Francesco - 5 months ago 28

Scala Question

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.

Answer

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)))
```