{"id":2595,"date":"2010-09-17T13:50:22","date_gmt":"2010-09-17T13:50:22","guid":{"rendered":"https:\/\/blogs.mathworks.com\/pick\/2010\/09\/17\/sorting-structure-arrays-based-on-fields\/"},"modified":"2020-10-05T03:39:24","modified_gmt":"2020-10-05T07:39:24","slug":"sorting-structure-arrays-based-on-fields","status":"publish","type":"post","link":"https:\/\/blogs.mathworks.com\/pick\/2010\/09\/17\/sorting-structure-arrays-based-on-fields\/","title":{"rendered":"Sorting Structure Arrays based on Fields"},"content":{"rendered":"\r\n\r\n

Jiro<\/a>'s pick this week is Nested Struct Sort<\/a> by Jake Hughey<\/a>.<\/p>

MATLAB has a nice function sortrows<\/tt><\/a> 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<\/a>, there are corresponding functions for dataset arrays<\/a> and ordinal arrays<\/a>.<\/p>

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.<\/p>

A = struct(...<\/span>\r\n    'name'<\/span>, {'mike'<\/span>, 'doug'<\/span>, 'steve'<\/span>, 'loren'<\/span>, 'jiro'<\/span>, 'brett'<\/span>, 'seth'<\/span>}, ...<\/span>\r\n    'year'<\/span>, {2005, 2001, 1993, 1987, 2006, 2005, 1998}, ...<\/span>\r\n    'day'<\/span>, {'Mon'<\/span>, 'Fri'<\/span>, 'Wed'<\/span>, 'Fri'<\/span>, 'Mon'<\/span>, 'Mon'<\/span>, 'Mon'<\/span>})\r\n\r\nfor<\/span> id = 1:length(A)\r\n    fprintf('%d\\n'<\/span>,id)\r\n    disp(A(id))\r\nend<\/span>\r\n<\/pre>
A = \r\n  1×7 struct array with fields:\r\n    name\r\n    year\r\n    day\r\n1\r\n    name: 'mike'\r\n    year: 2005\r\n     day: 'Mon'\r\n2\r\n    name: 'doug'\r\n    year: 2001\r\n     day: 'Fri'\r\n3\r\n    name: 'steve'\r\n    year: 1993\r\n     day: 'Wed'\r\n4\r\n    name: 'loren'\r\n    year: 1987\r\n     day: 'Fri'\r\n5\r\n    name: 'jiro'\r\n    year: 2006\r\n     day: 'Mon'\r\n6\r\n    name: 'brett'\r\n    year: 2005\r\n     day: 'Mon'\r\n7\r\n    name: 'seth'\r\n    year: 1998\r\n     day: 'Mon'\r\n<\/pre>
Let's sort the array by the field \"name\". First you need to convert to a cell array:<\/p>
Afields = fieldnames(A);\r\nAcell = struct2cell(A);\r\nsz = size(Acell)            % Notice that the this is a 3 dimensional array.<\/span>\r\n                            % For MxN structure array with P fields, the size<\/span>\r\n                            % of the converted cell array is PxMxN<\/span>\r\n<\/pre>
sz =\r\n     3     1     7\r\n<\/pre>
Once it's a cell array, you can sort using sortrows<\/tt>:<\/p>
% Convert to a matrix<\/span>\r\nAcell = reshape(Acell, sz(1), []);      % Px(MxN)<\/span>\r\n\r\n% Make each field a column<\/span>\r\nAcell = Acell';                         % (MxN)xP<\/span>\r\n\r\n% Sort by first field \"name\"<\/span>\r\nAcell = sortrows(Acell, 1)\r\n<\/pre>
Acell =\r\n  7×3 cell array\r\n    {'brett'}    {[2005]}    {'Mon'}\r\n    {'doug' }    {[2001]}    {'Fri'}\r\n    {'jiro' }    {[2006]}    {'Mon'}\r\n    {'loren'}    {[1987]}    {'Fri'}\r\n    {'mike' }    {[2005]}    {'Mon'}\r\n    {'seth' }    {[1998]}    {'Mon'}\r\n    {'steve'}    {[1993]}    {'Wed'}\r\n<\/pre>
And convert it back to a structure array:<\/p>
% Put back into original cell array format<\/span>\r\nAcell = reshape(Acell', sz);\r\n\r\n% Convert to Struct<\/span>\r\nAsorted = cell2struct(Acell, Afields, 1);\r\n\r\nfor<\/span> id = 1:length(Asorted)\r\n    fprintf('%d\\n'<\/span>,id)\r\n    disp(Asorted(id))\r\nend<\/span>\r\n<\/pre>
1\r\n    name: 'brett'\r\n    year: 2005\r\n     day: 'Mon'\r\n2\r\n    name: 'doug'\r\n    year: 2001\r\n     day: 'Fri'\r\n3\r\n    name: 'jiro'\r\n    year: 2006\r\n     day: 'Mon'\r\n4\r\n    name: 'loren'\r\n    year: 1987\r\n     day: 'Fri'\r\n5\r\n    name: 'mike'\r\n    year: 2005\r\n     day: 'Mon'\r\n6\r\n    name: 'seth'\r\n    year: 1998\r\n     day: 'Mon'\r\n7\r\n    name: 'steve'\r\n    year: 1993\r\n     day: 'Wed'\r\n<\/pre>
Simpler Approach<\/b><\/p>
Do you want an easier way of doing it? Just use Jake's nestedSortStruct<\/tt>:<\/p>
B = nestedSortStruct(A, 'name'<\/span>);\r\n\r\nisequal(B, Asorted)\r\n<\/pre>
ans =\r\n  logical\r\n   1\r\n<\/pre>
You can even sort by multiple fields. For example, let's sort by \"year\" and then by \"name\":<\/p>
C = nestedSortStruct(A, {'year'<\/span>, 'name'<\/span>});\r\n\r\nfor<\/span> id = 1:length(C)\r\n    fprintf('%d\\n'<\/span>,id)\r\n    disp(C(id))\r\nend<\/span>\r\n<\/pre>
1\r\n    name: 'loren'\r\n    year: 1987\r\n     day: 'Fri'\r\n2\r\n    name: 'steve'\r\n    year: 1993\r\n     day: 'Wed'\r\n3\r\n    name: 'seth'\r\n    year: 1998\r\n     day: 'Mon'\r\n4\r\n    name: 'doug'\r\n    year: 2001\r\n     day: 'Fri'\r\n5\r\n    name: 'brett'\r\n    year: 2005\r\n     day: 'Mon'\r\n6\r\n    name: 'mike'\r\n    year: 2005\r\n     day: 'Mon'\r\n7\r\n    name: 'jiro'\r\n    year: 2006\r\n     day: 'Mon'\r\n<\/pre>
Thanks for this entry, Jake. One recommendation I have is to include a couple of examples in the HELP text in the code.<\/p>
Edit (R2017a)<\/b><\/p>
From R2017a, you can use sortrows<\/tt> with tables<\/a> to accomplish a similar task cleanly and efficiently.<\/p>
Atable = struct2table(A);\r\nAtable_sorted = sortrows(Atable,'name'<\/span>)\r\n<\/pre>
Atable_sorted =\r\n  7×3 table\r\n      name       year      day  \r\n    _________    ____    _______\r\n    {'brett'}    2005    {'Mon'}\r\n    {'doug' }    2001    {'Fri'}\r\n    {'jiro' }    2006    {'Mon'}\r\n    {'loren'}    1987    {'Fri'}\r\n    {'mike' }    2005    {'Mon'}\r\n    {'seth' }    1998    {'Mon'}\r\n    {'steve'}    1993    {'Wed'}\r\n<\/pre>
Comments<\/b><\/p>
Give this a try and et us know what you think here<\/a> or leave a comment<\/a> for Jake.<\/p>