From time to time, I get asked or see queries about how to concatenate twostructarrays to merge the fields. There's even a section in thedocumentationcovering this topic. I thought I'd show it here to help people out.

Contents

Example Data

Suppose I've got some system for which I have collected information on a couple of individuals, including their names and ages.

s1.name ='fred'; s1.age = 42; s1(2).name ='alice'; s1(2).age = 29;

Later, I go back and collect the individual's heights (in cm).

s2.height = 170; s2(2).height = 160;

It would be great to merge these arrays now.

s1 s2

s1 = 1x2 struct array with fields: name age s2 = 1x2 struct array with fields: height

Let me collect the field names.

fn1 = fieldnames(s1); fn2 = fieldnames(s2); fn = [fn1; fn2];

Next, I ensure the fieldnames are unique.

ufn = length(fn) == unique(length(fn))

ufn = 1

Next let me convert the data in my structs into cell arrays usingstruct2cell.

c1 = struct2cell(s1) c2 = struct2cell(s2)

c1(:,:,1) = 'fred' [ 42] c1(:,:,2) = 'alice' [ 29] c2(:,:,1) = [170] c2(:,:,2) = [160]

Next I merge the data from the 2 cell arrays.

c = [c1;c2]

c(:,:,1) = 'fred' [ 42] [ 170] c(:,:,2) = 'alice' [ 29] [ 160]

And finally, I construct the new struct usingcell2struct.

s = cell2struct(c,fn,1)

s = 1x2 struct array with fields: name age height

I can check the output now.

s(1) s(2)

ans = name: 'fred' age: 42 height: 170 ans = name: 'alice' age: 29 height: 160

mergeStruct

Here's the functionmergeStructthat I created to encapsulate the steps in this process, including some error checks.

dbtypemergeStruct

1 function sout = mergestruct(varargin) 2 %MERGESTRUCT Merge structures with unique fields. 3 4 % Copyright 2009 The MathWorks, Inc. 5 6 % Start with collecting fieldnames, checking implicitly 7 % that inputs are structures. 8 fn = []; 9 for k = 1:nargin 10 try 11 fn = [fn ; fieldnames(varargin{k})]; 12 catch MEstruct 13 throw(MEstruct) 14 end 15 end 16 17 % Make sure the field names are unique. 18 if length(fn) ~= length(unique(fn)) 19 error('mergestruct:FieldsNotUnique',... 20 'Field names must be unique'); 21 end 22 23 % Now concatenate the data from each struct. Can't use 24 % structfun since input structs may not be scalar. 25 c = []; 26 for k = 1:nargin 27 try 28 c = [c ; struct2cell(varargin{k})]; 29 catch MEdata 30 throw(MEdata); 31 end 32 end 33 34 % Construct the output. 35 sout = cell2struct(c, fn, 1);

Do You Need to Merge struct data?

Do you ever need to merge data stored in structs when you gather new information? Or are your structs static in nature? If the information you collect is always the same sort of information, then you probably know the form of the structure when you start, even if all the data isn't available at first.

There iscodeon the File Exchange to concatenate structures; I confess I have not tried it, but it is highly rated. Does this post or the File Exchange contribution help your work?

Get the MATLAB code

Published with MATLAB® 7.9