I have some data wrapped into a MyData[data_Association] "structure".

My Association contains some big arrays and I do not want to copy them each time I modify it.

By example, if I want to add a field, does the following code

foo[MyData[data_Association]] :=
   datacpy = data;
   datacpy["extraField"] = 1;

data=<|"bigArray"->{{}}|>; (* {{}} not empty in real situation! *)


MyData[<|"bigArray" -> {{}}, "extraField" -> 1|>]

copy the "bigArray" field even if it is not modified? (or does it use some binding/reference mechanism).

  • $\begingroup$ Maybe store the associations not in an array but in a dictionary? (like dict[key]=key->value) and store dictionary globally? $\endgroup$
    – Kagaratsch
    May 7, 2019 at 19:54
  • $\begingroup$ I find the "wrapping" trick SomeSymbol[data...] very useful and I wanted to keep using this simple approach. But thanks for the suggestion. $\endgroup$ May 7, 2019 at 19:57
  • $\begingroup$ I feel this is a dupe, but feel free to object. $\endgroup$ May 10, 2019 at 12:18

3 Answers 3


I think you're asking whether the foo operation will store two independent copies of the big array. That is, given:

ds = MyData @ Association["BigArray"->RandomReal[1,10^8]];



Will your foo operation basically double the memory in use? Here is your foo function (rewritten to eliminate the superfluous Return that should never be used at the end of a Module or Block):

foo[MyData[data_Association]] := Block[{datacpy = data},
    datacpy["extraField"] = 1;





I think the above shows that the answer is no.

  • $\begingroup$ I feel really uncomfortable not knowing about Return[] usage -> I just found this mathematica.stackexchange.com/questions/58059/… Hopefully for my mood the post starts with "Return is surprisingly under-documented" $\endgroup$ May 7, 2019 at 20:26
  • 1
    $\begingroup$ Return is almost useless. It isn't really under-documented, but if you expect it to be like "return" in other languages, you'll find nothing to support such usage. $\endgroup$
    – John Doty
    May 7, 2019 at 20:40
  • $\begingroup$ @JohnDoty thanks, this is indeed the trap I fell into. $\endgroup$ May 7, 2019 at 21:00

Here's a method you could use if you really want to directly modify your association yourself. To do that, you need to store the association in a variable and use Hold attributes to prevent it from evaluating. In your case, it seems like it's easiest to just make MyData HoldFirst:

SetAttributes[MyData, HoldFirst]

You can now define your function as:

foo[MyData[data_Symbol?AssociationQ]] := (
  data["extraField"] = 1;

The AssociationQ pattern test will ensure that the symbol data refers to an association. You can use the function like so:

actualData = <|"bigArray" -> {{}}|>;

Out[15]= MyData[actualData]

Out[16]= <|"bigArray" -> {{}}, "extraField" -> 1|>

Note that MyData[actualData] does not show the association because of the HoldFirst attribute. You could define custom formatting rules to make it show a preview of the data. You could also try to turn MyData into a sort of constructor that will automatically create a new symbol to hold an association for you. For example:

SetAttributes[MyData, HoldFirst]
MyData[stuff : Except[_Symbol]] := With[{
    evaluatedStuff = stuff
   Module[{storeVar = evaluatedStuff},
   ] /; AssociationQ[evaluatedStuff]
MyData[AssociationThread[Range[10], RandomReal[1, 10]]]


<|1 -> 0.236334, 2 -> 0.354161, 3 -> 0.314371, 4 -> 0.738186, 5 -> 0.916299, 6 -> 0.0289776, 7 -> 0.831803, 8 -> 0.533609, 9 -> 0.316124, 10 -> 0.211526|>

All of this is roughly how Association itself actually works. Note the attributes on Association:

In[29]:= Attributes[Association]

{HoldAllComplete, Protected}

This is what is meant when people say that Association is a constructor and a container. You can tell that some sort of construction step takes place when you define an association:

Unevaluated[<||>] === <||>


  • $\begingroup$ very interesting and clearly explained, thanks $\endgroup$ May 8, 2019 at 15:07
  • $\begingroup$ If you are interested, I asked a related question here mathematica.stackexchange.com/q/198105/42847 $\endgroup$ May 10, 2019 at 10:47
  • $\begingroup$ You can be even slicker with this (rather than just checking for a Symbol) but checking for the NoEntry or Valid bits. These give you a way to build a constructor that can even take a Symbol as input and check that this is an Association in a very clean and efficient way. These bits can also be applied all over the place to tell that you have a proper constructed object and not just something that syntactically matches. That's a powerful property to have. $\endgroup$
    – b3m2a1
    May 10, 2019 at 19:55

It all depends on how you want to do the modification. In general, Mathematica is actually very efficient about handling pass-by-value calls. If you look at memory consumption "bigArray" will not be copied:

struct[] :=
struct[a_]@"Add"[field_ , val_] :=
  struct[Append[a, field -> val]];
struct[a_]@"Modify"[field_ , fn_] :=
  struct[ReplacePart[a, field -> fn@a[field]]];

myStruct = struct[];

Block[{arr = RandomReal[{}, {500, 500}]},
 (* done to prevent the memory getting stuck to Out *)
 myStruct = myStruct@"Add"["bigArray", arr];
 {arr // ByteCount, memPrev = MemoryInUse[]}

{2000152, 974574520}

 (* done to prevent the memory getting stuck to Out *)
 myStruct = myStruct@"Add"["empty", None];
 MemoryInUse[] - memPrev


And in fact "bigArray" is really stored as an object with an internal ID as can be seen in the many low-level functions that depend on explicit expression identity.

Of course, you could do this with a direct mutable OOP approach, too, but I won't get into that here.


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