In general, your task will be easier if you start by filling in those blank IDs. I will use data to refer to your table.
%%Fill in blanks (assumes first row is not blank)
for ii = 1:height(data)
ID = data.ContactID{ii};
if ~isempty(ID) % First occurance
prev = ID;
else % Not first occurance
data.ContactID{ii} = prev;
end
end
Then you can gather the IDs and select table subsets by common ID
[ID, ~, index] = unique(data.ContactID);
grouped = table();
grouped.ContactID = ID;
grouped.ActivityData = cell(height(grouped), 1);
for ii = 1:length(ID)
grouped.ActivityData{ii} = data(index == ii, 2:end);
end
You could also do this in a containers.Map object instead of a table.
grouped = containers.Map();
for ID = unique(data.ContactID)'
grouped(ID{1}) = data(strcmp(data.ContactID, ID{1}), 2:end);
end
