variable parameters in Scala constructor - scala

Question

I would like to write a Matrix class in Scala from that I can instantiate objects like this:
val m1 = new Matrix( (1.,2.,3.),(4.,5.,6.),(7.,8.,9.) )
val m2 = new Matrix( (1.,2.,3.),(4.,5.,6.) )
val m3 = new Matrix( (1.,2.),(3.,4.) )
val m4 = new Matrix( (1.,2.),(3.,4.),(5.,6.) )
I have tried this:
class Matrix(a: (List[Double])* ) { }
but then I get a type mismatch because the matrix rows are not of type List[Double].
Further it would be nice to just have to type Integers (1,2,3) instead of (1.,2.,3.) but still get a double matrix.
How to solve this?
Thanks!
Malte

Answer 1

(1.0, 2.0) is a Tuple2[Double, Double] not a List[Double]. Similarly (1.0, 2.0, 3.0) is a Tuple3[Double, Double, Double].
If you need to handle a fixed number of cardinality, the simplest solution in plain vanilla scala would be to have
class Matrix2(rows: Tuple2[Double, Double]*)
class Matrix3(rows: Tuple3[Double, Double, Double]*)
and so on.
Since there exist an implicit conversion from Int to Double, you can pass a tuple of ints and it will be automatically converted.
new Matrix2((1, 2), (3, 4))
If you instead need to abstract over the row cardinality, enforcing an NxM using types, you would have to resort to some more complex solutions, perhaps using the shapeless library.
Or you can use an actual list, but you cannot restrict the cardinality, i.e. you cannot ensure that all rows have the same length (again, in vanilla scala, shapeless can help)
class Matrix(rows: List[Double]*)
new Matrix(List(1, 2), List(3, 4))
Finally, the 1. literal syntax is deprecated since scala 2.10 and removed in scala 2.11. Use 1.0 instead.

Answer 2

If you need support for very large matrices, consider using an existing implementation like Breeze. Breeze has a DenseMatrix which probably meets your requirements. For performance reasons, Breeze offloads more complex operations into native code.
Getting Matrix algebra right is a difficult exercise and unless you are specifically implementing matrix to learn/assignment, better to go with proven libraries.
Edited based on comment below:
You can consider the following design.
class Row(n : Int*)
class Matrix(rows: Row*) {...}
Usage:
val m = new Matrix(Row(1, 2, 3), Row(2,3,4))
You need to validate that all Rows are of the length and reject the input if they are not.

Answer 3

I have hacked a solution in an - I think - a bit unscala-ish way
class Matrix(values: Object*) { // values is of type WrappedArray[Object]
var arr : Array[Double] = null
val rows : Integer = values.size
var cols : Integer = 0
var _arrIndex = 0
for(row <- values) {
// if Tuple (Tuple extends Product)
if(row.isInstanceOf[Product]) {
var colcount = row.asInstanceOf[Product].productIterator.length
assert(colcount > 0, "empty row")
assert(cols == 0 || colcount == cols, "varying number of columns")
cols = colcount
if(arr == null) {
arr = new Array[Double](rows*cols)
}
for(el <- row.asInstanceOf[Product].productIterator) {
var x : Double = 0.0
if(el.isInstanceOf[Integer]) {
x = el.asInstanceOf[Integer].toDouble
}
else {
assert(el.isInstanceOf[Double], "unknown element type")
x = el.asInstanceOf[Double]
}
arr(_arrIndex) = x
_arrIndex = _arrIndex + 1
}
}
}
}
works like
object ScalaMatrix extends App {
val m1 = new Matrix((1.0,2.0,3.0),(5,4,5))
val m2 = new Matrix((9,8),(7,6))
println(m1.toString())
println(m2.toString())
}
I don't really like it. What do you think about it?