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How to create an empty array of structs?

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I would like to make a loop that accumulates an array of structures, such as
array=struct([]); % The docs imply that this should work
for i=1:n
st=CreateAStruct(i);
array(i)=st;
end;
But...this doesn't work, I get the error, "Subscripted assignment between dissimilar structures." on the first pass through the loop. Instead the only way I've found to do this is the following.
for i=1:n
st=CreateAStruct(i);
if i==1
array=st;
else
array(i)=st;
end;
end;
Is there a nicer way to do this?
  4 Comments
Gustavo Delfino
Gustavo Delfino on 4 Oct 2016
Did you ever find the answer to this question?
Stephen23
Stephen23 on 19 Oct 2021
Edited: Stephen23 on 19 Oct 2021
Several "Answers" on this thread were written without a clear understanding of the actual problem and task.
As a demonstration and simple test case I wrote this function (below). To keep it simple it must be called in monotonic sequence with step 1 or -1, i.e. either 1, 2, ... N-1, N or N, N-1, ... 2, 1.
% do not attempt to preallocate array
for k = 7:-1:1
array(k) = CreateAStruct(k);
end
display(array)
array = 1×7 struct array with fields:
CUT ARW MMB QFA RUP
array = CreateAStruct(1);
for k = 2:7
array(k) = CreateAStruct(k);
end
display(array)
array = 1×7 struct array with fields:
CAZ YIW IUG UUZ OZT
Simple test function:
function sso = CreateAStruct(itr)
nmf = 5; % number of fields
nmc = 3; % number of characters per fieldname
persistent fnm prv
if isempty(fnm) || abs(itr-prv)~=1
fnm = cellstr(char(randi([65,90],nmf,nmc)));
end
sso = cell2struct(num2cell(rand(nmf,1)),fnm,1);
prv = itr;
end

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Accepted Answer

Walter Roberson
Walter Roberson on 2 Aug 2011
You are correct, struct() is a struct with no fields, and that is a distinct structure that is not the same as a structure with any defined fields.
Workaround:
T = arrayfun(@(K) CreateAsStruct(K), 1:n, 'UniformOutput',0);
array = horzcat(T{:});
clear T
Also, if I recall correctly, there is a MATLAB File Exchange contribution to do assignment between dissimilar structures.
  4 Comments
Fred Sigworth
Fred Sigworth on 27 May 2020
Thank you! I guess I'll finally have to learn how to use arrayfun and anonymous functions :) but it looks cool.
tommsch
tommsch on 20 Jul 2020
@Fred No need to learn arrayfun and anonymous functions. Those are magnitudes slower than plain for loops.

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More Answers (11)

Philip Borghesani
Philip Borghesani on 13 Jan 2017
Actualy the simplest and fastest solution to this problem is to not attempt to create an empty struct. Run the loop backwards to allocate the full structure array on the first loop:
% do not attempt to preallocate array
for i=n:-1:1
array(i)=CreateAStruct(i);
end
  2 Comments
Stephen23
Stephen23 on 12 Jun 2017
+1 nice and simple. Just make sure that the struct is not defined in the workspace before the loop.
Jeff Miller
Jeff Miller on 25 Jan 2018
Unfortunately this doesn't work with parfor, because its range must be increasing consecutive integers. But this seems OK:
array(n)=CreateAStruct(n);
parfor i=1:n-1
array(i) = CreateAStruct(i)
end

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Fernando Freitas Alves
Fernando Freitas Alves on 27 May 2020
Edited: Fernando Freitas Alves on 27 May 2020
Since R2008a, you can do:
array = struct.empty(n,0);
Once you cannot assign dissimilar structs and this struct has no field, this is useless.
A better approach would be:
array(n,1) = struct('field1',[],'field2',[],...);

Dien Nguyen
Dien Nguyen on 11 Apr 2018
Simple solution, use repmat to "repeat" "n" struct(s) as shown:
array = repmat(struct(field1, [], field2, [], ..., fieldN, []), n);
  1 Comment
Walter Roberson
Walter Roberson on 11 Apr 2018
This does not satisfy the original requirement that the struct entry be the result of executing CreateAStruct with argument equal to the index.

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Sean de Wolski
Sean de Wolski on 2 Aug 2011
st = 1:10;
for ii = 1:10
array(ii).st = st(ii);
end
You need to set the value to a field of the struct since that's how structs are indexed. You could also look into using cell arrays:
doc cell
  3 Comments
Sean de Wolski
Sean de Wolski on 2 Aug 2011
why don't you use a cell array of structs?
Nathaniel Jones
Nathaniel Jones on 12 Jun 2017
Sean, using a cell array of structs results in the following error when attempting to assign structs as elements of the cell array:
Conversion to cell from struct is not possible.
At this point, you might want to use
cell2struct()
to convert from a cell array to an array of structs. However, Romesh's answer is a better option.

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Samuel
Samuel on 3 Dec 2013
It's easy. test(10,10) = struct; This creates an 10*10 empty structs.
  1 Comment
Walter Roberson
Walter Roberson on 11 Apr 2018
This does not satisfy the original requirement that the struct entry be the result of executing CreateAStruct with argument equal to the index.

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DAEHO KIM
DAEHO KIM on 5 Apr 2021
when I pre-allocate the struct array, I do as follows
array(1: n)= struct;
for iter= 1: n
array(iter).a= "anything"
array(iter).n= "nothing"
end
  3 Comments
DAEHO KIM
DAEHO KIM on 6 Apr 2021
Edited: DAEHO KIM on 6 Apr 2021
Thank you.
There is an application version.
% pre-allocate array structure.
array(1: n)= struct;
for iter1= 1: n
array(iter1).a= "anything";
array(iter1).n= "nothing";
% pre-allocate array2 structure in the array structure.
array(iter1).array2(1: m)= struct;
for iter2= 1: m
array(iter1).array2(iter2).e= "everything";
end
end
Stephen23
Stephen23 on 19 Oct 2021
Edited: Stephen23 on 19 Oct 2021
This requires that the structure fields are known in advance, which is not what the question requested.
Case in point: the output of DIR, whose fields have changed over different MATLAB versions.

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David Young
David Young on 21 Jan 2014
For a description of the different kinds of empty structs, and a function that allows you to create each kind easily, see my File Exchange submission emptyStruct
  1 Comment
Stephen23
Stephen23 on 25 Jan 2023
This requires that the structure fields are known in advance, which is not what the question requested.

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Francesco Onorati
Francesco Onorati on 13 Jan 2017
Edited: Francesco Onorati on 13 Jan 2017
array(n)=struct(field1, [], field2, [], ..., fieldN, []); % <-- as CreateAStruct struct
for i:n
array(i)=CreateAStruct(var1(i), var2(i));
end
  1 Comment
Stephen23
Stephen23 on 19 Oct 2021
This requires that the structure fields are known in advance, which is not what the question requested.

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Bruno Luong
Bruno Luong on 20 Jul 2020
Edited: Bruno Luong on 20 Jul 2020
Been there, done that. The most generic way I deal with such situation is like that using a function CATSTRUCT I have created (attached here).
Usage is typically like this:
cellresult = cell(1,n)
for i=1:n
% do something first
% ...
% call iteration subtask that returns a structure or structure array
cellresult{i} = myfun(i, var1, var2, etc);
% do something else
% ...
end
dim = 2; % whatever elongation of structresult you want to get
structresult = catstruct(dim, cellresult); % function mfiles attached
The function CATSTRUCTS can deal with a list of structures that are all dissimilar, so very generic possible usage. The function MYFUN is allowed to return disimilar structures from iteration to iteration. This of course have some speed penalty when structure are concatenated at the last statement compared to stock functions such as horzcat, vertcat, cat(dim, c{: )).
The solution I propose does not require to know in advance the fieldnames of the structure.
PS: TMW can inspire of my small utilities and include in their next MATLAB releases if they wish.
  1 Comment
Bruno Luong
Bruno Luong on 28 Feb 2022
Anotherway is to use the attached file AllocateStruct with the structure element has identical fileds
for i=1:n
s = myfun(i, var1, var2, etc);
if i == 1 % ~exist('sarray', 'var')
sarray = AllocateStruct(s, [1 n]);
end
sarray(i) = s;
end

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Owen Claxton
Owen Claxton on 19 Oct 2021
Minimum working example:
struct_array_col = [struct()];
struct_array_row = [struct()];
n_structs = 10;
for i = 1 : n_structs
struct_array_col(i,1).name = num2str(round(rand(1) .* 100, 3));
struct_array_row(i).name = num2str(round(rand(1) .* 100, 3));
end
disp(size(struct_array_col))
disp(size(struct_array_row))
  2 Comments
Stephen23
Stephen23 on 19 Oct 2021
Edited: Stephen23 on 19 Oct 2021
@Owen Claxton: no, this does not assign a (scalar) structure within the loop, as the original question requires.
Also: square brackets are a concatenation operator, so in your code they are completely superfluous.
Owen Claxton
Owen Claxton on 11 Nov 2021
Edited: Owen Claxton on 11 Nov 2021
Thanks Stephen, after encounting the problem actually described in the question I realised my error. Leaving my answer up just in case it helps someone. Personally, I went with the preallocation approach as I knew the struct fields (thus I could make an array with similar objects):
section_pieces = struct('type', '', 'slength', 0, 'radius', 0, 'sectionID', 0);
for i = 1 : num_sections
section_pieces(i) = sectionSpec(section_types{i}, section_lengths(i), section_radii(i), i);
% sectionSpec creates a struct with type (char array), slength (int),
% radius (int), sectionID (int) fields using some input arrays
end

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Lihan Xie
Lihan Xie on 25 Feb 2022
Edited: Lihan Xie on 25 Feb 2022
The most simple way to get the struct array :
array=[];
for i=1:n
st=CreateAStruct(i);
array=[array st];
end

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