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

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.

[Sara]

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)?

Answer 1

[James Tursa]

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:

http://www.mathworks.com/matlabcentral/fileexchange/45135-emptystruct

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.

[Shrinivas Iyengar]

What a beautiful solution! Thanks a lot 😀

Answer 2

[Stephen23]

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

[Joe V]

Brilliant! Recommending MathWorks include this in the MATLAB documentation.

[Aryaman Mohapatra]

^

[Emily T. Griffiths]

Love the simplicity of this. Thanks!

Answer 3

[Ben Oeveren]

I often use

fields = {'field1','field2','field2'} 
c = cell(length(fields),1);
s = cell2struct(c,fields);

[Donghoon Yeo]

Easy and perfectly works for me

[KAE]

I think the line should be

fields = {'field1','field2','field3'}

So you get

s = 
  struct with fields:
    field1: []
    field2: []
    field3: []

Answer 4

[Binxu]

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

[Stephen23]

+1 neat solution