Francesco
Francesco

Reputation: 413

Scala: count entries from a list in a matrix

I first need to say that I am quite new to Scala's collections' map reduce kind of functions.

What I am trying to do is counting for each row of a matrix, how many times (all) the elements of a list (in this case numbers from 0 to n) appear. The ideal output would be a matrix with the same number of rows of the input matrix and as many columns as the length of the list. For example:

val list = List(0,1,2)

val matrix = Array(
     Array(0,1,2,2),
     Array(0,0,1,0),
     Array(2,0,2,0))

in this case I would like to obtain such a matrix

1 1 2
3 1 0
2 0 2

What I came up with so far is the following:

val counts = matrix.map(r => r.map(x => (x,r.count(_ == x))).groupBy(_._1))

where the first part (up to groupBy) gives, for each row, couples of the kind (element, count). Then I tried to group this, but it doesn't quite work as I expected.

This is of course very far away from what I'd like to achieve,as it just counts the elements in the matrix (so would never give a 0).

Also, being in topic, is Array[Array[Type]] the best practice for dealing with matrices in Scala (given that no linear algebra should be applied, but rather counts and reading)?

Thank you.

EDIT: By rows was solved, but what if the same needs to be done on the columns? I guess transpose should be avoided if the matrix is very large.

Upvotes: 1

Views: 635

Answers (3)

Tzach Zohar
Tzach Zohar

Reputation: 37832

For each row r in matrix, map each item i in list into the number of its occurrences in r:

matrix.map(r => list.map(i => r.count(_ == i)))

Upvotes: 1

pltc325
pltc325

Reputation: 75

Using foldLeft:

matrix.map { row => {row.foldLeft(Array(0,0,0))((r,e)=> {if(list.contains(e)) {r(e) += 1}; r})}}

Upvotes: 0

Francesco
Francesco

Reputation: 413

In the meanwhile I came up with such solution, but I'm pretty sure there is a more compact and idiomatic way to do so.

def count_list_row(row: Array[Int], tot_elements: Int): Array[Int] ={
      val counts = new Array[Int](tot_elements)
      for(element <- 0 until tot_elements){
        counts(element) = row.count(_ == element)
      }
      return counts
    }

val counts = matrix.map(r => count_list_row(r,2))

Upvotes: 0

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