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listscalacountdistinct-valuesfind-occurrences

How to print occurrences of numbers in Scala?

I am new to Scala. Kindly help me with the expected output

    val list =List(1,123,2,3,123,1,2) 
    val result = aList.map(x => aList.count(_==x)))
    println(result.distinct)

Expected Output: Number of occurrence of 1 is 2 Number of occurrence of 2 is 2 Number of occurrence of 123 is 2 Number of occurrence of 3 is 1

Solution

  • groupBy works well for this. It returns a Map of elements grouped by a discriminator function which in the case is the element itself also known as identity

    scala.collection.immutable.Map[Int,List[Int]] = HashMap(1 -> List(1, 1), 2 -> List(2, 2), 3 -> List(3), 123 -> List(123, 123))

    Which we can work from. Then map each key/value pair to a key and the size of the values List which gives what you are looking for.

    list.groupBy(identity).map({case(k,v) => (k, v.size)})
scala.collection.immutable.Map[Int,Int] = HashMap(1 -> 2, 2 -> 2, 3 -> 1, 123 -> 2)

    With Scala 2.13 we can use groupMapReduce as LuisMiguelMejíaSuárez notes.

    list.groupMapReduce(identity)(_ => 1)(_ + _)

    Similar to the approach above, the first parameter is given the identity function to group the values, the second parameter maps each value to 1, and the third is given a function _ + _ to add them together.

    This gives a similar answer as the original approach: scala.collection.immutable.Map[Int,Int] = Map(1 -> 2, 2 -> 2, 3 -> 1, 123 -> 2)