describe views in the basic interface tutorial (#255)

* describe views

* Update basic_interface.md

* Update basic_interface.md

Project.toml

@@ -1,7 +1,7 @@
 name = "SummationByPartsOperators"
 uuid = "9f78cca6-572e-554e-b819-917d2f1cf240"
 author = ["Hendrik Ranocha"]
-version = "0.5.54"
+version = "0.5.55"
 
 [deps]
 ArgCheck = "dce04be8-c92d-5529-be00-80e4d2c0e197"

docs/src/tutorials/basic_interface.md

@@ -31,7 +31,9 @@ D * u
 
 As you can see above, calling `D * u` allocates a new vector for the
 result. If you want to apply an SBP operator multiple times and need
-good performance, you should consider using an in-place update instead.
+good performance, you should consider using pre-allocating the output
+and using in-place update instead. This strategy is also described in
+the [performance tips in the Julia manual](https://docs.julialang.org/en/v1/manual/performance-tips/#Pre-allocating-outputs).
 Julia provides the function `mul!` for this purpose.
 
 ```@repl
@@ -161,3 +163,50 @@ difference = D * x.^3 - 3 * x.^2
 
 error_l2 = sqrt(integrate(abs2, difference, D))
 ```
+
+
+## Multi-dimensional cases or multiple variables
+
+If you want to work with multiple space dimensions, you can still use
+the 1D operators provided by
+[SummationByPartsOperators.jl](https://github.com/ranocha/SummationByPartsOperators.jl)
+if you apply them in a tensor product fashion along each space dimension.
+
+```@repl
+using SummationByPartsOperators
+
+D = derivative_operator(MattssonNordström2004(),
+                        derivative_order = 1, accuracy_order = 4,
+                        xmin = 0.0, xmax = 1.0, N = 9)
+
+x = y = grid(D)
+
+u = x .* y'.^2 # u(x, y) = x y^2
+
+let du_dx = zero(u)
+    for j in axes(u, 2)
+        mul!(view(du_dx, :, j), D, view(u, :, j))
+    end
+    # The derivative of x*y^2 with respect to x is just y^2.
+    # Thus, the result is constant in each column and varies
+    # in the rows.
+    du_dx
+end
+
+let du_dy = zero(u)
+    for i in axes(u, 1)
+        mul!(view(du_dy, i, :), D, view(u, i, :))
+    end
+    # The derivative of x*y^2 with respect to y is 2*x*y.
+    du_dy
+end
+
+2 .* x .* y'
+```
+
+Here, we have used `view`s to interpret parts of the memory of the
+multi-dimensional arrays as one-diemnsional vectors that can be used
+together with the operators of
+[SummationByPartsOperators.jl](https://github.com/ranocha/SummationByPartsOperators.jl).
+You can use the same trick if you collect values of multiple variables
+in a multi-dimensional array.