Concatenation of variables inside a struct

Hi,

I have an array of structs with two fields on the struct. So for each element of the array I have:

% str is an array of structs
str(1).latlon = [10 20 30 40;3 4 5 6].'; % latlon is a X x 2 matrix, X is variable for each str array index
str(1).val = 200; % val is always a scalar
str(2).latlon = [50 60;3 4].';
str(2).val = 234;

latlon_all = cat(1,str.latlon);

I can concat all the latlon matrices easily with the instruction above. The problem is that I want to concat the val field in the struct with the number of rows in the latlon matrix. As a result, I would like a variable val_all to be:

val_all = [200 200 200 200 234 234];

programatically. The idea is to create the variable val_all in one shot after all the elements of the str array have been created, similarly to what it is done with latlon_all. Of course I could make loops or could modify the way the variable val is set for each element in the array during assignment, but with this situation, is there a way to do the concatenation taking into account the number of rows in the latlon matrix?

Thanks very much

Answer 1

[Stephen23]

Method one: size and repelem:

>> cnt = arrayfun(@(s)size(s.latlon,1),str);
>> out = repelem(cat(1,str.val),cnt)
out =
   200
   200
   200
   200
   234
   234

Method two: repmat and cell2mat:

>> fun = @(s)repmat(s.val,size(s.latlon,1),1);
>> out = cell2mat(arrayfun(fun,str(:),'uni',0))
out =
   200
   200
   200
   200
   234
   234

[Albert]

Hi,

Thanks for the reply. An added problem I forgot to mention is that some of the fields are empty. So that for example the following may occur:

% str is an array of structs
str(1).latlon = [10 20 30 40;3 4 5 6].'; % latlon is a X x 2 matrix, X is variable for each str array index
str(1).val = 200; % val is always a scalar
str(2).latlon = [50 60;3 4].';
str(2).val = 234;
str(3).latlon = [];
str(3).val = [];

In this case the code you proposed fails.

Thanks again

[Stephen23]

"In this case the code you proposed fails."

The first method is very easy to adapt for zero-row arrays (note the cnt>0 logical index):

>> out = repelem(cat(1,str.val),cnt(cnt>0))
out =
   200
   200
   200
   200
   234
   234

As an aside, I do not see any good reason why repelem should not accept zero replications. Seems rather inconsistent to me, as zero-length vectors are perfectly reasonable and MATLAB normally handles them without complaint.

[Albert]

With this simple modification the code works magically perfect! Excellent!

many many thanks