RFC: Make matrix transpose default for permutedims

NEWS.md

@@ -430,6 +430,12 @@ Library improvements
     definition relies on `ncodeunits` however, so for optimal performance you may need to
     define a custom method for that function.
 
+  * `permutedims(m::AbstractMatrix)` is now short for `permutedims(m, (2,1))`, and is now a
+    more convenient way of making a "shallow transpose" of a 2D array. This is the
+    recommended approach for manipulating arrays of data, rather than the recursively
+    defined, linear-algebra function `transpose`. Similarly,
+    `permutedims(v::AbstractVector)` will create a row matrix ([#24839]).
+
 Compiler/Runtime improvements
 -----------------------------
 

base/linalg/transpose.jl

@@ -147,7 +147,11 @@ end
 """
     transpose(A::AbstractMatrix)
 
-The transposition operator (`.'`).
+The transposition operator (`.'`). Note that the transposition is applied recursively to
+elements.
+
+This operation is intended for linear algebra usage - for general data manipulation see
+[`permutedims`](@ref), which is non-recursive.
 
 # Examples
 ```jldoctest

base/operators.jl

@@ -744,7 +744,11 @@ fldmod1(x::T, y::T) where {T<:Integer} = (fld1(x,y), mod1(x,y))
 """
     adjoint(A)
 
-The conjugate transposition operator (`'`).
+The conjugate transposition operator (`'`). Note that `adjoint` is applied recursively to
+elements.
+
+This operation is intended for linear algebra usage - for general data manipulation see
+[`permutedims`](@ref).
 
 # Examples
 ```jldoctest

base/permuteddimsarray.jl

@@ -2,7 +2,8 @@
 
 module PermutedDimsArrays
 
-export permutedims, PermutedDimsArray
+import Base: permutedims, permutedims!
+export PermutedDimsArray
 
 # Some day we will want storage-order-aware iteration, so put perm in the parameters
 struct PermutedDimsArray{T,N,perm,iperm,AA<:AbstractArray} <: AbstractArray{T,N}
@@ -77,11 +78,10 @@ end
 @inline genperm(I, perm::AbstractVector{Int}) = genperm(I, (perm...,))
 
 """
-    permutedims(A, perm)
+    permutedims(A::AbstractArray, perm)
 
 Permute the dimensions of array `A`. `perm` is a vector specifying a permutation of length
-`ndims(A)`. This is a generalization of transpose for multi-dimensional arrays. Transpose is
-equivalent to `permutedims(A, [2,1])`.
+`ndims(A)`.
 
 See also: [`PermutedDimsArray`](@ref).
 
@@ -108,11 +108,26 @@ julia> permutedims(A, [3, 2, 1])
  6  8
 ```
 """
-function Base.permutedims(A::AbstractArray, perm)
+function permutedims(A::AbstractArray, perm)
     dest = similar(A, genperm(indices(A), perm))
     permutedims!(dest, A, perm)
 end
 
+"""
+    permutedims(m::AbstractMatrix)
+
+Permute the dimensions of the matrix `m`, by flipping the elements across the diagonal of
+the matrix. Differs from [`transpose`](@ref) in that the operation is not recursive.
+"""
+permutedims(A::AbstractMatrix) = permutedims(A, (2,1))
+
+"""
+    permutedims(v::AbstractVector)
+
+Reshape vector `v` into a `1 × length(v)` row matrix.
+"""
+permutedims(v::AbstractVector) = reshape(v, (1, length(v)))
+
 """
     permutedims!(dest, src, perm)
 
@@ -124,7 +139,7 @@ regions.
 
 See also [`permutedims`](@ref).
 """
-function Base.permutedims!(dest, src::AbstractArray, perm)
+function permutedims!(dest, src::AbstractArray, perm)
     Base.checkdims_perm(dest, src, perm)
     P = PermutedDimsArray(dest, invperm(perm))
     _copy!(P, src)

test/arrayops.jl

@@ -577,6 +577,12 @@ end
         @test isequal(A,permutedims(permutedims(A,perm),invperm(perm)))
         @test isequal(A,permutedims(permutedims(A,invperm(perm)),perm))
     end
+
+    m = [1 2; 3 4]
+    @test permutedims(m) == [1 3; 2 4]
+
+    v = [1,2,3]
+    @test permutedims(v) == [1 2 3]
 end
 
 @testset "circshift" begin