Changes in / [91:e28fc773f3c0:93:f857981306ea] in lemon-main

Files:

• lemon/random.h (modified) (1 diff)
• test/random_test.cc (modified) (1 diff)

lemon/random.h

@@ -804 +804 @@
     }  
       
+    /// Poisson distribution
+
+    /// This function generates a Poisson distribution random number with
+    /// parameter \c lambda.
+    /// 
+    /// The probability mass function of this distribusion is
+    /// \f[ \frac{e^{-\lambda}\lambda^k}{k!} \f]
+    /// \note The algorithm is taken from the book of Donald E. Knuth titled
+    /// ''Seminumerical Algorithms'' (1969). Its running time is linear in the
+    /// return value.
+    
+    int poisson(double lambda)
+    {
+      const double l = std::exp(-lambda);
+      int k=0;
+      double p = 1.0;
+      do {
+        k++;
+        p*=real<double>();
+      } while (p>=l);
+      return k-1;
+    }  
+      
     ///@}
     

test/random_test.cc

@@ -34 +34 @@
   //Does gamma work with integer k?
   a=lemon::rnd.gamma(4.0,0);
+  a=lemon::rnd.poisson(.5);
 }