# Sorting Structure Arrays based on Fields5

Posted by Jiro Doke,

Jiro's pick this week is Nested Struct Sort by Jake Hughey.

MATLAB has a nice function sortrows to sort matrices (numeric or cells) based on specific column(s). I use it often when I am doing some data management. If you have Statistics Toolbox, there are corresponding functions for dataset arrays and ordinal arrays.

Sometimes, I want to do a similar action on structure arrays. I want to be able to sort according to specific fields. In order to do that, it requires some data manipulations. Let's take a look at an example.

A = struct(...
'name', {'mike', 'doug', 'steve', 'loren', 'jiro', 'brett', 'seth'}, ...
'year', {2005, 2001, 1993, 1987, 2006, 2005, 1998}, ...
'day', {'Mon', 'Fri', 'Wed', 'Fri', 'Mon', 'Mon', 'Mon'})

for id = 1:length(A)
fprintf('%d\n',id)
disp(A(id))
end
A =
1x7 struct array with fields:
name
year
day
1
name: 'mike'
year: 2005
day: 'Mon'
2
name: 'doug'
year: 2001
day: 'Fri'
3
name: 'steve'
year: 1993
day: 'Wed'
4
name: 'loren'
year: 1987
day: 'Fri'
5
name: 'jiro'
year: 2006
day: 'Mon'
6
name: 'brett'
year: 2005
day: 'Mon'
7
name: 'seth'
year: 1998
day: 'Mon'


Let's sort the array by the field "name". First you need to convert to a cell array:

Afields = fieldnames(A);
Acell = struct2cell(A);
sz = size(Acell)            % Notice that the this is a 3 dimensional array.
% For MxN structure array with P fields, the size
% of the converted cell array is PxMxN
sz =
3     1     7


Once it's a cell array, you can sort using sortrows:

% Convert to a matrix
Acell = reshape(Acell, sz(1), []);      % Px(MxN)

% Make each field a column
Acell = Acell';                         % (MxN)xP

% Sort by first field "name"
Acell = sortrows(Acell, 1)
Acell =
'brett'    [2005]    'Mon'
'doug'     [2001]    'Fri'
'jiro'     [2006]    'Mon'
'loren'    [1987]    'Fri'
'mike'     [2005]    'Mon'
'seth'     [1998]    'Mon'
'steve'    [1993]    'Wed'


And convert it back to a structure array:

% Put back into original cell array format
Acell = reshape(Acell', sz);

% Convert to Struct
Asorted = cell2struct(Acell, Afields, 1);

for id = 1:length(Asorted)
fprintf('%d\n',id)
disp(Asorted(id))
end
1
name: 'brett'
year: 2005
day: 'Mon'
2
name: 'doug'
year: 2001
day: 'Fri'
3
name: 'jiro'
year: 2006
day: 'Mon'
4
name: 'loren'
year: 1987
day: 'Fri'
5
name: 'mike'
year: 2005
day: 'Mon'
6
name: 'seth'
year: 1998
day: 'Mon'
7
name: 'steve'
year: 1993
day: 'Wed'


Simpler Approach

Do you want an easier way of doing it? Just use Jake's nestedSortStruct:

B = nestedSortStruct(A, 'name');

isequal(B, Asorted)
ans =
1


You can even sort by multiple fields. For example, let's sort by "year" and then by "name":

C = nestedSortStruct(A, {'year', 'name'});

for id = 1:length(C)
fprintf('%d\n',id)
disp(C(id))
end
1
name: 'loren'
year: 1987
day: 'Fri'
2
name: 'steve'
year: 1993
day: 'Wed'
3
name: 'seth'
year: 1998
day: 'Mon'
4
name: 'doug'
year: 2001
day: 'Fri'
5
name: 'brett'
year: 2005
day: 'Mon'
6
name: 'mike'
year: 2005
day: 'Mon'
7
name: 'jiro'
year: 2006
day: 'Mon'


Thanks for this entry, Jake. One recommendation I have is to include a couple of examples in the HELP text in the code.

Give this a try and et us know what you think here or leave a comment for Jake.

Get the MATLAB code

Published with MATLAB® 7.11

Tom replied on : 1 of 5

It seems easier to just make use of the index output of sort().

[tmp ind]=sort({A.name});
A=A(ind);

jiro replied on : 2 of 5

Nice one Tom! That does seem easier. And you can expand to multiple field sorting by doing this:

% sort by "year" then "name"
[tmp ind] = sortrows([{A.name}',{A.year}'], [2 1]);
A = A(ind);


The only issue I have is the scalability. If I need to sort by additional fields, I have to dynamically build the SORTROWS command, perhaps using convoluted loops or EVALs.

An M replied on : 3 of 5

great !! thanks !!

[tmp ind]=sort({A.name});
A=A(ind);

pr replied on : 4 of 5

I get the following error.

x = {Transactions.trtime};
[tmp ind]=sort(x);
??? Error using ==> cell.sort at 55
Input argument must be a cell array of strings

nuno replied on : 5 of 5

hello pr
if you have that error, use :

sort([struc.field]);