Add a field to an existing structure

I have a structure called "Events", containing a field called "Efix" for each trial in my data. Efix contains a subfield called "duration" and I want to add a field that copies this duration data. I'd like the new field to be called "duration_copy".

I've tried lots of different iterations, some of which are:

>> Events.Efix.duration_copy = Events.Efix.duration;
>> Events.Efix(:).duration_copy = Events.Efix(:).duration;
>> Events.Efix.duration_copy = Events.Efix.duration{:};

For these, I receive the error: "Expected one output from a curly brace or dot indexing expression, but there were 6 results."

I also tried:

>> [Events.Efix(:).duration_copy] = Events.Efix.duration; 
>> [Events.Efix(:).duration_copy] = Events.Efix(:).duration;
>> [Events.Efix(:).duration_copy] = Events.Efix{:}.duration;

For these, I receive the error: "Scalar structure required for this assignment."

What code would accomplish this?

Edited to add screenshots:

Answer 1

[madhan ravi]

https://www.mathworks.com/matlabcentral/answers/379250-adding-a-new-field-to-a-struct-array#answer_302008

[Al_G]

Hi Madhan,

I saw this answer before I posted and tried it with my own structure:

[Events.Efix.(duration_copy)] = Events.Efix.duration{:};

And received the error: "Undefined function or variable 'duration_copy'."

I also tried:

[Events.Efix.duration_copy] = Events.Efix.duration{:};

Received the error: "Scalar structure required for this assignment."

Is there an error in the way I've written my code?

[Walter Roberson]

[Events.Efix.duration_copy] = Events.Efix.duration;

[Al_G]

Hi Walter,

I again get the error "Scalar structure required for this assignment."

Any idea what could be the problem and how to resolve it?

Edit: I should add, I tried just putting the "Events.Efix.duration" data into a test variable:

>> a = Events.Efix.duration;

And I get an error: "Expected one output from a curly brace or dot indexing expression, but there were 6 results."

[Stephen23]

@Al_G: read these to know what is going on with your code:

https://www.mathworks.com/help/matlab/matlab_prog/comma-separated-lists.html

https://www.mathworks.com/matlabcentral/answers/320713-how-to-operate-on-comma-separated-lists

[Walter Roberson]

Which MATLAB release are you using? In R2018b,

>> A.B = struct('address', {'abc', 'pqr', 'stu'})

A =

struct with fields:

B: [1×3 struct]

>> A.B.address

ans =

'abc'

ans =

'pqr'

ans =

'stu'

>> [A.B.address_copy] = A.B.address

A =

struct with fields:

B: [1×3 struct]

[Al_G]

Hi Walter,

I'm using 2015b

[Al_G]

I updated to 2018b, and get the same error: "Scalar structure required for this assignment."

I also tried to step through the same steps you show to compare outputs:

>> Events

Events = 

  1×6 struct array with fields:

    Messages
    Efix
    Esacc
    Eblink
    

But there's again an error when I try to duplicate your step below:

>> A.B.address
ans =
    'abc'
ans =
    'pqr'
ans =
    'stu'

When I do it, I get:

    >> Events.Efix.duration
 Expected one output from a curly brace or dot indexing expression, but there were 6 results.

Is the issue that "Events" has other fields in addition to "Efix"? What am I missing here?

Screenshots:

[Walter Roberson]

The difference is that your top level structure is non-scalar, each entry of which contains a field Efix which all happen to be scalar struct (it looks like.) The test I show was for the case of a scalar top-level struct that had a non-scalar struct Efix inside it.

The only way that might work without a loop for you is https://www.mathworks.com/help/matlab/ref/setfield.html setfield(), but my reading of https://www.mathworks.com/help/matlab/ref/setfield.html#mw_412d6afc-f292-45a8-95be-f96d6bae8fb9

Indices for S and fields 1 through N-1 specify individual elements of structure arrays. Indices for field N specify one or more elements of the array in that field, which can be of any type.

is that you cannot do it one setfield call -- because in order to do it for nonscalar S your first index would have to be to specify something that was not an individual element of the array, which is not permitted.

So I think you are going to need to use a loop.

[Al_G]

Ah! Ok, that makes some sense. I'm self-taught with matlab (and programming), and still learning about its intricaies. I couldn't put my finger on what the difference was until you explained it. Many thanks, Walter!

For anyone running into a similar problem, this worked for me:

for iTrial=1:6
    Events(iTrial).Efix.duration_copy = Events(iTrial).Efix.duration; 
end