# How to create an empty struc with fields of a given struct?

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Stefanie on 18 Mar 2014
Commented: Stephen Cobeldick on 22 Apr 2020
I have a struct A with many fields:
A.a
A.b
...
A.z
Now I want to create a struct B, with the same fields:
B.a
B.b
...
B.z
With
B=struct(A)
B has also the same size as A, and all values of A are also included. But I want to have an empty struct with 0x1 dimension.
Background: in a loop I want to sort out some entries of A, that fit a certain criteria and write it into B. If I do
B=struct([])
B(n)=A(m)
then the structs do not match. Therefore I want to create B in advance and then assign it.

#### 1 Comment

Sara on 18 Mar 2014
A is not an array, so I don't see how you can do B(n)=A(m). Do you mean you want to do something like: 1) B.f = A.a, or 2) B.a(n) = A.b(m)?

James Tursa on 18 Mar 2014
Edited: James Tursa on 18 Mar 2014
One way, which will give you a 0x0 struct, is:
f = fieldnames(A)';
f{2,1} = {};
B = struct(f{:});
Method basically obtained from this FEX submission by David Young:
CAUTION: From the description you give, it sounds like you will be dynamically increasing the size of B inside a loop. That will cause repeated copying that will slow down performance. If the number of elements of B turns out to be small, then no big deal. But if the number of elements of B turns out to be large, then you might experience a performance drag. In that case it might make sense to preallocate a size for B at the start (instead of 0x0) that is large enough to hold all the results, and then lop off the tail that you don't need after the loop is done.

Ben Oeveren on 17 Aug 2018
Edited: Ben Oeveren on 17 Aug 2018
I often use
fields = {'field1','field2','field2'}
c = cell(length(fields),1);
s = cell2struct(c,fields);

Donghoon Yeo on 23 May 2019
Easy and perfectly works for me
KAE on 20 Dec 2019
I think the line should be
fields = {'field1','field2','field3'}
So you get
s =
struct with fields:
field1: []
field2: []
field3: []

Stephen Cobeldick on 23 May 2019
Edited: Stephen Cobeldick on 23 May 2019
The simplest and most efficient solution to the question posed is to just use indexing, e.g.:
>> A(1).x = 1;
>> A(1).y = 2;
>> A(2).x = 3;
>> A(2).y = 4
A =
1x2 struct array with fields:
x
y
>> B = A([])
B =
0x0 struct array with fields:
x
y
Using indexing we can also trivially obtain exactly the size requested in the original question:
>> B = A([],1)
B =
0x1 struct array with fields:
x
y

#### 1 Comment

Joe V on 17 Apr 2020
Brilliant! Recommending MathWorks include this in the MATLAB documentation.

Binxu on 22 Apr 2020
Actually I just fond a super simple way similar to Stephen Cobeldick 's answer
It seems to me that repmat is a super useful tool for initializing many data structure.
summary =
struct with fields:
anova_F: 1.0172
anova_p: 0.4279
anova_F_bsl: 1.0156
anova_p_bsl: 0.4335
t: 8.7424
t_p: 5.9749e-18
t_CI: [2×1 double]
stats = repmat(summary,0,0);
>> repmat(summary,0,0)
ans =
0×0 empty struct array with fields:
anova_F
anova_p
anova_F_bsl
anova_p_bsl
t
t_p
t_CI

#### 1 Comment

Stephen Cobeldick on 22 Apr 2020
+1 neat solution