Clojure - Get key with max value ArrayMap

Question

I have a data set that looks like this:

({"1880" 5} {"1951" 6} {"1952" 5} {"1976" 10} {"1902" 7} {"1919" 7} {"1949" 12} {"1814" 4} {"1930" 11})

I am trying to get the key with the highest value. So in the case above I want to get the value "1949" back. I believe my answer lies with max-key , however I don't fully understand how max-key works. For clarity as one answer was about looking at the string value:

I want the string "1949" as the result because it has the highest number associated with it of 12

Answer 1

Just use max-key , with a function to grab the val from each map:

(def data
  [{"1880" 5} {"1951" 6} {"1952" 5} {"1976" 10} {"1902" 7} {"1919" 7} {"1949" 12} {"1814" 4} {"1930" 11}])

(apply max-key #(val (first %)) data) => {"1949" 12}

You need the first function to convert each single element map into a MapEntry . You can then use the val function to grab value out of the MapEntry:

     (first {"1880" 5})    =>  <#clojure.lang.MapEntry ["1880" 5]>

(val (first {"1880" 5}))   =>  <#java.lang.Long 5>

Be sure to bookmark The Clojure CheatSheet and peruse it often!

---

PS Why first works for this:

Note that you can convert a map into sequence of MapEntry's using either seq or vec :

some-map               => <#clojure.lang.PersistentArrayMap {:a 1, :b 2}> 

(seq some-map)         => <#clojure.lang.PersistentArrayMap$Seq ([:a 1] [:b 2])>
(vec some-map)         => <#clojure.lang.PersistentVector [[:a 1] [:b 2]]>

You then need the first item from this seq/vector, which is where first comes in:

(first (vec some-map)) => <#clojure.lang.MapEntry [:a 1]>

Note, however, that first implicitly calls seq on whatever you pass to it, so we can skip the conversion and let first implicitly convert the map into a seq of MapEntry's for us:

(first some-map)  => <#clojure.lang.MapEntry [:a 1]>

Answer 2

You can sort your list of maps by the value of each map within it.

(last (sort-by (comp second first) data))
=> {"1949" 12}

But looking at the data, I'm wondering it wouldn't just be a single map rather than a sequence of maps. So I'm going to make the assumption that your data will never have duplicate keys, and then we can work with just a single map structure and it's easier:

(into {} data)
=> {"1919" 7, "1880" 5, "1814" 4, "1902" 7, "1951" 6, "1949" 12, "1976" 10, "1930" 11, "1952" 5}

Then you can get the same answer like this:

(last (sort-by second (into {} data)))
=> ["1949" 12]

You can call first with these outputs to get just the string "1949" .

Here's another way to do it, sorting descending with a custom/reversed comparator:

(->> (into {} data)
     (sort-by second #(compare %2 %1))
     (ffirst))
 => "1949"

Answer 3

Since your keys aren't numbers (they are strings) you can't use max-key without casting to a number.

You could achieve your desired result with this:

(last (sort (mapcat keys ({"1889" 1} {"1990" 2}))))