add array getter/setter

include/ginkgo/core/base/array.hpp

@@ -110,6 +110,44 @@ class const_array_view {
      */
     const value_type* get_const_data() const noexcept { return data_; }
 
+    /**
+     * Returns a single value from the array.
+     * This involves a bounds check, polymorphic calls and potentially a
+     * device-to-host copy, so it is not suitable for accessing many elements
+     * with high performance.
+     *
+     * @param index  the array element index.
+     * @return the value at index.
+     */
+    value_type get_value(size_type index) const
+    {
+        if (index < 0 || index >= this->get_num_elems()) {
+            throw OutOfBoundsError{__FILE__, __LINE__, index,
+                                   this->get_num_elems()};
+        }
+        return this->get_executor()->copy_val_to_host(this->get_const_data() +
+                                                      index);
+    }
+
+    /**
+     * Sets a single entry in the array to a new value.
+     * This involves a bounds check, polymorphic calls and potentially a
+     * host-to-device copy, so it is not suitable for accessing many elements
+     * with high performance.
+     *
+     * @param index  the array element index.
+     * @param value  the new value.
+     */
+    void set_value(size_type index, value_type value) const
+    {
+        if (index < 0 || index >= this->get_num_elems()) {
+            throw OutOfBoundsError{__FILE__, __LINE__, index,
+                                   this->get_num_elems()};
+        }
+        this->get_executor()->copy_from(this->get_executor()->get_master(), 1,
+                                        &value, this->get_data() + index);
+    }
+
     /**
      * Returns the Executor associated with the array view.
      *