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).

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!

score 7 · Accepted Answer · answered Feb 12 '13 at 14:47

7

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

answered Feb 12 '13 at 14:47

H.Muster

9,297
1
35
46

Thanks for this nice solution. Could you perhaps explain why this works, but why `[S.c] = deal(0)` does not? – Dennis Jaheruddin Apr 05 '13 at 19:53
1

Please pardon me, but actually, I do not know why this is the case. – H.Muster Apr 05 '13 at 20:37
2

Interested readers can check my follow up question where the issue is addressed: http://stackoverflow.com/questions/16342688/difference-between-empty-matlab-struct-s-and-all-elements-s – Dennis Jaheruddin May 10 '13 at 12:14

Shai · Answer 2 · 2013-02-12T10:40:00.533

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.

edited Feb 12 '13 at 10:40

answered Feb 12 '13 at 10:33

Shai

111,146
38
238
371

I guess that I could use an if statement to see if it is indeed empty and then use this, but I still hope that there is a way to do it without the check first. – Dennis Jaheruddin Feb 12 '13 at 10:35

score 2 · Answer 3 · answered Jun 27 '15 at 08:03

2

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

answered Jun 27 '15 at 08:03

Peter Biggelaar

56
3

Very nice, I don't see this possibility in the documentation but it seems to work the same as the accepted solution. -- Whilst looking at this, I stumbled upon one of these rare methods to crash Matlab (2012b): `S = struct(); S(:)=[]; S().e=[]; S(3,3).f=[]; S().g=[];` – Dennis Jaheruddin Jul 01 '15 at 19:02
1

This code gives me `Expected one output from a curly brace or dot indexing expression, but there were 0 results.` (R2016b). – nekomatic Oct 18 '16 at 11:43

score 1 · Answer 4 · edited May 23 '17 at 12:14

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

@down-voter: why, care to comment? If there is a problem with the code, I'm happy to improve it... It works fine on my end! — Amro, Nov 03 '14 at 17:05

Adding a field to an empty struct

4 Answers4

addfield.c

Linked