creating new fields in multiple structures with loops

Hi all

I have some data in the form of structures, eg strA, strB e.t.c. All structures have the same fields, let's say f1 and f2. I want to create the same new fields in every structure, for example:

strA.mean = mean(strA.f1);
strA.std = std(strA.f1);

Is there a way to do this with a loop so I don't have to write the above two lines of code for each structure? I tried the following but it doesn't work:

structnames = {'strA','strB','strC','strD','strF','strG','strK'}
n = length(structnames)
for i = 1:n
    structnames{i}.mean = mean(structnames{i}.f1);
    structnames{i}.std = std(structnames{i}.f1);
end

Any ideas?

Answer 1

[Ameer Hamza]

It is never a good idea to name your variables like strA, strB, strC, ... There is a done of discussion on this topic explaining why it is an inefficient and dangerous: https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval. A better strategy is to create an array of struct accessible as str(1), str(2), str(3), ...

Although it is possible to do what you asked in your question, I will not be a part of this sin ?and only show you a way to create a struct array. After that, you can easily add a new field to all the fields to all the elements of the array.

strA.f1 = rand(1,10);
strA.f2 = rand(1,10);

strB.f1 = rand(1,10);
strB.f2 = rand(1,10);

% create the array of struct
clear str
vars = whos('str*');
str(numel(vars)) = struct('f1', [], 'f2', []);
for i=1:numel(vars)
    str(i) = eval(vars(i).name);
end

% see how easy it is now
for i=1:numel(str)
    str(i).mean = mean(str(i).f1);
    str(i).var = var(str(i).f1);
end

[Manthos Vogiatzoglou]

Dear Ameer

Thanks a lot for the answer. There is only one small problem with it. In my case the structure names explain the structure's content. Let me give you an example. What if strA is lions, strB = tigers e.t.c with fields sex, length, weight, region and others:

lions.sex = {'M','M','F','F','F','M','F','F','M','F','F','F','M'};
lions.region = [1,1,1,1,1,1,2,2,2,2,2,2,3];

tigers.sex = {'F','M','F','M','F','M','F','F','F','M','F','M','F'}
tigers.region = [1,2,2,3,3,1,2,2,2,2,2,2,1];

If I name them str(1) str(2) etc I lose the reference to context.

Anyway, thanks alot for the time you consumed to provide the answer.

[Ameer Hamza]

It that case, it is still a better strategy to create struct arrays and save the name of the animals as a field of each struct. For example, try this.

lions.sex = {'M','M','F','F','F','M','F','F','M','F','F','F','M'};
lions.region = [1,1,1,1,1,1,2,2,2,2,2,2,3];
tigers.sex = {'F','M','F','M','F','M','F','F','F','M','F','M','F'};
tigers.region = [1,2,2,3,3,1,2,2,2,2,2,2,1];

% create the array of struct
vars = {'lions', 'tigers'};
clear animals
for i=1:numel(vars)
    animals(i) = eval(vars{i});
end
% see how easy it is now
for i=1:numel(animals(i))
    animals(i).name = vars{i};
    animals(i).mean = mean(animals(i).region);
    animals(i).var = var(animals(i).region);
end

[Ameer Hamza]

Now in case, it is absolutely necessary to keep the names of variables, and you have no other choice, then you can try this.

lions.sex = {'M','M','F','F','F','M','F','F','M','F','F','F','M'};
lions.region = [1,1,1,1,1,1,2,2,2,2,2,2,3];
tigers.sex = {'F','M','F','M','F','M','F','F','F','M','F','M','F'};
tigers.region = [1,2,2,3,3,1,2,2,2,2,2,2,1];

structnames = {'lions', 'tigers'};
for i=1:numel(structnames)
    % create the actual command as a string
    eval([structnames{i} '.mean = mean(' structnames{i} '.region);']);
    eval([structnames{i} '.var = var(' structnames{i} '.region);']);
end

This will keep the variables names the same as before and add new fields, as shown inside the eval() function.

Answer 2

[Fangjun Jiang]

This is the time to use structure array, not to use strA, strB, strC, ...

Just like use array A=1:3, not to use A1=1, A2=2, A3=3.

If you had strA, strB, strC, etc. from somewhere else, then you could do

str=[strA;strB;strC] and follow the code by @Ameer Hamza