diff --git a/eo/src/eoRanking.h b/eo/src/eoRanking.h
index 63a031071..212074aae 100644
--- a/eo/src/eoRanking.h
+++ b/eo/src/eoRanking.h
@@ -28,6 +28,7 @@
 #define eoRanking_h
 
 #include "eoPerf2Worth.h"
+#include <vector>
 
 /** An instance of eoPerfFromWorth
  *  COmputes the ranked fitness: fitnesses range in [m,M]
@@ -40,73 +41,93 @@ template <class EOT>
 class eoRanking : public eoPerf2Worth<EOT> // false: do not cache fitness
 {
 public:
-
-    using eoPerf2Worth<EOT>::value;
+  using eoPerf2Worth<EOT>::value;
 
   /* Ctor:
    @param _p selective pressure (in (1,2]
    @param _e exponent (1 == linear)
   */
-  eoRanking(double _p=2.0, double _e=1.0):
-    pressure(_p), exponent(_e) {}
+  eoRanking(double _p = 2.0, double _e = 1.0) : pressure(_p), exponent(_e), cached_pSize(0) {}
 
   /* helper function: finds index in _pop of _eo, an EOT * */
-  int lookfor(const EOT *_eo, const eoPop<EOT>& _pop)
+  /*
+  int lookfor(const EOT *_eo, const eoPop<EOT> &_pop)
+  {
+    typename eoPop<EOT>::const_iterator it;
+    for (it = _pop.begin(); it < _pop.end(); it++)
     {
-      typename eoPop<EOT>::const_iterator it;
-      for (it=_pop.begin(); it<_pop.end(); it++)
-        {
-          if (_eo == &(*it))
-            return it-_pop.begin();
-        }
-      throw eoException("Not found in eoLinearRanking");
+      if (_eo == &(*it))
+        return it - _pop.begin();
     }
+    throw eoException("Not found in eoLinearRanking");
+  }
+  */
 
   /* COmputes the ranked fitness: fitnesses range in [m,M]
      with m=2-pressure/popSize and M=pressure/popSize.
      in between, the progression depstd::ends on exponent (linear if 1).
    */
-  virtual void operator()(const eoPop<EOT>& _pop)
+  virtual void operator()(const eoPop<EOT> &_pop)
+  {
+
+    unsigned pSize = _pop.size();
+
+    if (pSize <= 1)
+      throw eoPopSizeException(pSize, "cannot do ranking with population of size <= 1");
+
+    // value() refers to the std::vector of worthes (we're in an eoParamvalue)
+    value().resize(pSize);
+
+    // Cache only if population size changed
+    if (pSize != cached_pSize)
     {
-      std::vector<const EOT *> rank;
-      _pop.sort(rank);
-      unsigned pSize =_pop.size();
-      unsigned int pSizeMinusOne = pSize-1;
-
-      if (pSize <= 1)
-        throw eoPopSizeException(pSize,"cannot do ranking with population of size <= 1");
-
-      // value() refers to the std::vector of worthes (we're in an eoParamvalue)
-      value().resize(pSize);
-
-      double beta = (2-pressure)/pSize;
-      if (exponent == 1.0)         // no need for exponetial then
-        {
-          double alpha = (2*pressure-2)/(pSize*pSizeMinusOne);
-          for (unsigned i=0; i<pSize; i++)
-            {
-              int which = lookfor(rank[i], _pop);
-              value()[which] = alpha*(pSize-i)+beta; // worst -> 1/[P(P-1)/2]
-            }
-        }
-      else                                 // exponent != 1
-        {
-          double gamma = (2*pressure-2)/pSize;
-          for (unsigned i=0; i<pSize; i++)
-            {
-              int which = lookfor(rank[i], _pop);
-              // value in in [0,1]
-              double tmp = ((double)(pSize-i))/pSize;
-              // to the exponent, and back to [m,M]
-              value()[which] = gamma*pow(tmp, exponent)+beta;
-            }
-        }
+      cached_pSize = pSize;
+      cached_pSizeMinusOne = pSize - 1;
+      cached_beta = (2 - pressure) / pSize;
+      cached_gamma = (2 * pressure - 2) / pSize;
+      cached_alpha = (2 * pressure - 2) / (pSize * cached_pSizeMinusOne);
     }
- private:
-  double pressure;	// selective pressure
+
+    std::vector<const EOT *> rank;
+    _pop.sort(rank);
+
+    // vector of indices sorted by fitness
+    std::vector<size_t> indices(pSize);
+    for (size_t i = 0; i < pSize; ++i)
+      indices[i] = i;
+
+    if (exponent == 1.0) // no need for exponetial then
+    {
+      for (unsigned i = 0; i < pSize; i++)
+      {
+        int which = indices[i];
+        ;
+        value()[which] = cached_alpha * (pSize - i) + cached_beta; // worst -> 1/[P(P-1)/2]
+      }
+    }
+    else // exponent != 1
+    {
+      for (unsigned i = 0; i < pSize; i++)
+      {
+        int which = indices[i];
+        // value in in [0,1]
+        double tmp = ((double)(pSize - i)) / pSize;
+        // to the exponent, and back to [m,M]
+        value()[which] = cached_gamma * pow(tmp, exponent) + cached_beta;
+      }
+    }
+  }
+
+private:
+  double pressure; // selective pressure
   double exponent;
+
+  // Cached values
+  unsigned cached_pSize;
+  unsigned cached_pSizeMinusOne;
+  double cached_alpha;
+  double cached_beta;
+  double cached_gamma;
 };
 
-
-
 #endif