Attempting to modify parts of a multi-level association in-place

Question

Generate a simple two-level association

In[1]:= m2 = <|"M" -> <|1 -> {1, 2, 3}, 2 -> {2, 3}|>, "F" -> <|1 -> {}, 2 -> {}|>|>

Out[1]= <|M-><|1->{1,2,3},2->{2,3}|>,F-><|1->{},2->{}|>|>

Show that m2["M"] is an L-value (assignable) of type association

In[2]:= Head[m2["M"]]

Out[2]= Association

In[3]:= m2["M"] = <|1 -> {1}, 2 -> {1, 2}|>; m2

Out[3]= <|M-><|1->{1},2->{1,2}|>,F-><|1->{},2->{}|>|>

AssociateTo is HoldFirst, it should pass m2["M"] unevaluated to be treated as an L-value

In[4]:= Attributes[AssociateTo]

Out[4]= {HoldFirst, Protected}

In[5]:= AssociateTo[m2["M"], "b" -> 3]; m2

During evaluation of In[5]:= AssociateTo::rvalue: m2(M) is not a variable with a value, so its value cannot be changed. >>

Out[5]= <|M-><|1->{1},2->{1,2}|>,F-><|1->{},2->{}|>|>

I don't understand exactly why this fails. I am trying to do an in-place modification of the bottom level of a nested association. I was hoping to use the ideas of Use Scan vs. Map to accomplish this, but it appears that AssociateTo is behaving in an unexpected way. Is there a way to accomplish this, perhaps using "Replacing parts of held expression?".

Answer 1

You can create/assign a key/value directly by using the resetting values syntax.

m2 = <|"M" -> <|1 -> {1, 2, 3}, 2 -> {2, 3}|>, "F" -> <|1 -> {}, 2 -> {}|>|>;

m2["M"]["b"] = 3;

m2
(* <|"M" -> <|1 -> {1, 2, 3}, 2 -> {2, 3}, "b" -> 3|>, "F" -> <|1 -> {}, 2 -> {}|>|> *)

This is the last example case in the Basic Examples subsection of the Examples section of the Association documentation.

Hope this helps