 10**(n-1)
int int_to_ten_power(int n) {
	if (n == 0) {
		return 0;
	}
	int one_and_zeros = 1;
	for (int i = 1; i < n; i++) {
		one_and_zeros *= 10;
	}
	return one_and_zeros;
}

// returns the yyyy part of an integer, as in yyyymmdd
int get_year(int d) {
	return d/((int_to_ten_power(get_int_len(d))/int_to_ten_power(4)));
}

// returns the mm part of an integer, as in yyyymmdd
int get_month(int d) {
	return (d - get_year(d) * 10000) / 100;
}

// returns the dd part of an integer, as in yyyymmdd
int get_day(int d) {
	return (d - get_year(d) * 10000) - (get_month(d) * 100);
}

// demo
int main() {
	int date = 20190413;
	printf("%i-0%i-%i\n", get_year(date), get_month(date), get_day(date));
}