LFE Friday - calendar:day_of_the_week/3

This week's LFE Friday was translated with permission from the Erlang Thursday series by Steven Proctor. This week's translator: Robert Virding.

Today’s LFE Friday covers calendar:day_of_the_week/3.

calendar:day_of_the_week/3 allows you to get the day of the week a day occurred on when passed a year, a month and a day.

The first argument represents the year and must be a non-negative integer. The second argument is the month and must be an integer value between 1 and 12 inclusive, representing the 12 months of the Gregorian Calendar, with January being month one. The final argument to calendar:day_of_the_week/3 is the date, and is expected to be a integer between 1 and 31 inclusive.

calendar:day_of_the_week/3 returns an integer value in between 1 and 7 inclusive, with a return value of 1 representing Monday, and a 7 being Sunday.

> (calendar:day_of_the_week 1 1 1)
1
> (calendar:day_of_the_week 1970 1 1)
4
> (calendar:day_of_the_week 1999 12 31)
5
> (calendar:day_of_the_week 0 1 1)     
6

As you can see that the publish date of this LFE Friday post¹, 2015-04-10, returns a 5, which is in fact a Friday according to Erlang.

> (calendar:day_of_the_week 2015 4 10)
5

There is also a calendar:day_of_the_week/1 function which has all the same constraints but instead of taking three arguments, takes one argument that is a three-tuple of the year, month, and day.

> (calendar:day_of_the_week #(2015 4 10))
5
> (calendar:day_of_the_week #(1970 1 1)) 
4
> (calendar:day_of_the_week #(1999 12 31))
5

And in the spirit of helping to recognize error messages when we see them, let’s take a look at what we get when we pass some invalid inputs to calendar:day_of_the_week/3.

> (calendar:day_of_the_week 0 0 0)        
exception error: function_clause
  in (: calendar date_to_gregorian_days 0 0 0)
  in calendar:day_of_the_week/3 (calendar.erl, line 151)
> (calendar:day_of_the_week 1970 2 31)   
exception error: if_clause
  in calendar:date_to_gregorian_days/3 (calendar.erl, line 116)
  in calendar:day_of_the_week/3 (calendar.erl, line 151)
> (calendar:day_of_the_week 1970 13 2)
exception error: function_clause
  in (: calendar last_day_of_the_month1 1970 13)
  in calendar:date_to_gregorian_days/3 (calendar.erl, line 115)
  in calendar:day_of_the_week/3 (calendar.erl, line 151)

If you look at the error messages you see that calendar:day_of_the_week/3 calls calendar:date_to_gregorian_days/3, which we will cover in next week’s LFE Friday.

–Proctor, Robert