Converting a cell array of dissimilar structs to an array of structs

In R2013b, say I have an cell array of structs like the following

% Some Mock Data
data{1} = struct('day', 'Monday', 'Temp', '35');
data{2} = struct('day', 'Tuesday', 'Temp', '34');
% Notice that data{3} has an extra field/value pair
data{3} = struct('day', 'Wednesday', 'Temp', '37', 'Comment', 'Rain');

I want to convert this into a struct array with 3 elements, but I want to "fill in" the missing field (Comment) in mystructarray(1) and mystructarray(2) with the empty array. Specifically, I want to end up with:

mystructarray(1)
ans = 
    day: 'Monday'
    Temp: '35'
    Comment: []    
mystructarray(2)
ans = 
    day: 'Tuesday'
    Temp: '34'
    Comment: []    
mystructarray(3)
ans = 
    day: 'Wednesday'
    Temp: '37'
    Comment: 'Rain'

I have tried the following, which is attractive for its readability, but it works ONLY if the 3 structures have the same field names.

mystructarray = [data{:}];

But because of the 'Comment' field in cell 3, I get the error:

Error using horzcat
Number of fields in structure arrays being concatenated do not match.
Concatenation of structure arrays requires that these arrays have the same set
of fields.

Is there an elegant way to build the desired structure array and "fill in" the missing fields to get what I want? I've tried CELLFUN but haven't gotten far with that function either.

Thank you. -Matt

Answer 1

[Geoff Hayes]

Matthew - using your example, you could first try getting a list of all the unique fields from all structures of the cell array

 >> uniqueFields = unique(char(cellfun(@(x)char(fieldnames(x)),data,...
                    'UniformOutput',false)),'rows')

 uniqueFields =

    Comment
    Temp   
    day

With the above, we create a character array of all fields within each structure (using cellfun and *fieldnames), and then treat every row as a single entity, and find those that are unique.

Now with that we should be able to iterate through the cell array, and figure out which fields are missing

 for k=1:length(data)
     for u=1:size(uniqueFields,1)
         fieldName = strtrim(uniqueFields(u,:));
         if ~isfield(data{k}, fieldName)
             data{k}.(fieldName) = [];
         end
     end
 end

And so,

 >> mystructarray = [data{:}]

 mystructarray = 

 1x3 struct array with fields:

    day
    Temp
    Comment

Try the above and see what happens!

[Matthew Crema]

Short and clever solution.

It works perfectly with the example data set and also with my actual, much larger data structures (which do not contain nested structs).

Thanks!