GatherBy Dateobject instance

Questions
1

How can I gather instances of DateObject like ‘{Month:Sep 2018}’ into sublist by ‘First’?
I tried

Flatten[GatherBy[{2018,9}],First]

but it shows error.

2

GatherBy[{{2018, 9}, {2019, 9}, {2019, 10}, {2020, 9}}, First]
(* {{{2018, 9}}, {{2019, 9}, {2019, 10}}, {{2020, 9}}} *)

CountsBy[{{2018, 9}, {2019, 9}, {2019, 10}, {2020, 9}}, First]
(* <|2018 -> 1, 2019 -> 2, 2020 -> 1|> *)

GroupBy[{{2018, 9}, {2019, 9}, {2019, 10}, {2020, 9}}, First]
(* <|2018 -> {{2018, 9}}, 2019 -> {{2019, 9}, {2019, 10}}, 2020 -> {{2020, 9}}|> *)
3

I am using it in solving an exercise and it is still not working. I added DateObject
Flatten[GatherBy[DateObject[{2018,9}],First]

4

If you are dealing with DateObject then

GatherBy[
 {DateObject[{2018, 9}], DateObject[{2019, 9}], 
  DateObject[{2019, 10}], DateObject[{2020, 9}]},
 DateValue[#, "Year"] &]

(*
{{DateObject[{2018, 9}, "Month"]}, {DateObject[{2019, 9}, "Month"], 
  DateObject[{2019, 10}, "Month"]}, {DateObject[{2020, 9}, "Month"]}}
*)

Don’t understand why you are flattening the result, doing so will undo the GatherBy.

5

Screenshot2021-12-18at11.16.38.png
Screenshot2021-12-18at11.16.38.png881×103 12.5 KB

![with flatten I get the same, but exercise does not pass it.

6

Please post code that can be copied/pasted, not images. The function needs to be DateValue[#, "Year"] &, not "Year". See the code in my answer.

7

GatherBy[{DateObject[{2015, 9}], DateObject[{2015, 1}],
DateObject[{2015, 5}], DateObject[{2017, 8}], DateObject[{2017, 5}],
DateObject[{2017, 8}], DateObject[{2018, 1}],
DateObject[{2018, 8}], DateObject[{2018, 3}], DateObject[{2018, 2}],
DateObject[{2018, 4}], DateObject[{2018, 9}],
DateObject[{2015, 6}], DateObject[{2015, 10}],
DateObject[{2015, 6}]}, “Year”]

and

GatherBy[{DateObject[{2015, 9}], DateObject[{2015, 1}], 
  DateObject[{2015, 5}], DateObject[{2017, 8}], DateObject[{2017, 5}],
   DateObject[{2017, 8}], DateObject[{2018, 1}], 
  DateObject[{2018, 8}], DateObject[{2018, 3}], DateObject[{2018, 2}],
   DateObject[{2018, 4}], DateObject[{2018, 9}], 
  DateObject[{2015, 6}], DateObject[{2015, 10}], 
  DateObject[{2015, 6}]}, DateValue[#, "Year"] &]

yield same result and exercise doesn’t pass any

8

Thanks Rohit. I found the error. I am curious to know how do you approach to such problem. I didn’t come near vicinity to it

9

GatherBy calls the specified function for each element in the list. All elements for which the function gives the same result are put in a sublist. So, the key is to write the function so it returns the value that should be grouped. If the elements in the list are DateObject and you want to group by year then the function should return the year part of the DateObject, the best way to do that is to use DateValue to extract the year.

Alternatively, you can extract the year from the FullForm of DateObject.

DateObject[{2015, 9}] // First
(* {2015, 9} *)

DateObject[{2015, 9}] // First // First
(* 2015 *)

So you could give GatherBy the function First /* First and the result would be the same. Or to gather by month First /* Last.

If you just have a list of {year, month} then you can use the function First.

GatherBy[{{2015, 9}, {2015, 1}, {2015, 5}, {2017, 8}, {2017, 5},
 {2017, 8}, {2018, 1}, {2018, 8}, {2018, 3}, {2018, 2}, {2018, 4}, 
 {2018, 9}, {2015, 6}, {2015, 10}, {2015, 6}}, First]