Merge pull request #16564 from JuliaLang/sb/sub-macro

Create `@view` macro for creating SubArrays via indexing.

base/exports.jl

@@ -1414,6 +1414,7 @@ export
     @enum,
     @label,
     @goto,
+    @view,
 
 # SparseArrays module re-exports
     SparseArrays,

base/subarray.jl

@@ -318,3 +318,76 @@ function parentdims(s::SubArray)
     end
     dimindex
 end
+
+"""
+    replace_ref_end!(ex)
+
+Recursively replace occurences of the symbol :end in a "ref" expression (i.e. A[...]) `ex`
+with the appropriate function calls (`endof`, `size` or `trailingsize`). Replacement uses
+the closest enclosing ref, so
+
+    A[B[end]]
+
+should transform to
+
+    A[B[endof(B)]]
+
+"""
+function replace_ref_end!(ex,withex=nothing)
+    if isa(ex,Symbol) && ex == :end
+        withex == nothing && error("Invalid use of end")
+        return withex
+    elseif isa(ex,Expr)
+        if ex.head == :ref
+            S = ex.args[1] = replace_ref_end!(ex.args[1],withex)
+            # new :ref, so redefine withex
+            nargs = length(ex.args)-1
+            if nargs == 0
+                return ex
+            elseif nargs == 1
+                # replace with endof(S)
+                ex.args[2] = replace_ref_end!(ex.args[2],:(Base.endof($S)))
+            else
+                n = 1
+                J = endof(ex.args)
+                for j = 2:J-1
+                    exj = ex.args[j] = replace_ref_end!(ex.args[j],:(Base.size($S,$n)))
+                    if isa(exj,Expr) && exj.head == :...
+                        # splatted object
+                        exjs = exj.args[1]
+                        n = :($n + length($exjs))
+                    elseif isa(n, Expr)
+                        # previous expression splatted
+                        n = :($n + 1)
+                    else
+                        # an integer
+                        n += 1
+                    end
+                end
+                ex.args[J] = replace_ref_end!(ex.args[J],:(Base.trailingsize($S,$n)))
+            end
+        else
+            # recursive search
+            for i = eachindex(ex.args)
+                ex.args[i] = replace_ref_end!(ex.args[i],withex)
+            end
+        end
+    end
+    ex
+end
+
+"""
+    @view A[inds...]
+
+Creates a `SubArray` from an indexing expression. This can only be applied directly to a
+reference expression (e.g. `@view A[1,2:end]`), and should *not* be used as the target of
+an assignment (e.g. `@view(A[1,2:end]) = ...`).
+"""
+macro view(ex)
+    if isa(ex, Expr) && ex.head == :ref
+        ex = replace_ref_end!(ex)
+        Expr(:&&, true, esc(Expr(:call,:(Base.view),ex.args...)))
+    else
+        throw(ArgumentError("Invalid use of @view macro: argument must be a reference expression A[...]."))
+    end
+end

doc/stdlib/arrays.rst

@@ -383,6 +383,12 @@ Indexing, Assignment, and Concatenation
 
    Like :func:`getindex`\ , but returns a view into the parent array ``A`` with the given indices instead of making a copy.  Calling :func:`getindex` or :func:`setindex!` on the returned :obj:`SubArray` computes the indices to the parent array on the fly without checking bounds.
 
+.. function:: @view A[inds...]
+
+   .. Docstring generated from Julia source
+
+   Creates a ``SubArray`` from an indexing expression. This can only be applied directly to a reference expression (e.g. ``@view A[1,2:end]``\ ), and should *not* be used as the target of an assignment (e.g. ``@view(A[1,2:end]) = ...``\ ).
+
 .. function:: parent(A)
 
    .. Docstring generated from Julia source

test/subarray.jl

@@ -465,3 +465,29 @@ end
 # the following segfaults with LLVM 3.8 on Windows, ref #15417
 @test collect(view(view(reshape(1:13^3, 13, 13, 13), 3:7, 6:6, :), 1:2:5, :, 1:2:5)) ==
     cat(3,[68,70,72],[406,408,410],[744,746,748])
+
+
+
+# tests @view (and replace_ref_end!)
+X = reshape(1:24,2,3,4)
+Y = 4:-1:1
+
+@test isa(@view(X[1:3]), SubArray)
+
+
+@test X[1:end] == @view X[1:end]
+@test X[1:end-3] == @view X[1:end-3]
+@test X[1:end,2,2] == @view X[1:end,2,2]
+@test X[1,1:end-2] == @view X[1,1:end-2]
+@test X[1,2,1:end-2] == @view X[1,2,1:end-2]
+@test X[1,2,Y[2:end]] == @view X[1,2,Y[2:end]]
+@test X[1:end,2,Y[2:end]] == @view X[1:end,2,Y[2:end]]
+
+u = (1,2:3)
+@test X[u...,2:end] == @view X[u...,2:end]
+@test X[(1,)...,(2,)...,2:end] == @view X[(1,)...,(2,)...,2:end]
+
+# test macro hygiene
+let size=(x,y)-> error("should not happen")
+    @test X[1:end,2,2] == @view X[1:end,2,2]
+end