Vector iteration Mahout.

TestSequentialAccessSparseVector.java

  @Test
  public void testVectorIteration() {
    Vector vector = new SequentialAccessSparseVector(100);
    vector.set(0, 1);
    vector.set(2, 2);
    vector.set(4, 3);
    vector.set(6, 4);
    Iterator<Vector.Element> vectorIterator = vector.iterateNonZero();
    Vector.Element element = null;
    int i = 0;
    while (vectorIterator.hasNext()) {
      if (i % 2 == 0) {
        element = vectorIterator.next();
      }
      System.out.printf("%d %d %f\n", i, element.index(), element.get());
      ++i;
    }
  }

Currently you can't hold on to a copy of Element when iterating, all your references will share same offset. I'd suggest to change SequentialAccessSparseVector.java

  private final class NonDefaultElement implements Element {

    private final int offset;

      private NonDefaultElement(int offset) {
          this.offset = offset;
      }

      @Override
    public double get() {
      return values.getValues()[offset];
    }

    @Override
    public int index() {
      return values.getIndices()[offset];
    }

    @Override
    public void set(double value) {
      invalidateCachedLength();
      values.setValueAt(offset, value);
    }
  }


  private final class NonDefaultIterator extends AbstractIterator<Element> {

    private int offset = -1;

    @Override
    protected Element computeNext() {
      int numMappings = values.getNumMappings();
      if (numMappings <= 0 || offset + 1 >= numMappings) {
        return endOfData();
      }
      offset++;
      return new NonDefaultElement( offset);
    }

  }