Add Fields to an existing Structure
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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
Accepted Answer
S.Level0=0;
f = {'SubLevel0','SubLevel1','SubLevel2','Text'};
cvar = '1234';
S = setfield(S,'Level1',f{:},cvar);
S.Level0
S.Level1.SubLevel0.SubLevel1.SubLevel2.Text
4 Comments
Image Analyst
on 11 Jun 2022
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
on 11 Jun 2022
+1 this is definitely the way to go. How it works:
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
on 12 Jun 2022
More Answers (2)
Image Analyst
on 11 Jun 2022
0 votes
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.
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.
2 Comments
Paul Mitchell
on 12 Jun 2022
Image Analyst
on 12 Jun 2022
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:
And Stephen's tutorial:
Dror Liran
on 20 Dec 2023
Edited: Dror Liran
on 24 Dec 2023
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";
3 Comments
Image Analyst
on 20 Dec 2023
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
on 24 Dec 2023
I have tried it and it works for me on matlab 2023b
Stephen23
on 24 Dec 2023
"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:
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