If I've got a sequence of records defined by (defrecord Person [name age]) and I want to get the record of the person with the maximum age, is there an easier way to do this than
(reduce #(if (> (:age %1) (:age %2)) %1 %2) people)
That's the only way I've figured out to do it so far, but it seems like this has to be a common enough scenario that there must be some built-in library functions that make this easier and/or more generic.
clojure.core/max-key is a suitable tool for the job.
(apply max-key :age [{:age 12} {:age 20} {:age 30}]) ;; -> {:age 30}
(last (sort-by :age [{:age 12} {:age 20} {:age 30}]))
sort-by uses compare
Related
Let's say I have several vectors
(def coll-a [{:name "foo"} ...])
(def coll-b [{:name "foo"} ...])
(def coll-c [{:name "foo"} ...])
and that I would like to see if the names of the first elements are equal.
I could
(= (:name (first coll-a)) (:name (first coll-b)) (:name (first coll-c)))
but this quickly gets tiring and overly verbose as more functions are composed. (Maybe I want to compare the last letter of the first element's name?)
To directly express the essence of the computation it seems intuitive to
(apply = (map (comp :name first) [coll-a coll-b coll-c]))
but it leaves me wondering if there's a higher level abstraction for this sort of thing.
I often find myself comparing / otherwise operating on things which are to be computed via a single composition applied to multiple elements, but the map syntax looks a little off to me.
If I were to home brew some sort of operator, I would want syntax like
(-op- (= :name first) coll-a coll-b coll-c)
because the majority of the computation is expressed in (= :name first).
I'd like an abstraction to apply to both the operator & the functions applied to each argument. That is, it should be just as easy to sum as compare.
(def coll-a [{:name "foo" :age 43}])
(def coll-b [{:name "foo" :age 35}])
(def coll-c [{:name "foo" :age 28}])
(-op- (+ :age first) coll-a coll-b coll-c)
; => 106
(-op- (= :name first) coll-a coll-b coll-c)
; => true
Something like
(defmacro -op-
[[op & to-comp] & args]
(let [args' (map (fn [a] `((comp ~#to-comp) ~a)) args)]
`(~op ~#args')))
Is there an idiomatic way to do this in clojure, some standard library function I could be using?
Is there a name for this type of expression?
For your addition example, I often use transduce:
(transduce
(map (comp :age first))
+
[coll-a coll-b coll-c])
Your equality use case is trickier, but you could create a custom reducing function to maintain a similar pattern. Here's one such function:
(defn all? [f]
(let [prev (volatile! ::no-value)]
(fn
([] true)
([result] result)
([result item]
(if (or (= ::no-value #prev)
(f #prev item))
(do
(vreset! prev item)
true)
(reduced false))))))
Then use it as
(transduce
(map (comp :name first))
(all? =)
[coll-a coll-b coll-c])
The semantics are fairly similar to your -op- macro, while being both more idiomatic Clojure and more extensible. Other Clojure developers will immediately understand your usage of transduce. They may have to investigate the custom reducing function, but such functions are common enough in Clojure that readers can see how it fits an existing pattern. Also, it should be fairly transparent how to create new reducing functions for use cases where a simple map-and-apply wouldn't work. The transducing function can also be composed with other transformations such as filter and mapcat, for cases when you have a more complex initial data structure.
You may be looking for the every? function, but I would enhance clarity by breaking it down and naming the sub-elements:
(let [colls [coll-a coll-b coll-c]
first-name (fn [coll] (:name (first coll)))
names (map first-name colls)
tgt-name (first-name coll-a)
all-names-equal (every? #(= tgt-name %) names)]
all-names-equal => true
I would avoid the DSL, as there is no need and it makes it much harder for others to read (since they don't know the DSL). Keep it simple:
(let [colls [coll-a coll-b coll-c]
vals (map #(:age (first %)) colls)
result (apply + vals)]
result => 106
I don't think you need a macro, you just need to parameterize your op function and compare functions. To me, you are pretty close with your (apply = (map (comp :name first) [coll-a coll-b coll-c])) version.
Here is one way you could make it more generic:
(defn compare-in [op to-compare & args]
(apply op (map #(get-in % to-compare) args)))
(compare-in + [0 :age] coll-a coll-b coll-c)
(compare-in = [0 :name] coll-a coll-b coll-c)
;; compares last element of "foo"
(compare-in = [0 :name 2] coll-a coll-b coll-c)
I actually did not know you can use get on strings, but in the third case you can see we compare the last element of each foo.
This approach doesn't allow the to-compare arguments to be arbitrary functions, but it seems like your use case mainly deals with digging out what elements you want to compare, and then applying an arbitrary function to those values.
I'm not sure this approach is better than the transducer version supplied above (certainly not as efficient), but I think it provides a simpler alternative when that efficiency is not needed.
I would split this process into three stages:
transform items in collections into the data in collections you want to operate
on - (map :name coll);
Operate on transformed items in collections, returning collection of results - (map = transf-coll-a transf-coll-b transf-coll-c)
Finally, selecting which result in resulting collection to return - (first calculated-coll)
When playing with collections, I try to put more than one item into collection:
(def coll-a [{:name "foo" :age 43} {:name "bar" :age 45}])
(def coll-b [{:name "foo" :age 35} {:name "bar" :age 37}])
(def coll-c [{:name "foo" :age 28} {:name "bra" :age 30}])
For example, matching items by second char in :name and returning result for items in second place:
(let
[colls [coll-a coll-b coll-c]
transf-fn (comp #(nth % 1) :name)
op =
fetch second]
(fetch (apply map op (map #(map transf-fn %) colls))))
;; => false
In transducers world you can use sequence function which also works on multiple collections:
(let
[colls [coll-a coll-b coll-c]
transf-fn (comp (map :name) (map #(nth % 1)))
op =
fetch second]
(fetch (apply sequence (map op) (map #(sequence transf-fn %) colls))))
Calculate sum of ages (for all items at the same level):
(let
[colls [coll-a coll-b coll-c]
transf-fn (comp (map :age))
op +
fetch identity]
(fetch (apply sequence (map op) (map #(sequence transf-fn %) colls))))
;; => (106 112)
Not sure what is going on here, but I have this code, where the map function successfully executes in my repl without being wrapped in a defined function:
(def dogs '({:name "scout" :age 5} {:name "rux" :age 3} {:name "fenley" :age 2}))
(def ages (atom {:above-four '() :below-four '()}))
(map
#(if (> (get-in % [:age]) 4)
(swap! ages update-in [:above-four] merge %)
(swap! ages update-in [:below-four] merge %)) dogs)
#ages
=> {:above-four ({:name "scout", :age 5}), :below-four ({:name "fenley", :age 2} {:name "rux", :age 3})}
Yet, when I define the map function as such:
(def ages (atom {:above-four '() :below-four '()}))
(def dogs '({:name "scout" :age 5} {:name "rux" :age 3} {:name "fenley" :age 2}))
(defn test-dogs []
(map
#(if (> (get-in % [:age]) 4)
(swap! ages update-in [:above-four] merge %)
(swap! ages update-in [:below-four] merge %)) dogs)
#ages)
I get the following result:
=> {:above-four (), :below-four ()}
I'm very confused, because this function taken straight from the Clojure docs works just fine:
(def m1 (atom {:a "A" :b "B"}))
(defn update-m1 []
(swap! m1 assoc :a "Aaay")
#m1)
=> {:a "Aaay", :b "B"}
Because test-dogs uses map, it returns a lazy sequence. The elements of lazy sequences aren't realized until they're needed.
The problem with your set up is you're trying to use map to run a side effect (the call to swap!; an impure action), and never actually use the result of map. Because you never request results from map, the mapping function containing swap! never runs.
By using mapv (which returns a non-lazy vector), or doseq (which is meant to carry out side effects):
(doseq [dog dogs]
(let [k (if (> (:age dog) 4)
:above-four
:below-four)]
(swap! ages update k merge dog)))
You can force the side effects to run.
I cleaned up the code a bit. The -in versions you were using were unnecessary; as was the the call to get-in. I also got rid of the redundant calls to swap!.
Note though that at least in your example, use of atoms is entirely unnecessary. Even if you have a more complicated use case, make sure their use is justified. Mutable variables just aren't as common in languages like Clojure.
I'm not quite understanding what's wrong about
(defrecord Person [name age])
(defn make-person [& opts]
(let [defaults {:age 18}]
(map->Person (merge defaults opts))))
(make-person {:name "Jim"})
=> ClassCastException clojure.lang.PersistentArrayMap cannot be cast to java.util.Map$Entry clojure.lang.APersistentMap.cons (APersistentMap.java:42)
If I do:
(map->Person (merge {:age 18} {:name "Jim"}))
I can also get the make-person function working with a non optional argument.
(defn make-person [opts]
(let [defaults {:age 18}]
(map->Person (merge defaults opts))))
The solution I've settled on for what I want to do works like:
(defn make-person
([opts] (map->Person (merge {:age 18} opts)))
([] (map->Person {:age 18})))
So I guess I'm asking, what does & really do when destructuring function arguments?
The & opts returns a sequence, which is problematic if you pass a map in.
You probably want to destructure the sequence something like:
(defn make-person [& [opts]]
(map->Person (merge {:age 18} opts))
Which lets you do (make-person) or (make-person {:opt1 "foo" :opt2 "bar"})
you could take advantage of the special syntax to allow keyword args:
(defn make-person [ & {:as opts} ]
(map->Person (merge {:age 18} opts))
Allowing you to do (make-person :opt1 "foo" :opt2 "bar") but in my experience that makes calling make-person difficult if you want to do the merging outside the call (which you will one day) (i.e if you want (make-person (merge some-opts some-other-opts)
Clojure beginner here..
If I have a set of maps, such as
(def kids #{{:name "Noah" :age 5}
{:name "George":age 3}
{:name "Reagan" :age 1.5}})
I know I can get names like this
(map :name kids)
1) How do I select a specific map? For example
I want to get back the map where name="Reagan".
{:name "Reagan" :age 1.5}
Can this be done using a filter?
2) How about returning the name where age = 3?
Yes, you can do it with filter:
(filter #(= (:name %) "Reagan") kids)
(filter #(= (:age %) 3) kids)
There's clojure.set/select:
(clojure.set/select set-of-maps #(-> % :age (= 3)))
And similarly with name and "Reagan". The return value in this case will be a set.
You could also use filter without any special preparations, since filter calls seq on its collection argument (edit: as already described by ffriend while I was typing this):
(filter #(-> % :age (= 3))) set-of-maps)
Here the return value will be a lazy seq.
If you know there will only be one item satisfying your predicate in the set, some will be more efficient (as it will not process any additional elements after finding the match):
(some #(if (-> % :age (= 3)) %) set-of-maps)
The return value here will be the matching element.
Imagine you have a map like this:
(def person {
:name {
:first-name "John"
:middle-name "Michael"
:last-name "Smith" }})
What is the idiomatic way to change values associated with both :first-name and :last-name in one expression?
(Clarification: Let's say you want to set :first-name to "Bob" and :last-name to "Doe". Let's also say that this map has some other values in it that we want to preserve, so constructing it from scratch is not an option)
Here are a couple of ways.
user> (update-in person [:name] assoc :first-name "Bob" :last-name "Doe")
{:name {:middle-name "Michael", :last-name "Doe", :first-name "Bob"}}
user> (update-in person [:name] merge {:first-name "Bob" :last-name "Doe"})
{:name {:middle-name "Michael", :last-name "Doe", :first-name "Bob"}}
user> (update-in person [:name] into {:first-name "Bob" :last-name "Doe"})
{:name {:middle-name "Michael", :last-name "Doe", :first-name "Bob"}}
user> (-> person
(assoc-in [:name :first-name] "Bob")
(assoc-in [:name :last-name] "Doe"))
{:name {:middle-name "Michael", :last-name "Doe", :first-name "Bob"}}
Edit
update-in does recursive assocs on your map. In this case it's roughly equivalent to:
user> (assoc person :name
(assoc (:name person)
:first-name "Bob"
:last-name "Doe"))
The repetition of keys becomes more and more tedious as you go deeper into a series of nested maps. update-in's recursion lets you avoid repeating keys (e.g. :name) over and over; intermediary results are stored on the stack between recursive calls. Take a look at the source for update-in to see how it's done.
user> (def foo {:bar {:baz {:quux 123}}})
#'user/foo
user> (assoc foo :bar
(assoc (:bar foo) :baz
(assoc (:baz (:bar foo)) :quux
(inc (:quux (:baz (:bar foo)))))))
{:bar {:baz {:quux 124}}}
user> (update-in foo [:bar :baz :quux] inc)
{:bar {:baz {:quux 124}}}
assoc is dynamic (as are update-in, assoc-in, and most other Clojure functions that operate on Clojure data structures). If assoc onto a map, it returns a map. If you assoc onto a vector, it returns a vector. Look at the source for assoc and take a look in in RT.java in the Clojure source for details.
I'm not sure if my case is quite the same but I had list of changes to apply:
(def updates [[[:name :first-name] "Bob"]
[[:name :last-name] "Doe"]])
In that case you can reduce over that list with assoc-in like this:
(reduce #(assoc-in %1 (first %2) (second %2)) person updates)