Time for an 2014 update…

Task/Entropy/00DESCRIPTION

@@ -5,3 +5,5 @@ Entropy is the [[wp:Expected value|expected value]] of the measure of [[wp:Self-
 where the information content <math>I(x) = -\log_{b} P(x)</math>. If the base of the logarithm <math>b = 2</math>, the result is expressed in ''bits'', a [[wp:Units of information|unit of information]]. Therefore, given a string <math>S</math> of length <math>n</math> where <math>P(s_i)</math> is the relative frequency of each character, the entropy of a string in bits is:
 :<math>H(S) = -\sum_{i=0}^n P(s_i) \log_2 (P(s_i))</math>
 For this task, use "<tt>1223334444</tt>" as an example. The result should be around 1.84644 bits.
+
+Related Task: [[Fibonacci_word]]

Task/Entropy/Emacs-Lisp/entropy-1.l

@@ -0,0 +1,15 @@
+(defun shannon-entropy (input)
+  (let ((freq-table (make-hash-table))
+	(entropy 0)
+	(length (+ (length input) 0.0)))
+    (mapcar (lambda (x)
+	      (puthash x
+		       (+ 1 (gethash x freq-table 0))
+		       freq-table))
+	    input)
+    (maphash (lambda (k v)
+	       (set 'entropy (+ entropy
+			     (* (/ v length)
+				(log (/ v length) 2)))))
+	     freq-table)
+  (- entropy)))

Task/Entropy/Emacs-Lisp/entropy-2.l

@@ -0,0 +1,2 @@
+(shannon-entropy "1223334444")
+1.8464393446710154

Task/Entropy/Groovy/entropy-1.groovy

@@ -0,0 +1,6 @@
+String.metaClass.getShannonEntrophy = {
+    -delegate.inject([:]) { map, v -> map[v] = (map[v] ?: 0) + 1; map }.values().inject(0.0) { sum, v ->
+        def p = (BigDecimal)v / delegate.size()
+        sum + p * Math.log(p) / Math.log(2)
+    }
+}

Task/Entropy/Groovy/entropy-2.groovy

@@ -0,0 +1,11 @@
+[ '1223334444': '1.846439344671',
+  '1223334444555555555': '1.969811065121',
+  '122333': '1.459147917061',
+  '1227774444': '1.846439344671',
+  aaBBcccDDDD: '1.936260027482',
+  '1234567890abcdefghijklmnopqrstuvwxyz': '5.169925004424',
+  'Rosetta Code': '3.084962500407' ].each { s, expected ->
+
+    println "Checking $s has a shannon entrophy of $expected"
+    assert sprintf('%.12f', s.shannonEntrophy) == expected
+}

Task/Entropy/REXX/entropy-2.rexx

@@ -6,7 +6,7 @@
 * 22.05.2013 Walter Pachl (I won't analyze the minor differences)
 * 25.05.2013 I did now analyze and had to discover that
 *            'my' log routine is apparently incorrect
-*            Correction is yet to come
+* 25.05.2013 problem identified & corrected
 *********************************************************************/
 Call both '1223334444'
 Call both '1223334444555555555'

Task/Entropy/Seed7/entropy.seed7

@@ -0,0 +1,29 @@
+$ include "seed7_05.s7i";
+  include "float.s7i";
+  include "math.s7i";
+
+const func float: entropy (in string: stri) is func
+  result
+    var float: entropy is 0.0;
+  local
+    var hash [char] integer: count is (hash [char] integer).value;
+    var char: ch is ' ';
+    var float: p is 0.0;
+  begin
+    for ch range stri do
+      if ch in count then
+        incr(count[ch]);
+      else
+        count @:= [ch] 1;
+      end if;
+    end for;
+    for key ch range count do
+      p := flt(count[ch]) / flt(length(stri));
+      entropy -:= p * log(p) / log(2.0);
+    end for;
+  end func ;
+
+const proc: main is func
+  begin
+    writeln(entropy("1223334444") digits 5);
+  end func;