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unique identifier for structure

Asked by Ahmad

Ahmad (view profile)

on 5 Dec 2012

I have structure array, let's say, like this:

s.var1 = 10;
s.var2 = rand(3,3);
s.var3 = 'my structure';

I want to associate a unique identifier to structure s. That unique identifier should change whenever I change either the structure or the values of its fields.




Ahmad (view profile)

on 5 Dec 2012

@isakson: thanks ... do you know how can I mimic it with a class?

@Robeson: By changing the structure I meant I add a new field like:

s.var4 = 'new var'

or I change the value of an already existing variable:

s.var1 = 20;

Thanks for the help guys.


Babak (view profile)

on 5 Dec 2012

My understanding is that you want to define a new variable (like a pointer in C) that when you change that variable, it changes s? I've never heard of this capability in MATLAB.

Walter Roberson

Walter Roberson (view profile)

on 5 Dec 2012

Does it need a new identifier every time it changes? Or does it just need to be a different identifier for each distinct structure?

For example, if after you do

s.var1 = 20;

you then, without otherwise using s, do

s.var1 = 30;

then does the second change need to be have a different identifier than was the case after the first change?

If you are needing this for structure uniqueness, then the identifier would only have to change if there was another copy of the structure. And that is something that can be detected at a lower level by looking at the internal pointers, due to the way that copy-on-write happens in MATLAB.

But if you are instead trying to easily track whether the structure has changed somehow relative to when something last looked at it, then there can be other methods.

In the simple case you can set a "dirty" flag in the structure when you make a change to it, and then the referencing code can just examine the "dirty" flag to see if it needs to update something, clearing the "dirty" flag afterwards. In more complex cases, sometimes it is enough to keep a "generation number" for the structure; I have used that technique.


Ahmad (view profile)



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1 Answer

Answer by Jan Simon

Jan Simon (view profile)

on 6 Dec 2012

You can use FEX: DataHash to get a fingerprint for the contents of a struct.

Walter's suggestion of a "dirty" flag would be a clear and clean solution, which is applied freuqently.


Jan Simon

Jan Simon (view profile)

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