ENH: Add reduce method to polynomial.Chebyshev and

polynomial.Polynomial. This method behaves like truncate except
it takes the degree of the result instead of the number of
coefficients.

3 files changed, 50 insertions, 1 deletions

diff --git a/numpy/polynomial/polytemplate.py b/numpy/polynomial/polytemplate.py
index bf94d28ec..706f22c2d 100644
--- a/numpy/polynomial/polytemplate.py
+++ b/numpy/polynomial/polytemplate.py
@@ -304,9 +304,42 @@ class $name(pu.PolyBase) :
     #
 
     def degree(self) :
-        """The degree of the series."""
+        """The degree of the series.
+
+        Notes
+        -----
+        .. versionadded:: 2.0.0
+
+        """
         return len(self) - 1
 
+    def reduce(self, deg) :
+        """Reduce the degree of the series.
+
+        Reduce the degree of the $name series to `deg` by discarding the
+        high order terms. If `deg` is greater than the current degree a
+        copy of the current series is returned. This can be useful in least
+        squares where the coefficients of the high degree terms may be very
+        small.
+
+        Parameters:
+        -----------
+        deg : non-negative int
+            The series is reduced to degree `deg` by discarding the high
+            order terms. The value of `deg` must be a non-negative integer.
+
+        Returns:
+        -------
+        new_instance : $name
+            New instance of $name with reduced degree.
+
+        Notes
+        -----
+        .. versionadded:: 2.0.0
+
+        """
+        return self.truncate(deg + 1)
+
     def convert(self, domain=None, kind=None) :
         """Convert to different class and/or domain.
 
diff --git a/numpy/polynomial/tests/test_chebyshev.py b/numpy/polynomial/tests/test_chebyshev.py
index d61e8b07c..5662d0362 100644
--- a/numpy/polynomial/tests/test_chebyshev.py
+++ b/numpy/polynomial/tests/test_chebyshev.py
@@ -402,6 +402,14 @@ class TestChebyshevClass(TestCase) :
     def test_degree(self) :
         assert_equal(self.p1.degree(), 2)
 
+    def test_reduce(self) :
+        assert_raises(ValueError, self.p1.reduce, .5)
+        assert_raises(ValueError, self.p1.reduce, -1)
+        assert_equal(len(self.p1.reduce(3)), 3)
+        assert_equal(len(self.p1.reduce(2)), 3)
+        assert_equal(len(self.p1.reduce(1)), 2)
+        assert_equal(len(self.p1.reduce(0)), 1)
+
     def test_convert(self) :
         x = np.linspace(-1,1)
         p = self.p1.convert(domain=[0,1])
diff --git a/numpy/polynomial/tests/test_polynomial.py b/numpy/polynomial/tests/test_polynomial.py
index 530e5d949..15525fcd8 100644
--- a/numpy/polynomial/tests/test_polynomial.py
+++ b/numpy/polynomial/tests/test_polynomial.py
@@ -373,6 +373,14 @@ class TestPolynomialClass(TestCase) :
     def test_degree(self) :
         assert_equal(self.p1.degree(), 2)
 
+    def test_reduce(self) :
+        assert_raises(ValueError, self.p1.reduce, .5)
+        assert_raises(ValueError, self.p1.reduce, -1)
+        assert_equal(len(self.p1.reduce(3)), 3)
+        assert_equal(len(self.p1.reduce(2)), 3)
+        assert_equal(len(self.p1.reduce(1)), 2)
+        assert_equal(len(self.p1.reduce(0)), 1)
+
     def test_convert(self) :
         x = np.linspace(-1,1)
         p = self.p1.convert(domain=[0,1])