Support zero-length axis in .map_axis/_mut()

Cargo.toml

@@ -47,6 +47,7 @@ serde = { version = "1.0", optional = true }
 defmac = "0.2"
 quickcheck = { version = "0.7.2", default-features = false }
 rawpointer = "0.1"
+itertools = { version = "0.7.0", default-features = false, features = ["use_std"] }
 approx = "0.3"
 
 [features]

src/impl_methods.rs

@@ -2102,13 +2102,18 @@ where
     {
         let view_len = self.len_of(axis);
         let view_stride = self.strides.axis(axis);
-        // use the 0th subview as a map to each 1d array view extended from
-        // the 0th element.
-        self.index_axis(axis, 0).map(|first_elt| {
-            unsafe {
-                mapping(ArrayView::new_(first_elt, Ix1(view_len), Ix1(view_stride)))
-            }
-        })
+        if view_len == 0 {
+            let new_dim = self.dim.remove_axis(axis);
+            Array::from_shape_fn(new_dim, move |_| mapping(ArrayView::from(&[])))
+        } else {
+            // use the 0th subview as a map to each 1d array view extended from
+            // the 0th element.
+            self.index_axis(axis, 0).map(|first_elt| {
+                unsafe {
+                    mapping(ArrayView::new_(first_elt, Ix1(view_len), Ix1(view_stride)))
+                }
+            })
+        }
     }
 
     /// Reduce the values along an axis into just one value, producing a new
@@ -2130,12 +2135,17 @@ where
     {
         let view_len = self.len_of(axis);
         let view_stride = self.strides.axis(axis);
-        // use the 0th subview as a map to each 1d array view extended from
-        // the 0th element.
-        self.index_axis_mut(axis, 0).map_mut(|first_elt: &mut A| {
-            unsafe {
-                mapping(ArrayViewMut::new_(first_elt, Ix1(view_len), Ix1(view_stride)))
-            }
-        })
+        if view_len == 0 {
+            let new_dim = self.dim.remove_axis(axis);
+            Array::from_shape_fn(new_dim, move |_| mapping(ArrayViewMut::from(&mut [])))
+        } else {
+            // use the 0th subview as a map to each 1d array view extended from
+            // the 0th element.
+            self.index_axis_mut(axis, 0).map_mut(|first_elt| {
+                unsafe {
+                    mapping(ArrayViewMut::new_(first_elt, Ix1(view_len), Ix1(view_stride)))
+                }
+            })
+        }
     }
 }

tests/array.rs

@@ -13,7 +13,7 @@ use ndarray::{
 };
 use ndarray::indices;
 use defmac::defmac;
-use itertools::{enumerate, zip};
+use itertools::{enumerate, zip, Itertools};
 
 macro_rules! assert_panics {
     ($body:expr) => {
@@ -1833,6 +1833,27 @@ fn test_map_axis() {
     let c = a.map_axis(Axis(1), |view| view.sum());
     let answer2 = arr1(&[6, 15, 24, 33]);
     assert_eq!(c, answer2);
+
+    // Test zero-length axis case
+    let arr = Array3::<f32>::zeros((3, 0, 4));
+    let mut counter = 0;
+    let result = arr.map_axis(Axis(1), |x| {
+        assert_eq!(x.shape(), &[0]);
+        counter += 1;
+        counter
+    });
+    assert_eq!(result.shape(), &[3, 4]);
+    itertools::assert_equal(result.iter().cloned().sorted(), 1..=3 * 4);
+
+    let mut arr = Array3::<f32>::zeros((3, 0, 4));
+    let mut counter = 0;
+    let result = arr.map_axis_mut(Axis(1), |x| {
+        assert_eq!(x.shape(), &[0]);
+        counter += 1;
+        counter
+    });
+    assert_eq!(result.shape(), &[3, 4]);
+    itertools::assert_equal(result.iter().cloned().sorted(), 1..=3 * 4);
 }
 
 #[test]