Concatenating structs

October 15th, 2009

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

Example Data
mergeStruct
Do You Need to Merge struct data?

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 using struct2cell.

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 using cell2struct.

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 function mergeStruct that I created to encapsulate the steps in this process, including some error checks.

dbtype mergeStruct

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 is code on 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? Let me know here.

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Published with MATLAB® 7.9

By Loren Shure

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11 Responses to “Concatenating structs”

Ben replied on October 16th, 2009 at 07:00 UTC :

My team has used just this idea for some time now. For a while, we had our own function named merge_structs (note the underscore :-)) but we replaced our usage of it with an undocumented function that (I think) does the same thing, called setstructfields. Doing a quick find on my system, I see that the file is in this path in my MATLAB installation: $matlabroot/toolbox/signal/sigtools/setstructfields.m Any chance this could be moved into “supported and documented” side of things?

Loren replied on October 16th, 2009 at 11:29 UTC :

Ben-

I recommend you use the link on the right to make the suggestion to Technical Support.

–Loren

Pete Scotson replied on October 16th, 2009 at 11:50 UTC :

Loren,

Yes, I do need to merge structures. I have tried and customised the functions on file exchange but I usually want to merge structures in a very specific way with custom error handling etc.

Mostly I use structures as “contents addressable” containers for data. Such a format makes finding the record easy using dynamic field names in code, and works well for keyboard work (with tab completion).

tmp.age       = 42;
tmp.height    = 170;
mystruc.fred  = tmp;

tmp.age       = 29;
tmp.height    = 160;
mystruc.alice = tmp;

...

name='alice';
mystruc.(name).age

I rarely want to do statistical analysis across the records because I usually want to do computations on the data within the individual record. Structfun can be used to get the items such as the ages into an array.

Regards,
Pete

A different Ben replied on November 10th, 2009 at 16:49 UTC :

Hi Loren,

I’ve got a slight variation on the merger you’ve presented – I want to use a struct like a linked list for merger.

In your example, the heights in s2 are in the same order as the entries of s1. In my problem, I may not know the order of the second structure, and want to use one value as an index to add to the first structure. That is, given a structure that is like this:

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

and a second structure like this:

s2.name = 'alice';
s2.height = 160;
s2(2).name = 'fred';
s2(2).height = 170;

how can I use the name to find the appropriate height to add from s2 to the correct substructure of s1? I ‘d like to end up with a structure like this:

s1.name = 'fred';
s1.age = 42;
s1.height = 170;
s1(2).name = 'alice';
s1(2).age = 29;
s1(2).height = 160;

Is there an easy way to do such a thing?

Loren replied on November 10th, 2009 at 19:05 UTC :

Hello A Different Ben!

There is nothing ready to go for you, but there are some tools that should make your life easier. ismember is the main one that occurs to me now.

Here I sketch my thought process in how to do this. It might not be the most efficient, but it has all the right steps, I think.

Either know which fields are in common, e.g., .name, or write a little code to find the overlaps. For the overlap ones, you need to get the values for both the first and second structs, and you can then use ismember with multiple outputs to figure out which locations in struct2 match the list in struct1. You can then reorder the struct2 values according to the ismember match indices. You can then delete the matching field (e.g., name) from struct. And finally, merge struct2 into struct1.

Good luck!

–Loren

Neil replied on December 26th, 2009 at 23:04 UTC :

Loren, Helpful, yes. I use structures to hold configuration data, with some fields ignored, all the time. Liked your posts on concatenating structs & using empty arrays to trigger use of defaults. I went a different way. I tend to use structs of configuration parameters. In my cases, I want the provided fields to replace the default fields & the default ones to persist if not replaced. Functions know what to look for, if extra fields are there, simple function don’t look for them. More complicated function look for additional fields, eg nested configuration structures to pass on. Hence wrote a structureUnionRecursive. But, I have some issues. I’ll try to post on the forum & see if I’m being silly, or any others have some of my issues.

Guangping Zhang replied on March 28th, 2010 at 23:29 UTC :

I put this post in your another blog, but I find this blog fit my question better. So I put it again.

1X1 structure S contain M1, M2, M3….M30 which has various length, such as 54, 32, 61…42.
I write code to get the length of each M.
If I write
fl(1)=length(S.(fS{1}));
fl(2)=length(S.(fS{2}));
fl(3)=length(S.(fS{3}));
……….
I will get correct length.
But if I using for loop below, answer is wrong, all equal 0.

fn=fieldnames(S); % all the field name was put into cell fn;
n=length(fn); % get fn length;
for i=1:n %get each Mi’s length;
fl(i)=length(S.(fS{i}));
end

Dear Loren, could you please help me? Why the loop don’t work?

Loren replied on March 29th, 2010 at 00:50 UTC :

Guangping-

This works for me:


%%
S.M1 = rand(1,54)
S.M2 = rand(1,91)
S.M3 = rand(1,31)
%%
fS = fieldnames(S);
fl(1)=length(S.(fS{1}));
fl(2)=length(S.(fS{2}));
fl(3)=length(S.(fS{3}));
%%
fn = fieldnames(S);
n = length(fn);
fl = zeros(n,1);
%%
for i=1:n %get each Mi’s length;
    fl(i)=length(S.(fS{i}));
end

I wonder if you have fS defined correctly since you don’t show it.

–loren

Guangping Zhang replied on March 29th, 2010 at 03:02 UTC :

Loren, thank you, I fixed it out.

Guangping,

Davis Bennett replied on January 5th, 2012 at 18:06 UTC :

Loren, thanks for this guide. I wonder, when you check for uniqueness of the fieldnames with

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

do you mean to use

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

Loren Shure replied on January 5th, 2012 at 18:45 UTC :

Davis-

You are right – sorry about that!

–loren

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October 15th, 2009