How to update values of particular keys efficiently

Question

Say I have a list of Association,

SeedRandom[45]
test = Association[Table[i -> RandomInteger[30], {i, 6}]]

<|1 -> 18, 2 -> 19, 3 -> 12, 4 -> 28, 5 -> 22, 6 -> 3|>

How to efficiently update the values according to new rules/counts? I tried

AssociateTo[test, -Counts[{2, 1, 2, 6}]]

<|1 -> -1, 2 -> -2, 3 -> 12, 4 -> 28, 5 -> 22, 6 -> -1|>

which is obvisoult not what I want. Desired result is

<|1 -> 17, 2 -> 17, 3 -> 12, 4 -> 28, 5 -> 22, 6 -> 2|>

That is, to reduce the value of relevant keys by the number of counts.

I used -Counts[] hope it add to the old values, but it was obviously the wrong thing to use here.

Apart from setting up long winded rules to locate each key and change its value individually, I am sure there are more efficient ways to hand this?

I have long Association and needs to update quickly (efficiently).

Thanks.

Answer 1

You are looking to merge your two associations. Use Merge, and use Total to combine the paired up values from the two associations to sum them:

Merge[{test, -Counts[{2, 1, 2, 6}]}, Total]

(* Out: <|1 -> 17, 2 -> 17, 3 -> 12, 4 -> 28, 5 -> 22, 6 -> 2|> *)

To check how this works, take a look at the result of the plain combination, i.e. Merge[{test, -Counts[{2, 1, 2, 6}]}, Identity].