How to create an empty array of structs?

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?

[Andrei Bobrov]

array=CreateAStruct(1); % The docs imply that this should work

for i=2:n

array(i)=CreateAStruct(i);

end;

[Fred Sigworth]

Yes, exactly. But why can't I start with an empty struct and fill it in, just like I can do with an array, e.g.

vec=[];

for i=1:n

vec(i)=GetValue(i);

end;

[Gustavo Delfino]

Did you ever find the answer to this question?

[Stephen23]

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.

Demonstration using Philip Borghesani's very efficient solution:

% 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

Demonstration using Andrei Bobrov's very simple solution:

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

Answer 1

[Walter Roberson]

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.

[KAE]

Walter, I believe you are thinking of this FEX contribution to "concatenate structs with dissimilar fieldnames by adding empty missing fields".

[Walter Roberson]

Not back in 2011 I wasn't; that contribution was 2015. :)

[Fred Sigworth]

Thank you! I guess I'll finally have to learn how to use arrayfun and anonymous functions :) but it looks cool.

[tommsch]

@Fred No need to learn arrayfun and anonymous functions. Those are magnitudes slower than plain for loops.

Answer 2

[Philip Borghesani]

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

[Stephen23]

+1 nice and simple. Just make sure that the struct is not defined in the workspace before the loop.

[Jeff Miller]

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

Answer 3

[Fernando Freitas Alves]

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',[],...);

Answer 4

[Sean de Wolski]

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

[Fred Sigworth]

Thank you, Sean, your code works. But when I then try to assign the whole larger struct to one element of the array it gives an error:

s.a=1;

s.b=2;

array=([]);

avals=1:10;

for ii=1:10

array(ii).a=avals(ii);

array(ii)=s; % if I leave this line out it works fine.

end;

I want to be able to do this because I want to build up an array (don't know beforehand how many elements) of structs that have 20 or so fields..

From the blog pointed to by Fangjun it's beginning to look like the only way I can do what I want is by crummy code like what I showed initially.

[Sean de Wolski]

why don't you use a cell array of structs?

[Nathaniel Jones]

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.

Answer 5

[Samuel]

It's easy. test(10,10) = struct; This creates an 10*10 empty structs.

[Walter Roberson]

This does not satisfy the original requirement that the struct entry be the result of executing CreateAStruct with argument equal to the index.

Answer 6

[Dien Nguyen]

Simple solution, use repmat to "repeat" "n" struct(s) as shown:

array = repmat(struct(field1, [], field2, [], ..., fieldN, []), n);

[Walter Roberson]

This does not satisfy the original requirement that the struct entry be the result of executing CreateAStruct with argument equal to the index.

Answer 7

[DAEHO KIM]

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

[Fred Sigworth]

Wow, fantastic! Thanks so much.

[DAEHO KIM]

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]

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.

Answer 8

[David Young]

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

[Stephen23]

This requires that the structure fields are known in advance, which is not what the question requested.

Answer 9

[Francesco Onorati]

array(n)=struct(field1, [], field2, [], ..., fieldN, []); % <-- as CreateAStruct struct
for i:n
    array(i)=CreateAStruct(var1(i), var2(i));
end

[Stephen23]

This requires that the structure fields are known in advance, which is not what the question requested.

Answer 10

[Bruno Luong]

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.

[Bruno Luong]

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

Answer 11

[Owen Claxton]

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))

[Stephen23]

@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]

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

Answer 12

[Lihan Xie]

The most simple way to get the struct array :

array=[];
for i=1:n
    st=CreateAStruct(i);
    array=[array st];
end

[Walter Roberson]

That is inefficient, as it has to keep expanding the struct array. Philip's code at https://www.mathworks.com/matlabcentral/answers/12912-how-to-create-an-empty-array-of-structs#answer_250471 is more efficient.