Matlab mystery: Comparison is not defined between double and datetime arrays - matlab

Question

I'm new to Matlab, troubleshooting a comparison error by chopping it down as simple as can be. This test matrix of two rows works fine, then I copy/paste to add a third record and I get a conversion error (for the same data)!
Background: my data is for patient exams: patient, arrival time, exam start time, and I'm writing a program to determine how full the waiting room gets. If the patient isn't seen instantaneously I add them to a patient_queue (just saving their eventual start time as a placeholder). The data is in order of arrival time, so at each new row I check the queue to see if anyone's exam started in the meantime, remove them, and go from there.
Here's an example with only two patient rows (this works):
data_matrix = [1,735724.291666667,735724.322916667,735724.343750000,5;
2,735724.331250000,735724.340277778,735724.371527778,18];
patient_queue = [];
highest_wait_num = 0;
[rows, columns] = size(data_matrix);
for i = 1:rows
this_row_arrival = datetime (data_matrix(i, 2), 'ConvertFrom', 'datenum');
this_row_exam_start = datetime (data_matrix(i, 3), 'ConvertFrom', 'datenum');
now = this_row_arrival; %making a copy called 'now' for readability below
% check patient queue: if anyone left in the meantime, remove them
for j = 1:length(patient_queue)
if patient_queue{j} < now
patient_queue(j) = [];
end
end
% if new patient isn't seen immediately (tb > ta), add tb to the queue
if this_row_exam_start > this_row_arrival
patient_queue{end+1} = this_row_exam_start;
end
% get the current queue size
patient_queue_non_zero = (~cellfun('isempty',patient_queue));
indices = find(patient_queue_non_zero);
current_queue_count = length(indices);
% if the current queue size beats the highest we've seen, update it
if current_queue_count > highest_wait_num
highest_wait_num = current_queue_count;
end
end
patient_queue{j}
highest_wait_num
But when I use the full data set I get an error at the line:
if patient_queue{j} < now
Comparison is not defined between double and datetime arrays.
So I'm narrowing the problem, and I can even reproduce the error by taking my simple matrix of 2 records that worked, copy the second one to make a matrix of 3, like so- just swapping that in the code above makes the error(!!):
data_matrix = [1,735724.291666667,735724.322916667,735724.343750000,5;
2,735724.331250000,735724.340277778,735724.371527778,18;
2,735724.331250000,735724.340277778,735724.371527778,18]
What am I missing?

Answer 1

Here is a much shorter way to do this, without converting numbers to dates:
patient_queue = [];
highest_wait_num = 0;
rows = size(data_matrix,1);
for k = 1:rows
this_row_arrival = data_matrix(k, 2);
this_row_exam_start = data_matrix(k, 3);
now = data_matrix(k, 2); %making a copy called 'now' for readability below
% check patient queue: if anyone left in the meantime, remove them
patient_queue(patient_queue < now) = [];
% if new patient isn't seen immediately (tb > ta), add tb to the queue
if this_row_exam_start > this_row_arrival
patient_queue(end+1) = this_row_exam_start;
end
% get the current queue size
current_queue_count = numel(nonzeros(patient_queue));
% if the current queue size beats the highest we've seen, update it
highest_wait_num = max(current_queue_count,highest_wait_num);
end

Answer 2

What difference the right braces make :/
I had declared this as a vector instead of a cell array:
patient_queue = [];
As a result, I couldn't get to the value inside.
The answer was to:
- declare it this way instead: patient_queue = {};
- add one more (missing) datetime conversion for patient_queue{j}
This is a specific pair of mistakes I had, so I'll delete the post since it's unlikely to be useful for others