Adding a field to an empty struct - matlab

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).
Adding something to the struct to ensure it is not empty, adding the field, and making the struct empty again
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

Answer 2

How about
S = struct('a', {}, 'b', {}, 'c', {} );
To create an empty struct?
Another way is to use mex file with mxAddField as a workaround to the error you got:
A dot name structure assignment is illegal when the structure is empty.
Use a subscript on the structure.

Answer 3

You can use setfield to solve the problem.
S = struct('a', {}, 'b', {});
S = setfield(S, {}, 'c', [])
This results in
S =
0x0 struct array with fields:
a
b
c

Answer 4

Just to expand on #Shai's answer, here is a simple MEX-function you can use:
addfield.c
#include "mex.h"
void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[])
{
char *fieldname;
/* Check for proper number of input and output arguments */
if (nrhs != 2) {
mexErrMsgIdAndTxt("struct:nrhs", "Two inputs required.");
} else if (nlhs > 1) {
mexErrMsgIdAndTxt("struct:nlhs", "Too many output arguments.");
} else if (!mxIsStruct(prhs[0])) {
mexErrMsgIdAndTxt("struct:wrongType", "First input must be a structure.");
} else if (!mxIsChar(prhs[1]) || mxGetM(prhs[1])!=1) {
mexErrMsgIdAndTxt("struct:wrongType", "Second input must be a string.");
}
/* copy structure for output */
plhs[0] = mxDuplicateArray(prhs[0]);
/* add field to structure */
fieldname = mxArrayToString(prhs[1]);
mxAddField(plhs[0], fieldname);
mxFree(fieldname);
}
Example:
>> S = struct('a',{});
>> S = addfield(S, 'b')
S =
0x0 struct array with fields:
a
b