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evaluation - Side-effecting an array in an association?


I'm working on some classical data structures (stack, queue, etc.) and want to mimic oo style in MMA. As a first attempt, I want to store an array in an Association, like this:


q = <|elems -> ConstantArray[Null, 4]|>


<|elems -> {Null, Null, Null, Null}|>

Later, I want to side-effect my array, like this


q[elems][[2]] = 42;



Set::setps: q[elems] in the part assignment is not a symbol. >>



Ahh, yes, of course, I need a symbol... Next attempt is this:


q = Module[{storage = ConstantArray[Null, 4]},
  <|elems -> Hold[storage]|>]


<|elems -> Hold[storage$1987]|>


In[4]:= ReleaseHold[q[elems]][[2]] = 42


During evaluation of In[4]:= Set::setps: ReleaseHold[q[elems]] in the part assignment is not a symbol. >>


 42

Oh, yeah, that's not going to work.


I could do the following, but it's going to copy the array every time and defeat the purpose of implementing classical algorithms (that being "efficiency"):


q = <|elems -> ConstantArray[Null, 4]|>;

SetAttributes[setQ, HoldFirst];
setQ[q_, slot_, item_] :=
  Module[{newElems = q[elems]},
   newElems[[slot]] = item;
   q[elems] = newElems;
   q[elems]];
setQ[q, 2, 42]


{Null, 42, Null, Null}


It looks like I need some kind of variant of Part that doesn't evaluate a held symbol on its left-hand side -- a PartHoldFirst. I don't see a way to do this with stuff I know.



Answer



Maybe I don't understand what you're trying to do, but...


q = <|elems -> ConstantArray[Null, 4]|>

q[[1, 2]] = 42;

q



<|elems -> {Null, 42, Null, Null}|>

q[[Key[elems], 4]] = 99;


<|elems -> {Null, 42, Null, 99}|>

So whether you set by position or Key it works.


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