Merge pull request #2 from Aoishi28/AddingNum

Adding NUM class

src/NUM.py

@@ -0,0 +1,43 @@
+import math
+
+class NUM:
+    '''
+    Summarizes a stream of numbers
+    '''
+
+    def __init__(self):
+        self.n=0
+        self.mu=0
+        self.m2=0
+        self.lo=math.inf
+        self.hi=-math.inf
+
+
+    def add(self,n):
+        '''
+        Add 'n', update lo,hi and stuff needed for standard deviation
+        '''
+        if(n!='?'):
+            self.n=self.n+1
+            d=n-self.mu
+            self.mu=self.mu+d/self.n
+            self.m2=self.m2+d*(n-self.mu)
+            self.lo=math.min(n,self.lo)
+            self.hi=math.max(n,self.hi)
+
+
+    def mid(self):
+        '''
+        Return mean
+        '''
+        return self.mu
+
+    def div(self):
+        '''
+        Return standard deviation using Welford's algorithm http://t.ly/nn_W
+        '''
+        if(self.m2<0 or self.n<2):
+            return 0
+        else:
+            return math.pow((self.m2/(self.n-1)),0.5)
+