Question

Given:

(def my-vec [{:id 0 :a "foo" :b "bar"} {:id 1 :a "baz" :b "spam"} 
             {:id 2 :a "qux" :b "fred"}])

How can I idiomatically update * the item in my-vec with :id=1 to have values :a="baz2" and :b="spam2"?

_{*: I recognize that I wouldn’t actually be updating my-vec, but really returning a new vector that is identical to my-vec except for the replacement values.}

Answer 1

map a function over the vector of maps that either creates a modified map if the key matches or uses the original if the keys don’t match then turn the result back into a vector

(vec (map #(if (= (:id %) 1) 
             (assoc % :a "baz2" :b "spam2")
             %)))

It is possible to do this more succinctly though this one really shows where the structural sharing occurs.