BUG: fix NA handling in ewma adjustment. close pandas-dev#2128

RELEASE.rst

@@ -78,6 +78,7 @@ pandas 0.9.1
   - Raise Exception when xs view not possible of MultiIndex'd DataFrame (#2117)
   - Fix groupby(...).first() issue with datetime64 (#2133)
   - Better floating point error robustness in some rolling_* functions (#2114)
+  - Fix ewma NA handling in the middle of Series (#2128)
 
 pandas 0.9.0
 ============

pandas/src/moments.pyx

@@ -265,7 +265,10 @@ def ewma(ndarray[double_t] input, double_t com, int adjust):
     oldw = 1. - neww
     adj = oldw
 
-    output[0] = neww * input[0]
+    if adjust:
+        output[0] = neww * input[0]
+    else:
+        output[0] = input[0]
 
     for i from 1 <= i < N:
         cur = input[i]
@@ -282,10 +285,13 @@ def ewma(ndarray[double_t] input, double_t com, int adjust):
     if adjust:
         for i from 0 <= i < N:
             cur = input[i]
-            output[i] = output[i] / (1. - adj)
 
             if cur == cur:
+                output[i] = output[i] / (1. - adj)
                 adj *= oldw
+            else:
+                if i >= 1:
+                    output[i] = output[i - 1]
 
     return output
 

pandas/stats/tests/test_moments.py

@@ -299,6 +299,12 @@ def test_ewma(self):
         result = mom.ewma(arr, span=100, adjust=False).sum()
         self.assert_(np.abs(result - 1) < 1e-2)
 
+    def test_ewma_nan_handling(self):
+        s = Series([1.] + [np.nan] * 5 + [1.])
+
+        result = mom.ewma(s, com=5)
+        assert_almost_equal(result, [1] * len(s))
+
     def test_ewmvar(self):
         self._check_ew(mom.ewmvar)