p19.erl

-module(p19).

-export([answer/0]).

%% You are given the following information, but you may prefer to do some research for yourself.
%%    * 1 Jan 1900 was a Monday.
%%    * Thirty days has September, April, June and November.
%%      All the rest have thirty-one, Saving February alone,
%%      Which has twenty-eight, rain or shine.
%%      And on leap years, twenty-nine.
%%    * A leap year occurs on any year evenly divisible by 4,
%%      but not on a century unless it is divisible by 400.
%% How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

answer() ->
    length([ 1 || Y <- lists:seq(1901, 2000),
                  M <- lists:seq(1, 12),
                  calendar:day_of_the_week(Y,M,1) =:= 7 ]).