From 1a89e795741f21fd9acbdd78382ac017fb40aead Mon Sep 17 00:00:00 2001
From: conrad <conrad@localhost.localdomain>
Date: Thu, 29 Aug 2019 00:41:49 +1000
Subject: [PATCH 1/4] avoid writing zero via sparse matrix iterator

---
 .../methods/cf/normalization/z_score_normalization.hpp   | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/mlpack/methods/cf/normalization/z_score_normalization.hpp b/src/mlpack/methods/cf/normalization/z_score_normalization.hpp
index e65841d77a..bd3f7bd23a 100644
--- a/src/mlpack/methods/cf/normalization/z_score_normalization.hpp
+++ b/src/mlpack/methods/cf/normalization/z_score_normalization.hpp
@@ -92,11 +92,14 @@ class ZScoreNormalization
     arma::sp_mat::iterator it_end = cleanedData.end();
     for (; it != it_end; ++it)
     {
-      *it = (*it - mean) / stddev;
+      double tmp = (*it - mean) / stddev;
+      
       // The algorithm omits rating of zero. If normalized rating equals zero,
       // it is set to the smallest positive double value.
-      if (*it == 0)
-        *it = std::numeric_limits<double>::min();
+      if (tmp == 0)
+        tmp = std::numeric_limits<double>::min();
+      
+      *it = tmp;
     }
   }
 

From 23204880a328027b456d68646115d47db99ca84e Mon Sep 17 00:00:00 2001
From: conrad <conrad@localhost.localdomain>
Date: Thu, 29 Aug 2019 00:44:49 +1000
Subject: [PATCH 2/4] added TODO to consider using .transform() instead of
 iterators

---
 src/mlpack/methods/cf/normalization/z_score_normalization.hpp | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/src/mlpack/methods/cf/normalization/z_score_normalization.hpp b/src/mlpack/methods/cf/normalization/z_score_normalization.hpp
index bd3f7bd23a..9e0d7f43d9 100644
--- a/src/mlpack/methods/cf/normalization/z_score_normalization.hpp
+++ b/src/mlpack/methods/cf/normalization/z_score_normalization.hpp
@@ -88,6 +88,8 @@ class ZScoreNormalization
     }
 
     // Subtract mean from existing rating and divide it by stddev.
+    // TODO: consider using spmat::transform() instead of spmat iterators
+    // TODO: http://arma.sourceforge.net/docs.html#transform
     arma::sp_mat::iterator it = cleanedData.begin();
     arma::sp_mat::iterator it_end = cleanedData.end();
     for (; it != it_end; ++it)

From 0c7e9decc8b5ae99b3648f2554d59d0bedc187c3 Mon Sep 17 00:00:00 2001
From: Ryan Curtin <ryan@ratml.org>
Date: Thu, 29 Aug 2019 18:14:49 -0400
Subject: [PATCH 3/4] Fix style issues.

---
 src/mlpack/methods/cf/normalization/z_score_normalization.hpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/mlpack/methods/cf/normalization/z_score_normalization.hpp b/src/mlpack/methods/cf/normalization/z_score_normalization.hpp
index 9e0d7f43d9..7296e2de6e 100644
--- a/src/mlpack/methods/cf/normalization/z_score_normalization.hpp
+++ b/src/mlpack/methods/cf/normalization/z_score_normalization.hpp
@@ -95,12 +95,12 @@ class ZScoreNormalization
     for (; it != it_end; ++it)
     {
       double tmp = (*it - mean) / stddev;
-      
+
       // The algorithm omits rating of zero. If normalized rating equals zero,
       // it is set to the smallest positive double value.
       if (tmp == 0)
         tmp = std::numeric_limits<double>::min();
-      
+
       *it = tmp;
     }
   }

From 70c531dc2d823f90bec55077eecf5b9dee9f59c3 Mon Sep 17 00:00:00 2001
From: Ryan Curtin <ryan@ratml.org>
Date: Thu, 29 Aug 2019 18:15:16 -0400
Subject: [PATCH 4/4] Update history.

---
 HISTORY.md | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/HISTORY.md b/HISTORY.md
index 183f42886c..a2e38c1056 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -33,6 +33,9 @@
 
   * Improve KDE pruning by reclaiming not used error tolerance (#1954, #1984).
 
+  * Optimizations for sparse matrix accesses in z-score normalization for CF
+    (#1989).
+
 ### mlpack 3.1.1
 ###### 2019-05-26
   * Fix random forest bug for numerical-only data (#1887).