Pin last value to 1.0 in cumulative_distribution to address floating-point errors. (#8342)

* Renormalize cdf after summation to improve numerical stability.

* Removing numpy dependency from unit tests

* Removing numpy dependency from unit tests

* Adding missing assert to unit test

---------

Co-authored-by: Alejandro Candioti <amcandio@gmail.com>

Author: tillahoffmann

networkx/utils/random_sequence.py

@@ -131,10 +131,11 @@ def cumulative_distribution(distribution):
     """Returns normalized cumulative distribution from discrete distribution."""
 
     cdf = [0.0]
-    psum = sum(distribution)
-    for i in range(len(distribution)):
-        cdf.append(cdf[i] + distribution[i] / psum)
-    return cdf
+    cumulative = 0.0
+    for element in distribution:
+        cumulative += element
+        cdf.append(cumulative)
+    return [element / cumulative for element in cdf]
 
 
 @py_random_state(3)

networkx/utils/tests/test_random_sequence.py

@@ -47,3 +47,7 @@ def test_random_weighted_choice():
     c = nx.utils.weighted_choice(mapping, seed=1)
     c = nx.utils.weighted_choice(mapping)
     assert c == "a"
+
+
+def test_random_sequence_low_precision():
+    assert nx.utils.cumulative_distribution([0.1] * 100)[-1] == 1.0