Hash key sort week numbers using ruby - ruby

Question

I am having an hash whose keys are week numbers and values are attendance scores. I am tying to calculate the average attendance for each month based on the week number i.e.keys.
Below is the example of the hash
weekly_attendance = {31 => 40.0, 32 => 100.00, 33 => 34.00, 34 => 23.78, 35 => 56.79, 36 => 44.50, 37 => 67.00, 38 => 55.00 }
Since a month consists of 4 weeks and the beginning week of the month is divisible by 4, the attendance needs to be sorted as follows
Month 1 attendance consists of weeks 31,32 i.e. (40.00+100.00)/2 =70.0
Month 2 attendance consists of weeks 33,34,35,36
i.e. (34.00+23.78+56.79+44.50)/4 = 39.5
Month 3 attendance consists of weeks 37, 38 i.e. (67.00+55.00)/2 = 69.5
The output should be
monthly_attendance = [70.0,39.5,61]
I had tried each and select approaches and used the modulo operator condition i.e. week % 4 == 0 to add the attendance values. But could not effectively group them based on months
tmp = 0
monthly_attendance = []
weekly_attendance.select do |k,v|
tmp += v
monthly_attendance << tmp if k % 4 == 0
end
I am unable to sort the week number in ranges using the above code.

Answer 1

You can try something like this:
results = weekly_attendance.group_by { |week, value| (week + 3) / 4 }.map do |month, groups|
values = groups.map(&:last)
average = values.inject(0) { |sum, val| sum + val } / values.length
[month, average]
end.to_h
p results # {8=>70.0, 9=>39.7675, 10=>61.0}
But the logic of converting weeks to months is flawed here, it's better to use some calendar function instead of just division by 4.
You can get the real month numbers using:
require 'date'
weekly_attendance.group_by { |week, value| Date.commercial(Time.now.year, week, 1).month }
But the result will not match the result you expect, because for example week 31 is in July, while week 32 is in August (this year), instead of being the same month like you expect.

Answer 2

I assume that if x units are produced in a given week, x/7 units are produced on each day of that week. The code below could be easily changed if this assumption were changed.
First construct a hash whose keys are months (1-12) and whose values are hashes whose keys are weeks and whose values are the numbers of days in the given week for the given month. (Whew!)
require 'date'
def months_to_weeks(year)
day = Date.new(year)
days = day.leap? ? 365 : 364
days.times.with_object(Hash.new { |h,k| h[k] = Hash.new(0) }) do |_,h|
h[day.month][day.cweek] += 1
day = day.next
end
end
The doc for Hash#new provides an explanation of the statement:
Hash.new { |h,k| h[k] = Hash.new(0) }
In brief, this creates an empty hash with a default given by the block. If h is the hash that is created, and h does not have a key k, h[k] will cause the block to be executed, which adds that key to the hash and sets its value to an empty hash with a default value of 0. The latter hash is often referred to as a "counting hash". I realize this is still rather a mouthful for a Ruby newbie.
Let's generate this hash for the current year:
year = 2015
mon_to_wks = months_to_weeks(year)
#=> {1 =>{1 =>4, 2 =>7, 3 =>7, 4 =>7, 5=>6},
# 2 =>{5 =>1, 6 =>7, 7 =>7, 8 =>7, 9=>6},
# 3 =>{9 =>1, 10=>7, 11=>7, 12=>7, 13=>7, 14=>2},
# 4 =>{14=>5, 15=>7, 16=>7, 17=>7, 18=>4},
# 5 =>{18=>3, 19=>7, 20=>7, 21=>7, 22=>7},
# 6 =>{23=>7, 24=>7, 25=>7, 26=>7, 27=>2},
# 7 =>{27=>5, 28=>7, 29=>7, 30=>7, 31=>5},
# 8 =>{31=>2, 32=>7, 33=>7, 34=>7, 35=>7, 36=>1},
# 9 =>{36=>6, 37=>7, 38=>7, 39=>7, 40=>3},
# 10=>{40=>4, 41=>7, 42=>7, 43=>7, 44=>6},
# 11=>{44=>1, 45=>7, 46=>7, 47=>7, 48=>7, 49=>1},
# 12=>{49=>6, 50=>7, 51=>7, 52=>7, 53=>3}}
Because of how Date#cweek is defined, the weeks in this hash begin on Mondays. In January, for example, there 4 days are in week 1. These four days, Jan. 1-4, 2015, would be the first Thursday, Friday, Saturday and Sunday of 2015. (Check your calendar.)
If the first day of each week is to be a day other than Monday (Sunday, for example) the hash calculation would have to be changed slightly.
This shows, for example, that in January of 2015, there are 4 days in week 1, 7 days in weeks 2, 3 and 4 and 6 days in week 5. The remaining day of week 5 is the first day in February.
Once this hash has been constructed, it is a simple matter to compute the averages for each month:
weekly_attendance = {31 => 40.00, 32 => 100.00, 33 => 34.00, 34 => 23.78,
35 => 56.79, 36 => 44.50, 37 => 67.00, 38 => 55.00 }
prod_by_mon = (1..12).each_with_object(Hash.new(0)) do |i,h|
mon_to_wks[i].each do |week, days|
h[i] += (days/7.0)*weekly_attendance[week] if weekly_attendance.key?(week)
end
end
#=> {7=>28.571428571428573, 8=>232.3557142857143, 9=>160.14285714285714}
prod_by_mon.merge(prod_by_mon) { |_,v| v.round(2) }
#=> {7=>28.57, 8=>232.36, 9=>160.14}
This shows that production in month 7 was 27.57, and so on. Note that:
28.57 + 232.36 + 160.14 #=> 421.07
weekly_attendance.values.reduce(:+) #=> 421.07