all tasks

Task/Standard-deviation/Ruby/standard-deviation-1.rb

@@ -0,0 +1,25 @@
+class StdDevAccumulator
+  def initialize
+    @n, @sum, @sumofsquares = 0, 0.0, 0.0
+  end
+
+  def <<(num)
+    # return self to make this possible:  sd << 1 << 2 << 3 # => 0.816496580927726
+    @n += 1
+    @sum += num
+    @sumofsquares += num**2
+    self
+  end
+
+  def stddev
+    Math.sqrt( (@sumofsquares / @n) - (@sum / @n)**2 )
+  end
+
+  def to_s
+    stddev.to_s
+  end
+end
+
+sd = StdDevAccumulator.new
+i = 0
+[2,4,4,4,5,5,7,9].each {|n| puts "adding #{n}: stddev of #{i+=1} samples is #{sd << n}" }

Task/Standard-deviation/Ruby/standard-deviation-2.rb

@@ -0,0 +1,12 @@
+def sdaccum
+  n, sum, sum2 = 0, 0.0, 0.0
+  lambda do |num|
+    n += 1
+    sum += num
+    sum2 += num**2
+    Math.sqrt( (sum2 / n) - (sum / n)**2 )
+  end
+end
+
+sd = sdaccum
+[2,4,4,4,5,5,7,9].each {|n| print sd.call(n), ", "}