1. I would say that :qm is string not a number

Your function returns the result of doall. Which realizes and returns the sequence created by for. Sequence, where each item is the result of swap!. And swap! returns the value that was swapped in. I don't know how your code returns 10505267698496 - it should return a sequence. You can replace doall with something like this:

(apply + (map #(:qm (second %)) @zimmer))

2. The code is too complex IMO, you can simplify it like this:

(reduce (fn [acc [_ {:keys [qm]}]] (+ acc qm)) 0 @zimmer)

@U6HHQ05GB Your code is incorrect. You're missing something that in the original code was [_ %2].

apply above is the shortest solution. Could be made shorter if we knew that @zimmer is a map - then second could be replaced with val.

(should also be marginally faster that way)

I don’t think so, because of [_ {:keys [qm]}]

Oh, sorry - you're right! I just woke up and read it wrong three times in a row.

Be honest I didn’t try it in REPL , I wrote it just the top of my head 😄

Thank you Guys for your suggestions. I will report my progress.

Nice, I have got two working solutions 😊.

(rf/reg-sub
  :sum-up-roomsize
  (fn []
    (let [zimmer (rf/subscribe [:entry-point-rooms])]
      (reduce (fn [acc [_ {:keys [qm]}]] (+ acc qm)) 0 @zimmer)
      )
  )
)

(rf/reg-sub
  :sum-up-roomsize
  (fn []
    (let [zimmer (rf/subscribe [:entry-point-rooms])]
      (apply + (map #(:qm (second %)) @zimmer))
      )
  )
)