Add Array2::from_diag (#673)

* ENH Add Array2::from_diag

* Lint

* Address review comments

* Add example and panic conditions

* Remove cfg(feature = "approx")

* Add missing closing code fence

src/impl_constructors.rs

@@ -171,6 +171,29 @@ where
         }
         eye
     }
+
+    /// Create a 2D matrix from its diagonal
+    ///
+    /// **Panics** if `diag.len() * diag.len()` would overflow `isize`.
+    ///
+    /// ```rust
+    /// use ndarray::{Array2, arr1, arr2};
+    ///
+    /// let diag = arr1(&[1, 2]);
+    /// let array = Array2::from_diag(&diag);
+    /// assert_eq!(array, arr2(&[[1, 0], [0, 2]]));
+    /// ```
+    pub fn from_diag<S2>(diag: &ArrayBase<S2, Ix1>) -> Self
+    where
+        A: Clone + Zero,
+        S: DataMut,
+        S2: Data<Elem = A>,
+    {
+        let n = diag.len();
+        let mut arr = Self::zeros((n, n));
+        arr.diag_mut().assign(&diag);
+        arr
+    }
 }
 
 #[cfg(not(debug_assertions))]

tests/array.rs

@@ -1950,6 +1950,20 @@ fn test_array_clone_same_view() {
     assert_eq!(a, b);
 }
 
+#[test]
+fn test_array2_from_diag() {
+    let diag = arr1(&[0, 1, 2]);
+    let x = Array2::from_diag(&diag);
+    let x_exp = arr2(&[[0, 0, 0], [0, 1, 0], [0, 0, 2]]);
+    assert_eq!(x, x_exp);
+
+    // check 0 length array
+    let diag = Array1::<f64>::zeros(0);
+    let x = Array2::from_diag(&diag);
+    assert_eq!(x.ndim(), 2);
+    assert_eq!(x.shape(), [0, 0]);
+}
+
 #[test]
 fn array_macros() {
     // array