Fix 149 (#155)

* implement moment method for stats
* from PR discussion, the default should be to compute the raw/uncentered moments, which makes moment2 unnecessary.

docs/source/api/stats/xgi.stats.EdgeStat.rst

@@ -21,13 +21,14 @@
    .. autosummary::
       :nosignatures:
 
-      ~NodeStat.asdict
-      ~NodeStat.aslist
-      ~NodeStat.asnumpy
-      ~NodeStat.aspandas
-      ~NodeStat.max
-      ~NodeStat.mean
-      ~NodeStat.median
-      ~NodeStat.min
-      ~NodeStat.std
-      ~NodeStat.var
+      ~EdgeStat.asdict
+      ~EdgeStat.aslist
+      ~EdgeStat.asnumpy
+      ~EdgeStat.aspandas
+      ~EdgeStat.max
+      ~EdgeStat.mean
+      ~EdgeStat.median
+      ~EdgeStat.min
+      ~EdgeStat.std
+      ~EdgeStat.var
+      ~EdgeStat.moment

docs/source/api/stats/xgi.stats.NodeStat.rst

@@ -31,3 +31,4 @@
       ~NodeStat.min
       ~NodeStat.std
       ~NodeStat.var
+      ~NodeStat.moment

tests/stats/test_nodestats.py

@@ -542,3 +542,21 @@ def test_view_val(edgelist1, edgelist2):
     H = xgi.Hypergraph(edgelist2)
     assert H.nodes.degree._val == {1: 1, 2: 1, 3: 1, 4: 2, 5: 1, 6: 1}
     assert H.nodes([4, 5, 6]).degree._val == {4: 2, 5: 1, 6: 1}
+
+
+def test_moment(edgelist1, edgelist6):
+    H = xgi.Hypergraph(edgelist1)
+    deg = H.nodes.degree
+    assert round(deg.moment(), 3) == 1.375
+    assert round(deg.moment(2, center=False), 3) == 1.375
+    assert round(deg.moment(2, center=True), 3) == 0.109
+    assert round(deg.moment(3, center=False), 3) == 1.875
+    assert round(deg.moment(3, center=True), 3) == 0.082
+
+    H = xgi.Hypergraph(edgelist6)
+    deg = H.edges.size
+    assert round(deg.moment(), 3) == 9.0
+    assert round(deg.moment(2, center=False), 3) == 9.0
+    assert round(deg.moment(2, center=True), 3) == 0.0
+    assert round(deg.moment(3, center=False), 3) == 27.0
+    assert round(deg.moment(3, center=True), 3) == 0.0

xgi/stats/__init__.py

@@ -49,6 +49,7 @@
 
 import numpy as np
 import pandas as pd
+from scipy.stats import moment as spmoment
 
 from xgi.exception import IDNotFound
 
@@ -246,6 +247,20 @@ def var(self):
         """The variance of this stat."""
         return self.asnumpy().var(axis=0)
 
+    def moment(self, order=2, center=False):
+        """The statistical moments of this stat.
+
+        Parameters
+        ----------
+        order : int (default 2)
+            The order of the moment.
+        center : bool (default False)
+            Whether to compute the centered (False) or uncentered/raw (True) moment.
+
+        """
+        arr = self.asnumpy()
+        return spmoment(arr, moment=order) if center else np.mean(arr ** order)
+
     def dist(self):
         return np.histogram(self.asnumpy(), density=True)