Sum the next value of hash in an array of hashes - ruby-on-rails

Question

I have an array of hashes like this:
[{Mon, 09 May 2016 14:49:17 UTC +00:00=>12},
{Sun, 17 Apr 2016 14:08:40 UTC +00:00=>30},
{Sun, 16 Apr 2016 14:08:40 UTC +00:00=>18},
{Sun, 15 Apr 2016 14:03:33 UTC +00:00=>21}]
How can I sum the previous value from the oldest date to the current date, my expected output will be:
[{Mon, 09 May 2016 14:49:17 UTC +00:00=>81},
{Sun, 17 Apr 2016 14:08:40 UTC +00:00=>69},
{Sun, 16 Apr 2016 14:08:40 UTC +00:00=>39},
{Sun, 15 Apr 2016 14:03:33 UTC +00:00=>21}]
Thanks!

Answer 1

Assuming that the key of every hash in your array is a DateTime object, you can get what your want with this:
balance = [
{DateTime.parse('Mon, 09 May 2016 14:49:17 UTC +00:00')=>12},
{DateTime.parse('Sun, 17 Apr 2016 14:08:40 UTC +00:00')=>30},
{DateTime.parse('Sun, 16 Apr 2016 14:08:40 UTC +00:00')=>18},
{DateTime.parse('Sun, 15 Apr 2016 14:03:33 UTC +00:00')=>21}
] # => your original array
# Get expected array.
balance.map{ |h|
{
h.keys.first => balance.select{ |e|
e.keys.first <= h.keys.first }.map{ |s|
s[s.keys.first] }.reduce(:+)
}
}
I split the code in lines in order to improve readability.

Answer 2

Another approach would be to sort the array first and then use the map function to keep a running total to collect the required data.
# sort the balances by date
balance = balance.sort {|a, b| a.keys.first <=> b.keys.first }
# get running total and collect for each date
total = 0
balance.map do |entry|
date, value = entry.first
total += value
{date => total}
end

Answer 3

I assume your array is in lastest-to-earliest date order and looks something like arr below:
a = [{ "Mon, 09 May 2016 14:49:17 UTC +00:00"=>12 },
{ "Sun, 17 Apr 2016 14:08:40 UTC +00:00"=>30 },
{ "Sun, 16 Apr 2016 14:08:40 UTC +00:00"=>18 },
{ "Sun, 15 Apr 2016 14:03:33 UTC +00:00"=>21 }]
require 'date'
arr = a.map do |h|
(d, v) = h.to_a.first
{ DateTime.parse(d) => v }
end
#=> [{#<DateTime: 2016-05-09T14:49:17+00:00 ((2457518j,53357s,0n),+0s,2299161j)>=>12},
# {#<DateTime: 2016-04-17T14:08:40+00:00 ((2457496j,50920s,0n),+0s,2299161j)>=>30},
# {#<DateTime: 2016-04-16T14:08:40+00:00 ((2457495j,50920s,0n),+0s,2299161j)>=>18},
# {#<DateTime: 2016-04-15T14:03:33+00:00 ((2457494j,50613s,0n),+0s,2299161j)>=>21}]
We can then compute the required array as follows.
cumv = 0
arr.reverse.
map { |h| h.to_a.first }.
each_with_object([]) do |(d,v),a|
cumv += v
a << { d => cumv }
end.
reverse
#=> [{#<DateTime: 2016-05-09T14:49:17+00:00 ((2457518j,53357s,0n),+0s,2299161j)>=>81},
# {#<DateTime: 2016-04-17T14:08:40+00:00 ((2457496j,50920s,0n),+0s,2299161j)>=>69},
# {#<DateTime: 2016-04-16T14:08:40+00:00 ((2457495j,50920s,0n),+0s,2299161j)>=>39},
# {#<DateTime: 2016-04-15T14:03:33+00:00 ((2457494j,50613s,0n),+0s,2299161j)>=>21}]