[mlpack] 29/37: Fix usage of log2(), which does not exist in C99.

[Barak A. Pearlmutter]

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bap pushed a commit to tag mlpack-1.0.10
in repository mlpack.

commit b5cdc90aa77c7d1cca5fd1706a082066415f9f97
Author: Ryan Curtin <ryan at ratml.org>
Date:   Fri Aug 29 14:40:20 2014 +0000

    Fix usage of log2(), which does not exist in C99.
---
 src/mlpack/methods/decision_stump/decision_stump_impl.hpp | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/mlpack/methods/decision_stump/decision_stump_impl.hpp b/src/mlpack/methods/decision_stump/decision_stump_impl.hpp
index a042b21..7ed5130 100644
--- a/src/mlpack/methods/decision_stump/decision_stump_impl.hpp
+++ b/src/mlpack/methods/decision_stump/decision_stump_impl.hpp
@@ -428,14 +428,17 @@ double DecisionStump<MatType>::CalculateEntropy(arma::subview_row<LabelType> lab
   for (j = 0; j < labels.n_elem; j++)
     numElem(labels(j))++;
 
+  // The equation for entropy uses log2(), but log2() is from C99 and thus
+  // Visual Studio will not have it.  Therefore, we will use std::log(), and
+  // then apply the change-of-base formula at the end of the calculation.
   for (j = 0; j < numClass; j++)
   {
     const double p1 = ((double) numElem(j) / labels.n_elem);
 
-    entropy += (p1 == 0) ? 0 : p1 * log2(p1);
+    entropy += (p1 == 0) ? 0 : p1 * std::log(p1);
   }
 
-  return entropy;
+  return entropy / std::log(2.0);
 }
 
 }; // namespace decision_stump

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