Add Fields to an existing Structure

Hi,

I have structure, for example

S.Level0=0

I have a field structure I want to add to S namely

S.Level1.SubLevel0.SubLevel1.SubLevel2.Text = '1234';

where i have the above fields defined in a cell array

f = {'SubLevel0','SubLevel1','SubLevel2','Text'}

and the string cvar='1234' being the required value

is there an elegant way to produce

S.Level1.SubLevel0.SubLevel1.SubLevel2.Text='1234'

using f & cvar ?

Thank You

Answer 1

[Paul]

S.Level0=0;
f = {'SubLevel0','SubLevel1','SubLevel2','Text'};
cvar = '1234';
S = setfield(S,'Level1',f{:},cvar);
S.Level0

ans = 0

S.Level1.SubLevel0.SubLevel1.SubLevel2.Text

ans = '1234'

[Image Analyst]

Yep, still a bad idea. How are you going to refer to those fields later in your code if you don't know their names? If you knew their names, then you'd just use them. But you don't and making up names according to some strings in a cell array. Let's say you want to put the text in a plot title or something

title(S.Level1.SubLevel0.SubLevel1.SubLevel2.Text);

OK but if the string in "f" changed, then that won't work. It's not robust or general purpose.

Again, you can do this, with parentheses and dynamic field names, but it's universally frowned upon and I doubt you will see any expert MATLAB programmer do that.

[Stephen23]

+1 this is definitely the way to go. How it works:

https://www.mathworks.com/matlabcentral/answers/1656435-tutorial-comma-separated-lists-and-how-to-use-them

[Paul]

@Image Analyst

I don't know where the OP is going with this and so don't have a position on whether or not it's the right way to go. Just trying to answer the question. Having said that, getfield() can be used in a similar fashion to extract based on f.

S.Level0=0;
f = {'SubLevel0','SubLevel1','SubLevel2','Text'};
cvar = '1234';
S = setfield(S,'Level1',f{:},cvar);
plot(rand(3))
title(getfield(S.Level1,f{:}))

[Paul Mitchell]

Thank you very much for spending time to provide me with the solution. I thought I had tried everything possible !! Clearly I did not - Regards Paul

Answer 2

[Image Analyst]

Having field names that are unknown in advance, and their names are only determined by getting their name from a cell array variable sounds like a bad idea. It's probably one of the worst and most common ideas and it comes up every week.

However it is possible to do bad things in MATLAB and the FAQ will show you how to do it and explain why it's bad.

https://matlab.fandom.com/wiki/FAQ#How_can_I_create_variables_A1.2C_A2.2C....2C_A10_in_a_loop.3F

I'm sure Stephen will also chime in with additional reasons why you really don't want to do this even though you think you do.

[Paul Mitchell]

I know its bad, but the field names are unknown in advance (and could change - hence running the conversion process multiple times)

[Image Analyst]

The solution you accepted will do what you want but as you can see, to get this unknown field name you have to do some pretty cryptic stuff that makes the code almost unmaintainable. Would be better to use fixed and known names rather than dynamically varying names, as I hope someday you realize. But good luck with it. You might also like to read the FAQ:

https://matlab.fandom.com/wiki/FAQ#How_can_I_create_variables_A1,_A2,...,_A10_in_a_loop?

And Stephen's tutorial:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

Answer 3

[Dror Liran]

What you are after is called dynamic field names, the matlab documatation is here: https://www.mathworks.com/help/matlab/matlab_prog/generate-field-names-from-variables.html

Anyway the way to do it is something like:

names = ["name1" "name2"];
a.fld1 = 1;
a.fld2 = "a";
a.(names(1)) = "whatever";

[Image Analyst]

Yes, but you'll forever be referring to a.((names(1)). You can never refer to a.name1 because that field name is in a variable that presumably might change from run to run. If you did, then that means the field name never varies, and if it never varies, why not just simply use a fixed field name and not worry about the parentheses? Like

a.var = "whatever";

[Dror Liran]

I have tried it and it works for me on matlab 2023b

[Stephen23]

"I have tried it and it works for me on matlab 2023b"

It should work in R2023b, dynamic fieldnames were added more than twenty years ago in R13:

https://blogs.mathworks.com/loren/2005/12/13/use-dynamic-field-references/