Calculating Week Number Given a Date
In many applications, it's essential to determine the week number for a given date. This information is useful in industries like planning and scheduling. This article explores an algorithm and a code example to calculate the week number based on ISO 8601.
ISO 8601 Definition
The ISO 8601 standard defines a week number based on the following rules:
Algorithm
To calculate the week number, follow these steps:
C Code Example
#include <iostream> #include <chrono> using namespace std; // Date manipulation functions int getYear(tm* date); int getMonth(tm* date); int getDay(tm* date); int getWeek(tm* date); // Algorithm for calculating week number based on ISO 8601 int getWeek(tm* date) { // Calculate d1w1, the first day (Monday) of the first week (Week 1) of the year tm d1w1 = *date; d1w1.tm_mon = 0; // January (0-based month) d1w1.tm_mday = 1; // First day of the month d1w1.tm_wday = 1; // Monday (1-based day of the week) // Get the number of days between the given date and d1w1 time_t t1 = mktime(&d1w1); time_t t2 = mktime(date); int delta = int((t2 - t1) / (24 * 60 * 60)); // Calculate the week number int wn = delta / 7 + 1; // Handle edge cases (last week of previous year or first week of next year) int year = getYear(date); if (delta < 0) { // Given date is in the last week of the previous year year--; d1w1.tm_year = year - 1900; t1 = mktime(&d1w1); delta = int((t2 - t1) / (24 * 60 * 60)); wn = delta / 7 + 1; } else if (getDay(date) == 1 && getMonth(date) == 0 && wn == 1) { // Given date is on January 1st (Monday), so it's in the last week of the previous year year--; d1w1.tm_year = year - 1900; t1 = mktime(&d1w1); delta = int((t2 - t1) / (24 * 60 * 60)); wn = delta / 7 + 1; } return wn; } int main() { struct tm date; // Example date: January 10, 2008 date.tm_year = 2008 - 1900; // tm_year uses years since 1900 date.tm_mon = 0; // Months are 0-based date.tm_mday = 10; int weekNumber = getWeek(&date); cout << "Week number for January 10, 2008: " << weekNumber << endl; return 0; }
In this example, the given date is January 10, 2008, which falls in Week 2 of 2008, corresponding to the expected output. The code can be adapted to process different date formats and handle edge cases as needed.
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