I want to find unique value(s) based on a given conditions

I have 2 arrays a and b:

a = [datetime(2020,1,2), datetime(2020,1,5), datetime(2020,1,6), datetime(2020,2,3), datetime(2020,2,8),datetime(2020,3,4),datetime(2020,3,10),datetime(2020,4,11)]'
b = {'2020_01';'2020_01';'2020_01';'2020_02';'2020_02';'2020_03';'2020_03';'2020_04'}

I want to find the indices of unique element of b and if there are ties, choose the most recent based on a.

in the case, the indices that I would like to identify are the following:

indices = [0,0,1,0,1,0,1,1]'

I can of course do it with a loop but I am interested in a vectorize solution using unique, which I was not able to figure out so far.

Answer 1

[Jeffrey Clark]

@Tulkkas, this should do it (I added a randomize of your sorted data since first, if it would always be sorted skip as indicated):

a = [datetime(2020,1,2), datetime(2020,1,5), datetime(2020,1,6), datetime(2020,2,3) ...
   , datetime(2020,2,8),datetime(2020,3,4),datetime(2020,3,10),datetime(2020,4,11)]';
b = {'2020_01';'2020_01';'2020_01';'2020_02';'2020_02';'2020_03';'2020_03';'2020_04'};

% setup data assuming future datasets won't be sorted already
L = length(a);
I = randperm(L);
a = a(I);
b = b(I);

% sort datasets
[a,i] = sort(a);
b = b(i);

% extract unique data
[bu,ui] = unique(b,'last');
au = a(ui);

[Tulkkas]

Great that does the trick, thx!

[Jeffrey Clark]

@Tulkkas, or if you want the last two lines (the actual work) can be:

indices = [string(b(1:end-1))~=string(b(2:end));false];
indices(end) = indices(end-1);