MATLAB Answers

Sort a structure by the date field

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Adam Goldsmith
Adam Goldsmith on 18 Sep 2019
Commented: Stephen Cobeldick on 19 Sep 2019
I have a structure, He, which is a database containing a bunch of helium mass spec data, in addition to the date that it was collected. So each entry into the He structure will have various fields, for example, Date, PanName, ExtractPeriod, Data, etc. Because the entries into the He structure aren't always chronological, I want to be able to rearrange them by the Date field. I have tried exporting the dates into a new variable and sorting that, but MATLAB insists that it can't sort a cell array. I try to convert it from a cell array to a datetime array, then MATLAB insists that it isn't a cell array (it is, class() returns 'cell'). I have tried converting it to a datetime array using cell2mat, but again, it's insisting that it isn't a cell array (again, it is).
>> date_list = {He.Date}
>> sort(date_list)
Error using sort
Input argument must be a cell array of character vectors.
>> date_list = cell2mat(date_list)
Error using cell2mat (line 52)
CELL2MAT does not support cell arrays containing cell arrays or objects.
>> class(asdf)
ans =
'cell'
What's the easiest way to go about doing this and what am I doing wrong? Why does it tell me it's a cell on one line, then tell me it's not a cell in the next?? All I want to do is sort this database by date, but I can't even get to the point where I can sort a list of the dates...

  3 Comments

Rik
Rik on 18 Sep 2019
The sort function is telling you that not all cell elements contain a char vector. The cell2mat function tells you that some element contains a data type that it can't handle.
If you attach your struct in a mat file we might be able to help you generate a sorted list.
per isakson
per isakson on 18 Sep 2019
>> d = datetime
d =
datetime
19-Sep-2019 00:08:38
>> methods(d)
... sort ...
The value of the date field, what class is it ?
Adam Goldsmith
Adam Goldsmith on 19 Sep 2019
The class of the Date field is 'datetime'
Unfortunately, the file is larger than the 5 MB limit and can't be uploaded, however, you could really easily generate a dummy structure with the same issue:
examplestruct = struct;
for i = 1:5
examplestruct(i).data = rand(1,10)
end
examplestruct(1).date = datetime('09/01/2019','InputFormat','MM/dd/yyyy')
examplestruct(2).date = datetime('08/11/2019','InputFormat','MM/dd/yyyy')
examplestruct(3).date = datetime('09/12/2019','InputFormat','MM/dd/yyyy')
examplestruct(4).date = datetime('09/16/2019','InputFormat','MM/dd/yyyy')
examplestruct(5).date = datetime('09/05/2019','InputFormat','MM/dd/yyyy')

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Accepted Answer

Rik
Rik on 19 Sep 2019
Don't concatenate to a cell, but to an array:
examplestruct = struct;
for i = 1:5
examplestruct(i).data = rand(1,10);
end
examplestruct(1).date = datetime('09/01/2019','InputFormat','MM/dd/yyyy');
examplestruct(2).date = datetime('08/11/2019','InputFormat','MM/dd/yyyy');
examplestruct(3).date = datetime('09/12/2019','InputFormat','MM/dd/yyyy');
examplestruct(4).date = datetime('09/16/2019','InputFormat','MM/dd/yyyy');
examplestruct(5).date = datetime('09/05/2019','InputFormat','MM/dd/yyyy');
date_list =[examplestruct.date];
[~,order]=sort(date_list);
sorted=examplestruct(order);

  2 Comments

Adam Goldsmith
Adam Goldsmith on 19 Sep 2019
This works, thank you! I had seen using {struct.field} to turn the structure field into a cell array, but wasn't aware that I could do [struct.field] to convert it into an array. That's very helpful, thank you!
Stephen Cobeldick
Stephen Cobeldick on 19 Sep 2019
"I had seen using {struct.field} to turn the structure field into a cell array, but wasn't aware that I could do [struct.field] to convert it into an array"
Read these to know how it works and where you can use that syntax:

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More Answers (1)

xi
xi on 19 Sep 2019
It's easier to handle it in table:
T=struct2table(examplestruct);
sort(T.date)

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