Github user mengxr commented on a diff in the pull request:

    https://github.com/apache/spark/pull/2294#discussion_r17709085
  
    --- Diff: mllib/src/main/scala/org/apache/spark/mllib/linalg/Matrices.scala 
---
    @@ -59,11 +93,113 @@ trait Matrix extends Serializable {
      */
     class DenseMatrix(val numRows: Int, val numCols: Int, val values: 
Array[Double]) extends Matrix {
     
    -  require(values.length == numRows * numCols)
    +  require(values.length == numRows * numCols, "The number of values 
supplied doesn't match the " +
    +    s"size of the matrix! values.length: ${values.length}, numRows * 
numCols: ${numRows * numCols}")
     
       override def toArray: Array[Double] = values
     
    -  private[mllib] override def toBreeze: BM[Double] = new 
BDM[Double](numRows, numCols, values)
    +  private[mllib] def toBreeze: BM[Double] = new BDM[Double](numRows, 
numCols, values)
    +
    +  private[mllib] def apply(i: Int): Double = values(i)
    +
    +  private[mllib] def apply(i: Int, j: Int): Double = values(index(i, j))
    +
    +  private[mllib] def index(i: Int, j: Int): Int = i + numRows * j
    +
    +  private[mllib] def update(i: Int, j: Int, v: Double){
    +    values(index(i, j)) = v
    +  }
    +
    +  override def copy = new DenseMatrix(numRows, numCols, values.clone())
    +}
    +
    +/**
    + * Column-majored sparse matrix.
    + * The entry values are stored in Compressed Sparse Column (CSC) format.
    + * For example, the following matrix
    + * {{{
    + *   1.0 0.0 4.0
    + *   0.0 3.0 5.0
    + *   2.0 0.0 6.0
    + * }}}
    + * is stored as `values: [1.0, 2.0, 3.0, 4.0, 5.0, 6.0]`,
    + * `rowIndices=[0, 2, 1, 0, 1, 2]`, `colPointers=[0, 2, 3, 6]`.
    + *
    + * @param numRows number of rows
    + * @param numCols number of columns
    + * @param colPtrs the index corresponding to the start of a new column
    + * @param rowIndices the row index of the entry
    + * @param values non-zero matrix entries in column major
    + */
    +class SparseMatrix(
    +    val numRows: Int,
    +    val numCols: Int,
    +    val colPtrs: Array[Int],
    +    val rowIndices: Array[Int],
    +    val values: Array[Double]) extends Matrix {
    +
    +  require(values.length == rowIndices.length, "The number of row indices 
and values don't match! " +
    +    s"values.length: ${values.length}, rowIndices.length: 
${rowIndices.length}")
    +  require(colPtrs.length == numCols + 1, "The length of the column indices 
should be the " +
    +    s"number of columns + 1. Currently, colPointers.length: 
${colPtrs.length}, " +
    +    s"numCols: $numCols")
    +
    +  override def toArray: Array[Double] = {
    +    val arr = Array.fill(numRows * numCols)(0.0)
    +    var j = 0
    +    while (j < numCols) {
    +      var i = colPtrs(j)
    +      val indEnd = colPtrs(j + 1)
    +      val offset = j * numRows
    +      while (i < indEnd) {
    +        val rowIndex = rowIndices(i)
    +        arr(offset + rowIndex) = values(i)
    +        i += 1
    +      }
    +      j += 1
    +    }
    +    arr
    +  }
    +
    +  private[mllib] def toBreeze: BM[Double] =
    +    new BSM[Double](values, numRows, numCols, colPtrs, rowIndices)
    +
    +  private[mllib] def apply(i: Int): Double = values(i)
    --- End diff --
    
    ditto: remove this method and use `.values(i)` directly


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