Monday, October 5, 2009

How to simply sum values in a Map in Scala

Here is a code snippet that sums up the values in a map in Scala:
m.foldLeft(0)(_+_._2)

It may look cryptic at first but once you get the Scala feel, you will appreciate its conciseness and elegance. Here below is the explanation:
m stands for a Map, foldLeft is a method that takes an initial value (0 here since sum is 0 at the beginning) and feeds it to its next iteration on this map entry. Note that the first _ stands for the result of previous iteration result. And _+_._2 means:
take previous result and add it to the next map entry's value, which _._2 stands for. In Scala, Map values can be accessed as Tuples. _1 is the key and _2 is the value. Here is a working example:

scala> val m = Map("one"->1,"two"->2,"three"->3)
m: scala.collection.immutable.Map[java.lang.String,Int] = Map(one -> 1, two -> 2, three -> 3)
scala> m.foldLeft(0)(_+_._2)
res0: Int = 6

Note: You can change + to * and use 1 for the initial value to get the product of values.

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