zhengruifeng commented on a change in pull request #26550: 
[WIP][SPARK-29334][ML][MLLIB] Support for basic vector operators
URL: https://github.com/apache/spark/pull/26550#discussion_r353057261
 
 

 ##########
 File path: mllib-local/src/main/scala/org/apache/spark/ml/linalg/Vectors.scala
 ##########
 @@ -753,6 +799,24 @@ class SparseVector @Since("2.0.0") (
     }.unzip
     new SparseVector(selectedIndices.length, sliceInds.toArray, 
sliceVals.toArray)
   }
+
+  def +(v: Vector): Vector = {
+    require(this.size == v.size)
+    v match {
+      case dv: DenseVector =>
+        val arr = dv.values.clone
+        for (i <- this.indices) arr(i) += this(i)
+        Vectors.dense(arr)
+      case sv: SparseVector =>
 
 Review comment:
   Current impl: 1, needs distinct and sort, 2, complexity = O(nnz * log(nnz))
   It maybe faster to add two `SparseVector` in this way:
   ```scala
     def add(v: SparseVector): SparseVector = {
       require(size == v.size)
       if (v.indices.isEmpty) {
         copy
       } else if (indices.isEmpty) {
         v.copy
       } else {
         val thisIndices = indices
         val thatIndices = v.indices
         val thisValues = values
         val thatValues = v.values
   
         val indicesBuilder = mutable.ArrayBuilder.make[Int]
         val valuesBuilder = mutable.ArrayBuilder.make[Double]
         var thisCur = 0
         var thatCur = 0
   
         while (thisCur < thisIndices.length && thatCur < thatIndices.length) {
           val thisIndex = thisIndices(thisCur)
           val thatIndex = thatIndices(thatCur)
           val (index, value) = if (thisIndex == thatIndex) {
             val sum = thisValues(thisCur) + thatValues(thatCur)
             thisCur += 1
             thatCur += 1
             (thisIndex, sum)
           } else if (thisIndex < thatIndex) {
             val sum = thisValues(thisCur)
             thisCur += 1
             (thisIndex, sum)
           } else {
             val sum = thatValues(thatCur)
             thatCur += 1
             (thatIndex, sum)
           }
   
           if (value != 0.0) {
             indicesBuilder += index
             valuesBuilder += value
           }
         }
   
         if (thisCur < thisIndices.length) {
           while (thisCur < thisIndices.length) {
             val index = thisIndices(thisCur)
             val value = thisValues(thisCur)
             if (value != 0.0) {
               indicesBuilder += index
               valuesBuilder += value
             }
             thisCur += 1
           }
         } else if (thatCur < thatIndices.length) {
           while (thatCur < thatIndices.length) {
             val index = thatIndices(thatCur)
             val value = thatValues(thatCur)
             if (value != 0.0) {
               indicesBuilder += index
               valuesBuilder += value
             }
             thatCur += 1
           }
         }
   
         new SparseVector(size, indicesBuilder.result(), valuesBuilder.result())
       }
     }
   ```

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