Is there an error in this code from https://www.learn-clojurescript.com/section-3/lesson-20-capstone-3-contact-book/. IMU assoc returns a new map and so the new map is never used an ignored?

Hey! Quick question: How can I efficiently update every nth element in a vector? As in, I want something faster than iterating over every single element. Thanks!

Loop and update with assoc on the index

(loop [coll v, i 0]
  (if (< i (count v))
    (recur (assoc coll i (inc (nth coll i))) (+ i 10))
    coll))

Increments every 10th element

Thank you, Alex!

if you want to optimize for readability I took a stab at this using partition for fun 🙂

; every third indexed element is now the last element in a nested list
user=> (partition 3 [1 2 3 4 5 6 7])
((1 2 3) (4 5 6))

; increment every 3rd element by 1
user=> (->> (partition 3 [1 2 3 4 5 6 7])
  #_=>      (map (comp inc last)))
(4 7)

I want ot replace each item in a collection if the index is a second collection with a replacement value. Is their a nicer way than this?

(let [maze [:none :none :none :none :none :none :none :none :none :none]
      path #{0 2 4 6 8}
      replacement :path]
  (map-indexed (fn [idx tile] (if (path idx) replacement tile)) maze))

(let [maze [:none :none :none :none :none :none :none :none :none :none]
      path #{0 2 4 6 8}]
  (reduce #(assoc %1 %2 :path) maze path))

caveat, use a data structure that provides the access patterns that you need. Perhaps a sparse map {0 :path 2 :path 4 :path ...} style map, or whatever data structure allows for the read and write patterns you will need.

hmmm. I have a vector that represents a 2d maze (I store it as single dimension and at the UI partition it up), I'd lose the order if I use map, I think.

Hi team, I have a situation in my code that, I need to call SQL queries for each client id and get the results, so I am writing it in reduce function, but this is costly operation, Calling database query every time, is there a way where we can improve this?