Adding a field to an empty struct

问题

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!

回答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

回答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.

回答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

回答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

来源：https://stackoverflow.com/questions/14830370/adding-a-field-to-an-empty-struct

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matlab

struct

variable-assignment