Where are these `New Name` and `[]` coming from?

Question

I'm learning about structure iteration, and trying to output things in the loop

patient(1).name = 'John Doe';
patient(1).billing = 127.00;
patient(1).test = [79, 75, 73; 180, 178, 177.5; 220, 210, 205]; 
patient(2).name = 'Ann Lane';
patient(2).billing = 28.50;
patient(2).test = [68, 70, 68; 118, 118, 119; 172, 170, 169]; 

fields = fieldnames(patient)

%numel is number of elements
for i=1:numel(fields)
  fields(i)
  patient.(fields{i})
end

During this patient.(fields{i}), it gives 'New Name' and [] which are not part of my struct. Where are those values coming from?

The output is:

ans = 'name'
ans = John Doe
ans = Ann Lane
ans = New Name
ans = 'billing'
ans = 127
ans = 28.5000
ans = []
ans = 'test'
ans = 79.0000   75.0000   73.0000
     180.0000  178.0000  177.5000
     220.0000  210.0000  205.0000
ans = 68    70    68
     118   118   119
     172   170   169
ans = []

Are you sure that you didn't add a `patient(3).name = 'New Name'` during testing? Try `clear patient` to remove `patient` completely before assigning new data. — Suever, Jan 13 '17 at 20:20
Run `clear patient` and then run your code again and I guarantee you'll have no problems. — Suever, Jan 13 '17 at 20:21
Hey that works..., but I don't know why? Did previous values of my output get cached? — Plain_Dude_Sleeping_Alone, Jan 13 '17 at 20:22
@Suever Thanks, please you put it as an answer so that I can end my question. — Plain_Dude_Sleeping_Alone, Jan 13 '17 at 20:25
You must have assigned `patient(3).name = 'New Name'` at some point. Assigning to `patient(1).name` doesn't delete the previous `struct`, only `clear` can do that. — Suever, Jan 13 '17 at 20:25

Suever · Accepted Answer · 2017-01-13T20:32:40.850

You must have previously assigned patient(3).name = 'New Name' and since your code only over-writes the first and second elements of patient, the third element remains untouched and is therefore appearing during your looping.

You can check this by using size or numel

numel(patient) 
%   3

To prevent this, you can either initialize your struct to an empty struct prior to assignment

% Initialize it
patient = struct()

% Now populate
patient(1).name = 'whatever';

Or explicitly clear the variable clear patient to ensure that this doesn't happen.

clear patient

% Now populate it
patient(1).name = 'whatever';

Also, as a side-note, the reason that the other fields are [] is because if you add a new field to an existing struct array, then all struct entries in the array will receive [] as the value for the new field

clear patient

patient(2).name = 'New Name';
patient(1).name = 'Test';

% Add a new field only to patient(2)
patient(2).date = 'today';

% patient(1).date becomes []
patient(1).date
%   []

Where are these `New Name` and `[]` coming from?

1 Answers1