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scalaoctavescala-breeze

Scala Breeze expansion matrix

In Octave/Matlab I can expand an identity matrix as follows.

>> I = eye(3)
I =
Diagonal Matrix
  1   0   0
  0   1   0
  0   0   1

>> A = [ 3 2 3 2 2 1 3 2 2 1 ]

>> E = I(:, A)
E =
   0   0   0   0   0   1   0   0   0   1
   0   1   0   1   1   0   0   1   1   0
   1   0   1   0   0   0   1   0   0   0

How can I achieve the same thing (i.e. obtain E from A, possibly using I) with Scala/Breeze ?

Solution

  • Got it. Actually very similar to Octave. 

    scala> val I = DenseMatrix.eye[Int](3)
I: breeze.linalg.DenseMatrix[Int] =
1  0  0
0  1  0
0  0  1

scala> val A = DenseMatrix(2, 1, 2, 1, 1, 0, 2, 1, 1, 0) // zero based in breeze

scala> I(::, A.toArray.toSeq)
res26: breeze.linalg.SliceMatrix[Int,Int,Int] =
0  0  0  0  0  1  0  0  0  1
0  1  0  1  1  0  0  1  1  0
1  0  1  0  0  0  1  0  0  0

    The caveats are:

    • the matrices must contain Integers
    • indices are 0 based (as opposed to 1 based in Octave/Matlab)