Re: [scala-breeze] generics question

[David Hall]

So, one of the places where Breeze beats Scalala is at being generic,
in general. That said, in Breeze, Rows are not Vectors, but Matrices,
so it's easier to work on the transpose of the matrix. So that gives:


def iota[T](m: DenseMatrix[T], v: DenseVector[T]) : Int = {
for(i <- 0 until m.rows) {
val row = m.t(::, i)
if(row.equals(v))
return i
}
-1
}

iota(DenseMatrix((1.0,2.0,3.0), (4.0,5.0,6.0)), DenseVector(4.0, 5.0, 6.0))
res5: Int = 1


In Scalala the compiler error was:

scala> def iota[T](m: DenseMatrix[T], v: DenseVector[T]) : Int = {
| for(i <- 0 until m.numRows) {
| val row = m(i, ::)
| if(row.equals(v))
| return i
| }
| -1
| }
<console>:37: error: could not find implicit value for parameter bf:
scalala.generic.collection.CanSliceRow[scalala.tensor.dense.DenseMatrix[T],Int,That]
val row = m(i, ::)

Which means you need to provide the method with the CanSlice implicit:

scala> def iota[T](m: DenseMatrix[T], v: DenseVector[T])(implicit
csr: scalala.generic.collection.CanSliceRow[scalala.tensor.dense.DenseMatrix[T],Int,DenseVector[T]])
: Int = {
| for(i <- 0 until m.numRows) {
| val row = m(i, ::)
| if(row.equals(v))
| return i
| }
| -1
| }
iota: [T](m: scalala.tensor.dense.DenseMatrix[T], v:
scalala.tensor.dense.DenseVector[T])(implicit csr:
scalala.generic.collection.CanSliceRow[scalala.tensor.dense.DenseMatrix[T],Int,scalala.tensor.dense.DenseVector[T]])Int

scala> iota(DenseMatrix((1.0,2.0,3.0), (4.0,5.0,6.0)),
DenseVector(4.0, 5.0, 6.0))
res0: Int = 1

HTH,

David

On Thu, Apr 25, 2013 at 2:31 AM, Daniel Petyus <petyus...@gmail.com> wrote:
> Hi,
>
> I'm using scalala 2.9.0 but I think this would be a question for breeze to.
> I have this very simple method to find the index of a row in a matrix:
>
> def iota(m: DenseMatrix[Int], v: DenseVector[Int]) : Int = {
> for(i <- 0 until m.numRows) {
> val row = m(i, ::)
> if(row.equals(v))
> return i
> }
> -1
> }
>
> I've tried to make it generic by changing the signature to
> - def iota[T <% Numeric[T]](m: DenseMatrix[T], v: DenseVector[T]) : Int = {
> }
> or
> - def iota[T](m: DenseMatrix[T], v: DenseVector[T])(implicit num :
> Numeric[T] ) : Int = { }
>
> none of these works.
>
> Can you help me with this?
>
> Thanks,
> `Daniel
>
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[Daniel Petyus]

Hi David,

Thank you, it does help. Will check the solution in breeze.

`Daniel