# Dynamically update of Association

I try dynamically to add new keys and update the values of existing keys in given dataset (Association). I implement this by the following code:

test = Association[{a -> 1, b -> 2, c -> 3}]

(If[MissingQ[test[#]], AssociateTo[test, # -> 1],
AssociateTo[test, # -> (test[#] + 1)]]) & /@ {a, b, c, d, e}

{<|a -> 2, b -> 2, c -> 3|>, <|a -> 2, b -> 3, c -> 3|>, <|a -> 2,
b -> 3, c -> 4|>, <|a -> 2, b -> 3, c -> 4, d -> 1|>, <|a -> 2,
b -> 3, c -> 4, d -> 1, e -> 1|>}


But instead of updating the dataset the result is replications of the dataset with updates

## 1 Answer

Map (/@) returns the results of the iterations of AssociateTo in a list and that is confusing you. You can suppress the output by using Scan instead of Map. Actually, the output of the mapped function is not of interest to you. What matters is the value of test afterwards, as AssociateTo uses call by reference.

test = Association[{a -> 1, b -> 2, c -> 3}]
Scan[
(If[MissingQ[test[#]],
AssociateTo[test, # -> 1],
AssociateTo[test, # -> (test[#] + 1)]
]) &,
{a, b, c, d, e}
];
test


<|a -> 1, b -> 2, c -> 3|>

<|a -> 2, b -> 3, c -> 4, d -> 1, e -> 1|>

Using Scan instead of Map increases performance since for returning the intermediate results of AssociateTo requires copying them. But that is what you actually try to avoid by using AssociateTo instead of using Associate recursively. Here is an illustration of the performance difference:

n = 100000;
a = b = AssociationThread[Range[n], RandomInteger[10, n]];
rand = RandomInteger[n, n];
a == b
Scan[
(If[MissingQ[a[#]],
AssociateTo[a, # -> 1],
AssociateTo[a, # -> (a[#] + 1)]
]) &,
rand
]; // AbsoluteTiming //First
Map[
(If[MissingQ[b[#]],
AssociateTo[b, # -> 1],
AssociateTo[b, # -> (b[#] + 1)]
]) &,
rand
]; // AbsoluteTiming //First
a == b


0.30575

0.517976

True

• Thank you. It is a great solution; Scan function is parallelization function (as well as Map) then I can speed up the computation (I have a huge dataset ~500 MByte). – Kiril Danilchenko Apr 29 '18 at 10:07
• Hm. Beware: If you parallelize that, all subprocesses have to write into the same data structure (the association). That means that the subprocesses have to communicate a lot with each other in order to avoid race conditions. This can slow down parallelization so much that is even slower than unparallelized code. Try it but don't be surprised if it doesn't work. If it is really only about counting occurences of expressions in a list, you might also be interested in Tally. – Henrik Schumacher Apr 29 '18 at 10:17
• I thought so :) Anyways, if you are going to parallelize that, try to build one association per kernel and Merge the results in the end. – Henrik Schumacher Apr 29 '18 at 10:35
• thanks a lot. I agree with you that a parallelization may run slower than a sequence computation. I try to do something more interesting than to count the frequency of expressions in a list:). I want to create a bag of bigrams in given text (more specific a set of documents) and calculate the TF-IDF vector of each document. Maybe you can suggest me a good way to implement this problem? – Kiril Danilchenko Apr 29 '18 at 10:40
• Yes I can. Could you please start a new question for that, also shortly explaining what a bigram is and what you mean by TF-IDF vector? That would make it also easier for me to post code that does not into a comment. – Henrik Schumacher Apr 29 '18 at 10:48