Clojure - Working with list and date-time - list

Question

I am quite stuck in this scenario.
I have a list of atoms representing bank transactions.
(#<Ref#29a71299: {:desc "DESC1", :amount 150, :date #<LocalDate 2017-01-10>}>)
(#<Ref#5a4ebf03: {:desc "DESC2", :amount 250, :date #<LocalDate 2017-01-10>}>)
(#<Ref#5a4ebf03: {:desc "DESC3", :amount -250, :date #<LocalDate 2017-01-11>}>)
(#<Ref#5a4ebf03: {:desc "DESC4", :amount 50, :date #<LocalDate 2017-01-12>}>)
I need calculate the balance account in the end of the day, so I should grab all transactions separated per day to know the balance in the end of the day.
Someone did it before ? What is the best way to filter dates and do this math ? I am still noob/student in clojure.
obs. I am using this library to work with date Jodatime

Answer 1

A great way to approach problems in Clojure is to think:
How can I break this problem down (this is usually the hard part)
How can I solve each problem alone
How do I compose these solutions (this is usually the easy part)
Applying this to your problem I see these problems:
segmenting a list of maps by a property of one of the keys
(partition-by ... something ...)
summing all the values of one of the keys in each of a sequence of maps
(map (reduce ...))
making an output format with the data and the sum from each segment
(map ... something)
And the composing part is likely just nesting each of these as nested function calls. Nested function calls can be written using the thread-last maco and will look something like this:
(->> data
(... problem one solution ...)
(problem two solution)
(some output formatting for problem three))

Answer 2

You may want to break it down this way:
(defn per-day-balance [txns]
(->> txns
(partition-by :date)
(map (fn [[{date :date} :as txns]]
{:date date :balance (reduce + (map :amt txns))}))))
Find the daily balance assuming everyday starts with 0. Sample run:
(def txns [{:date 1 :amt 10}
{:date 1 :amt 3}
{:date 2 :amt 9}
{:date 2 :amt -11}
{:date 3 :amt 13}])
user> (per-day-balance txns)
=> ({:date 1, :balance 13} {:date 2, :balance -2} {:date 3, :balance 13})
Now add a reduction function to get the running total. The reduction function simply 'update' the cumulative balance:
(defn running-balance [bals]
(let [[day-1 & others] bals]
(reductions
(fn [{running :balance} e] (update e :balance + running))
day-1
others)))
Sample run:
user> (->> txns
per-day-balance
running-balance)
=> ({:date 1, :balance 13} {:date 2, :balance 11} {:date 3, :balance 24})
Note: You can use whatever data type for :date field. Secondly, I deliberately avoid atom to make the functions pure.

Answer 3

This ended up getting more complicated than I thought it would. I looked at partition-by, and you should almost definitely use that instead. It's perfectly suited for this problem. This is just an example of how it could be done with a loop:
(defn split-dates [rows]
(loop [[row & rest-rows] rows ; Split on the head
last-date nil
acc [[]]]
(if row
(let [current-date (last row)]
(recur rest-rows current-date
; If the day is the same as the previous row
(if (or (nil? last-date) (= current-date last-date))
; Update the current day list with the new row
(update acc (dec (count acc))
#(conj % row))
; Else, create a new list of rows for the new date
(conj acc [row]))))
acc)))
(clojure.pprint/pprint
(split-dates
[[0 1 "16.12.25"]
[2 3 "16.12.25"]
[4 5 "16.12.26"]
[6 7 "16.12.26"]
[8 9 "16.12.26"]
[1 9 "16.12.27"]]))
[[[0 1 "16.12.25"] [2 3 "16.12.25"]]
[[4 5 "16.12.26"] [6 7 "16.12.26"] [8 9 "16.12.26"]]
[[1 9 "16.12.27"]]]
Notes:
This assumes the dates are in the last column, and that the rows are sorted by date.
It returns [[]] when given given an empty list. This may or may not be what you want.