C# SparseMatrix.FoldByRow方法代码示例

        /// <summary>
        /// Convert one user's preference relations into position matrix
        /// See: Brun, A., Hamad, A., Buffet, O., & Boyer, A. (2010). 
        /// Towards preference relations in recommender systems. Workshop in ECML-PKDD.
        /// </summary>
        /// <param name="userPreferences"></param>
        /// <returns></returns>
        public static Vector<double> PreferencesToPositions(SparseMatrix userPreferences)
        {
            // Count for each preference type
            // Actually the original paper count the strict preferred and less preferred by exact match
            // but here we just compare with the EquallyPreferred, because the preferences can be 
            // scalar with Bradley-Terry model. However, it won't affect the result when it is discrete preference relations.
            SparseVector preferredCountByItem = SparseVector.OfEnumerable(userPreferences.FoldByRow((count, pref) =>
                    count + (pref == Config.Preferences.Preferred ? 1 : 0), 0.0));

            SparseVector lessPreferredCountByItem = SparseVector.OfEnumerable(userPreferences.FoldByRow((count, pref) =>
                    count + (pref == Config.Preferences.LessPreferred ? 1 : 0), 0.0));

            // Note that we assume the diagonal are left empty
            // otherwise the equally count needs to be offset by 1 for each, i.e. item itself does not count
            Debug.Assert(userPreferences.Trace() == 0);
            SparseVector equallyPreferredCountByItem = SparseVector.OfEnumerable(userPreferences.FoldByRow((count, pref) =>
                    count + (pref == Config.Preferences.EquallyPreferred  ? 1 : 0), 0.0));

            // Note that if the position is value zero then it won't appear in  positionByItem
            // because the use of SparseVector.OfVector() will ignore all zero values
            Vector<double> positionByItem =
     (preferredCountByItem - lessPreferredCountByItem)
     .PointwiseDivide(lessPreferredCountByItem + preferredCountByItem + equallyPreferredCountByItem) + Config.Preferences.PositionShift;

            Vector<double> indicatorOfSeenItems = userPreferences.RowSums();    // If zero, then this item has never been seen
            for(int i = 0; i < indicatorOfSeenItems.Count; i++)
            {
                if(indicatorOfSeenItems[i] == 0)
                {
                    positionByItem[i] = SparseVector.Zero;
                }
            }

            // Note that the positions do not add up to 0
            // because we shifted all values
            Debug.Assert(
                Math.Abs(positionByItem.Sum()- ((SparseVector)positionByItem).NonZerosCount * Config.Preferences.PositionShift)
                < 0.001);
            //+Config.Preferences.PositionShift;

            // TODO: May improve later. Some items have position 0 and we dont want to mix
            // up the position 0 and the 0 in sparsematrix.
            // So we use a constant to hold the space for position value 0
            // it should be reverted back when use
            //Vector<double> indicatorVector = userPreferences.RowSums();
            //for (int i = 0; i < indicatorVector.Count; i++)
            //{
                //if (indicatorVector[i] != 0 && positionByItem[i] != 0)
                //{
                //    Debug.Assert(true, "By using the PositionShift constant, we should not be in here.");
                 //   positionByItem[i] = Config.ZeroInSparseMatrix;
               // }
            //}

            return positionByItem;
        }