Count number of entries based on month and year

Question

I have the following sample data:

test={{{2020, 1, 1}, Brazil}, {{2020, 1, 2}, Brazil}, {{2020, 1, 1}, Brazil}, {{2020, 2, 1}, Brazil}, {{2020, 1, 1}, Brazil},{{2019, 1, 1}, Brazil}}

I would like to first group the data by month and year, and then count how many entries are in each group.

I would like the output to look something like this:

<|{{2020, 1},4}, {{2020, 2},1},{{2019, 1},1}|>

Carl Lange · Accepted Answer · 2021-11-01 09:23:35Z

1

You can use CountsBy for this:

CountsBy[test[[All, 1]], d |-> d[[;; 2]]]

<|{2020, 1} -> 4, {2020, 2} -> 1, {2019, 1} -> 1|>

answered Nov 1, 2021 at 9:23

Carl Lange

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$\begingroup$ Is there a way to not make it an association? $\endgroup$
– Julia Ballester
Commented Nov 1, 2021 at 10:03
$\begingroup$ You can use Normal on the association to transform it into a list of rules, or you can use Tally: Tally[test[[All, 1]], {d1, d2} |-> d1[[;; 2]] == d2[[;; 2]]] $\endgroup$
– Carl Lange
Commented Nov 1, 2021 at 10:26

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