Merge pull request numpy#12846 from tlatorre-uchicago/divmod-bugfix

BUG: fix signed zero behavior in npy_divmod
JianMeng · Feb 22, 2019 · 1bf2b94 · 1bf2b94
2 parents 04b4220 + 56bf38b
commit 1bf2b94
Show file tree

Hide file tree

Showing 3 changed files with 22 additions and 2 deletions.
diff --git a/doc/release/1.17.0-notes.rst b/doc/release/1.17.0-notes.rst
@@ -36,7 +36,21 @@ Casting from a different floating point precision to float16 used incorrect
 rounding in some edge cases. This means in rare cases, subnormal results will
 now be rounded up instead of down, changing the last bit (ULP) of the result.
 
+Signed zero when using divmod
+-----------------------------
 
+Starting in version 1.12.0, numpy incorrectly returned a negatively signed zero
+when using the ``divmod`` and ``floor_divide`` functions when the result was
+zero. For example::
+
+   >>> np.zeros(10)//1
+   array([-0., -0., -0., -0., -0., -0., -0., -0., -0., -0.])
+
+With this release, the result is correctly returned as a positively signed
+zero::
+
+   >>> np.zeros(10)//1
+   array([0., 0., 0., 0., 0., 0., 0., 0., 0., 0.])
 
 C API changes
 =============

diff --git a/numpy/core/src/npymath/npy_math_internal.h.src b/numpy/core/src/npymath/npy_math_internal.h.src
@@ -654,7 +654,7 @@ npy_divmod@c@(@type@ a, @type@ b, @type@ *modulus)
     }
     else {
         /* if mod is zero ensure correct sign */
-        mod = (b > 0) ? 0.0@c@ : -0.0@c@;
+        mod = npy_copysign@c@(0, b);
     }
 
     /* snap quotient to nearest integral value */
@@ -665,7 +665,7 @@ npy_divmod@c@(@type@ a, @type@ b, @type@ *modulus)
     }
     else {
         /* if div is zero ensure correct sign */
-        floordiv = (a / b > 0) ?  0.0@c@ : -0.0@c@;
+        floordiv = npy_copysign@c@(0, a/b);
     }
 
     *modulus = mod;

diff --git a/numpy/core/tests/test_umath.py b/numpy/core/tests/test_umath.py
@@ -273,6 +273,12 @@ def test_floor_division_complex(self):
         y = np.floor_divide(x**2, x)
         assert_equal(y, [1.e+110, 0], err_msg=msg)
 
+    def test_floor_division_signed_zero(self):
+        # Check that the sign bit is correctly set when dividing positive and
+        # negative zero by one.
+        x = np.zeros(10)
+        assert_equal(np.signbit(x//1), 0)
+        assert_equal(np.signbit((-x)//1), 1)
 
 def floor_divide_and_remainder(x, y):
     return (np.floor_divide(x, y), np.remainder(x, y))