From 019c8c9b2a7c084eb01cf4d8569799a5537d884d Mon Sep 17 00:00:00 2001
From: Miles Cranmer <miles.cranmer@gmail.com>
Date: Fri, 24 Jun 2022 08:24:22 -0400
Subject: BUG: Fix in1d for empty integer array as input (#21842)

* TST: Tests empty input to in1d of various types

* MAINT: Skip table method on empty input array

* MAINT: Check array size before call to min

* MAINT: Return early for kind="table" and empty ar2

* TST: Test other dtypes of empty arrays
---
 numpy/lib/arraysetops.py            |  7 +++++++
 numpy/lib/tests/test_arraysetops.py | 20 +++++++++++++++++++-
 2 files changed, 26 insertions(+), 1 deletion(-)

(limited to 'numpy/lib')

diff --git a/numpy/lib/arraysetops.py b/numpy/lib/arraysetops.py
index 9a44d7e44..fa332e7fd 100644
--- a/numpy/lib/arraysetops.py
+++ b/numpy/lib/arraysetops.py
@@ -636,6 +636,13 @@ def in1d(ar1, ar2, assume_unique=False, invert=False, *, kind=None):
             f"Invalid kind: '{kind}'. Please use None, 'sort' or 'table'.")
 
     if integer_arrays and kind in {None, 'table'}:
+
+        if ar2.size == 0:
+            if invert:
+                return np.ones_like(ar1, dtype=bool)
+            else:
+                return np.zeros_like(ar1, dtype=bool)
+
         ar2_min = np.min(ar2)
         ar2_max = np.max(ar2)
 
diff --git a/numpy/lib/tests/test_arraysetops.py b/numpy/lib/tests/test_arraysetops.py
index d91d36282..a21e5f640 100644
--- a/numpy/lib/tests/test_arraysetops.py
+++ b/numpy/lib/tests/test_arraysetops.py
@@ -232,12 +232,30 @@ class TestSetOps:
         assert_isin_equal(5, 6)
 
         # empty array-like:
-        if kind in {None, "sort"}:
+        if kind != "table":
+            # An empty list will become float64,
+            # which is invalid for kind="table"
             x = []
             assert_isin_equal(x, b)
             assert_isin_equal(a, x)
             assert_isin_equal(x, x)
 
+        # empty array with various types:
+        for dtype in [bool, np.int64, np.float64]:
+            if kind == "table" and dtype == np.float64:
+                continue
+
+            if dtype in {np.int64, np.float64}:
+                ar = np.array([10, 20, 30], dtype=dtype)
+            elif dtype in {bool}:
+                ar = np.array([True, False, False])
+
+            empty_array = np.array([], dtype=dtype)
+
+            assert_isin_equal(empty_array, ar)
+            assert_isin_equal(ar, empty_array)
+            assert_isin_equal(empty_array, empty_array)
+
     @pytest.mark.parametrize("kind", [None, "sort", "table"])
     def test_in1d(self, kind):
         # we use two different sizes for the b array here to test the
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