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% Create 1-by-14 cell array {'a','b', ..., 'n'}
str = 'a':'n';
c = split(str,'')';
c([1 end]) = [];
% Create all the 3-character selection
cmb = nchoosek(c,3);
cmb = strcat(cmb(:,1),cmb(:,2),cmb(:,3));
% Create 364 date and 3-character lookup table
date = datetime(2019,1,1:364)';
T = table(date,cmb,'VariableNames',{'date','char'});
The result is as follows:
>> T
T =
364×2 table
date char
__________ _______
2019/01/01 {'abc'}
2019/01/02 {'abd'}
2019/01/03 {'abe'}
....
2019/12/30 {'lmn'}
