CODEWITHSUNDEEP
clojure
Coding active
Coding active

Reputation: 1680

Clojure - update data values in array of hashes

I would like to update values in hashes, but I'm not sure how this can be done efficiently

I tried using a loop approach, but keeping the previous record's value also in account seems like a big challenge.

This is what I am trying to do,

Considering the records are sorted based on created_at in descending order, For example, 

[{:id 1, :created_at "2016-08-30 11:07:00"}{:id 2, :created_at "2016-08-30 11:05:00"}...]

] ; Basically in humanised form.

Could anyone share some ideas to achieve this? Thanks in advance.

Upvotes: 0

Views: 74

Answers (2)

Michiel Borkent
Michiel Borkent

Reputation: 34800

Simplified example:

(def data [{:value 10} {:value 8} {:value 3}])

(conj
 (mapv
  (fn [[m1 m2]] (assoc m1 :difference (- (:value m1) (:value m2))))
  (partition 2 1 data))
 (last data))

;;=> [{:value 10, :difference 2} {:value 8, :difference 5} {:value 3}]

Upvotes: 1

leetwinski
leetwinski

Reputation: 17859

what you need, is to iterate over all the pairs of consecutive records, keeping the first of them, adding the difference to it.

first some utility functions for dates handling:

(defn parse-date [date-str]
  (.parse (java.text.SimpleDateFormat. "yyyy-MM-dd HH:mm:ss") date-str))

(defn dates-diff [date-str1 date-str2]
  (- (.getTime (parse-date date-str1))
     (.getTime (parse-date date-str2))))

then the mapping part:

user> (def data [{:id 1, :created_at "2016-08-30 11:07:00"}
                 {:id 2, :created_at "2016-08-30 11:05:00"}
                 {:id 3, :created_at "2016-08-30 10:25:00"}])

user> (map (fn [[rec1 rec2]]
             (assoc rec1 :difference
                    (dates-diff (:created_at rec1)
                                (:created_at rec2))))
           (partition 2 1 data))
({:id 1, :created_at "2016-08-30 11:07:00", :difference 120000}
 {:id2, :created_at "2016-08-30 11:05:00", :difference 2400000})

notice that it doesn't contain the last item, since it was never the first item of a pair. So you would have to add it manually:

user> (conj (mapv (fn [[rec1 rec2]]
                    (assoc rec1 :difference
                           (dates-diff (:created_at rec1)
                                       (:created_at rec2))))
                  (partition 2 1 data))
            (assoc (last data) :difference ""))
[{:id 1, :created_at "2016-08-30 11:07:00", :difference 120000} 
 {:id 2, :created_at "2016-08-30 11:05:00", :difference 2400000} 
 {:id 3, :created_at "2016-08-30 10:25:00", :difference ""}]

now it's ok. The only difference with your desired variant, is that the diff is in millis, rather than formatted string. To do that you can add the formatting to the dates-diff function.

Upvotes: 0

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