Data commit

Task/Permutations-Derangements/Ruby/permutations-derangements.rb

@@ -0,0 +1,27 @@
+def derangements(n)
+  ary = (1 .. n).to_a
+  ary.permutation.select do |perm|
+    ary.zip(perm).all? {|a,b| a != b}
+  end
+end
+
+def subfact(n)
+  case n
+  when 0 then 1
+  when 1 then 0
+  else (n-1)*(subfact(n-1) + subfact(n-2))
+  end
+end
+
+puts "derangements for n = 4"
+derangements(4).each{|d|p d}
+
+puts "\n n   derange  subfact"
+(0..9).each do |n|
+  puts "%2d :%8d,%8d" % [n, derangements(n).size, subfact(n)]
+end
+
+puts "\nNumber of derangements"
+(10..20).each do |n|
+  puts "#{n} : #{subfact(n)}"
+end