在Clojure的集合中查找最长字符串的长度

Question

This seems like it should be comically easy but I somehow can't figure it out: how do you find the length of the longest string in a collection in Clojure?

eg (fn ["cat", "dog", "bear"]) => 4

Answer 1

user=> ((fn [strs] (apply max (map count strs))) ["cat" "dog" "bear"])
4

Answer 2

(max (count coll)) is counting the collection, which returns 3, and then taking the max of 3 (and nothing) results in 3. This is not what you want.

(apply max count coll) results in (max count "cat" "dog" "bear") which leads to comparing a symbol and strings with max , and that throws an exception because max expects one or more numbers.

(fn ["cat", "dog", "bear"]) is not valid syntax and results in an exception that fn does not conform to the spec. The special form fn takes as first argument a vector of symbols (binding-form) (or optionally the name and than third argument the binding-form) . Eg, (fn ident [n] n)) is a function with the name ident that takes an argument n and returns n , ((fn [n] n) 1) ;; => 1 ((fn [n] n) 1) ;; => 1 .

So taking these things in mind, one approach is to first determine the counts of the elements in the coll and then finding the max of that coll.

(def coll ["cat" "dog" "bear"]) (or use a set #{"cat" "dog" "bear"} if you don't need duplicates)

Here are a few ways to do so:

(apply max (map count coll)) ;; => 4 (apply max (map count coll)) ;; => 4 turn the collection in collection of counts [3 3 4] and apply max to get (max 3 3 4) .

(reduce max (map count coll)) ;; => 4 (reduce max (map count coll)) ;; => 4 Same as above but reduce the collection by taking max of first element only, then second element with max of previous result, and so on.

And depending on what you need you can do things like:

(group-by count coll) ;; => {3 ["cat" "dog"], 4 ["bear"]}

(sort-by count ["a" "aa" "aaa" "aa" "aaaaa" "a"]) ;; => ("a" "a" "aa" "aa" "aaa" "aaaaa")