MATLAB Answers

Rob Weh

Acces data in nested structure

Asked by Rob Weh
on 4 Jun 2019
Latest activity Commented on by Bjorn Gustavsson on 4 Jun 2019
Hi everyone,
I have a structure like this:
S.various_substructures.value1.x and S.various_substructures.value1.y
various_substructures are structures of varying fieldnames while the following substructures have the same names. The last structure contains arrays x and y.
I tried it with arrayfun without succes. Any idea to acces the data without a loop?
TIA, Rob


Would looping be all that bad in this case? If you need to do this in a general-usage data-reading context, maybe it is enough to just write this as very specific function(s)?
(Looks similar to what one sometimes has to go through when dealing with hd5-structures, I've had cases where I degenerated to more or less hard-coding the data extraction)
on 4 Jun 2019
The chosen representation of the data is not useful, as you see. This happens frequently in large programming jobs, because effects of the initial choice of the data structure are not clear in advance. Whenever you observe such a problem, you know that it is time for a refactoring of the complete code. If the program is clean and modular, or even better object oriented (or at least programmed in an object oriented style), there is a small number of locations, which need an adjustment of the objects.
@Jan: ...and we all have memories of thinking: "if - if only..."

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3 Answers

Answer by Stephen Cobeldick on 4 Jun 2019
Edited by Stephen Cobeldick on 4 Jun 2019
 Accepted Answer

"Any idea to acces the data without a loop?"
Not really.
You can either use an explicit for loop or an implicit loop using arrayfun or structfun:
>> S.A.value1.x = 1;
>> S.A.value1.y = 11;
>> S.B.value1.x = 2;
>> S.B.value1.y = 22;
>> S.C.value1.x = 3;
>> S.C.value1.y = 33;
>> X = structfun(@(s)s.value1.x,S)
X =
>> Y = structfun(@(s)s.value1.y,S)
Y =

  1 Comment

Rob Weh
on 4 Jun 2019
Damn, was trying structfun before but seems I messed something up... :\ Many thanks Stephen for your fast reply!!!

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Answer by Sayyed Ahmad on 4 Jun 2019

if your various_substructures is an array you can access the value like follow:
for i=1:5


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Answer by Raghunandan V on 4 Jun 2019

You can treat the variable name as string.
  • First you should store all the various_subStructures in an array(or cell) in form of string.
  • Make a loop and get the name of the variables that you want in string format.
  • Then convert the name in the form of string to variable name using commands like genvarname and eval like this
Raghunandan V


Do NOT do this!
Looping over nested structures will be simpler, more robust, and much more efficient.
Dynamically accessing variable names is one way that some beginners force themselves into writing slow, complex, buggy code that is hard to debug. Read this to know why:
Thanks, I didn't know this

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