Adding a field to an empty struct

Question

Assuming I have a struct S of size 0x1 with the fields a and b, what is the most elegant way to add a field c to it?

Usually I am able to do it like this:

S = struct('a',0,'b',0); %1x1 struct with fields a,b
S.c = 0

However, if I receive an empty struct this does not work anymore:

S = struct('a',0,'b',0);
S(1) = []; % 0x1 struct with fields a,b
S.c = 0;
% A dot name structure assignment is illegal when the structure is empty.  
% Use a subscript on the structure.

I have thought of two ways to deal with this, but both are quite ugly and feel like workarounds rather than solutions. (Note the possibility of a non-empty struct should also be dealt with properly).

1. Adding something to the struct to ensure it is not empty, adding the field, and making the struct empty again
2. Initializing a new struct with the required fieldnames, filling it with the data from the original struct, and overwriting the original struct

I realize that it may be odd that I care about empty structs, but unfortunately part of the code that is not managed by me will crash if the fieldname does not exist. I have looked at help struct, help subsasgn and also searched for the given error message but so far I have not yet found any hints. Help is therefore much appreciated!

Answer 1

You can use deal to solve this problem:

S = struct('a',0,'b',0);
S(1) = [];

[S(:).c] = deal(0);

This results in

S = 

1x0 struct array with fields:
    a
    b
    c 

This works also for non-empty structs:

S = struct('a',0,'b',0);

[S(:).c] = deal(0);

which results in

S = 

    a: 0
    b: 0
    c: 0