Getting the maximum key value pair in a Scala map by value

Question

I am trying to pull the maximum value form a map along with its key. For example:

val map = Map('a' -> 100, 'b' -> 23, ... 'z' -> 56)

Where 100 is the largest value, how would I go about pulling ('a',100)? I essentially want to use Map.max but search by value rather than key.

Solution 1

You can use maxBy with a function from the key-value pair to just the value:

val map = Map('a' -> 100, 'b' -> 23, 'z' -> 56)

map.maxBy(_._2)  // (a,100)

This is a short form for

map.maxBy { case (key, value) => value }

Solution 2

A slight modification in case the max value you are looking for is present more than once in the map:

// Find the entries with the max value in the map
val maxValue = map.maxBy(item => item._2)

// Filter the map and retain the entries with the max value
map.filter(item => item._2 == maxValue._2)

Comments

[pmaurais]

Perfect, that works! I do have to ask, where are you getting the syntax for the predicate passed to maxBy()?

[0__]

If you look into the API docs, you find for Map[A, B]: def maxBy[C](f: ((A, B)) ⇒ C)(implicit cmp: Ordering[C]): (A, B) - the doc formatting is broken, I'm afraid, so I wrote the correct form here with a fresh type parameter C. So you pass a function from the tuple of (key, value) to any other value for which an ordering exists. map.maxBy(_._2) then is just the short form for map.maxBy { case (key, value) => value }.

[Victor]

Hi, what about if 100 is present for more than one key? Is it possible to get all values instead of just one? Thanks!

[0__]

@Victor You'd determine the maximum value first and then filter the map; e.g. val map = Map('a' -> 100, 'b' -> 23, 'z' -> 56, 'c' -> 100); val maxVal = map.valuesIterator.max; map.filter(_._2 == maxVal)