Deprecate keyword argument "thin" in favor of "square" in lq methods.

NEWS.md

@@ -363,6 +363,10 @@ Deprecated or removed
   * The `cholfact`/`cholfact!` methods that accepted an `uplo` symbol have been deprecated
     in favor of using `Hermitian` (or `Symmetric`) views ([#22187], [#22188]).
 
+  * The `thin` keyword argument for orthogonal decomposition methods has
+    been deprecated in favor of `square`, which has the opposite meaning:
+    `thin == true` if and only if `square == false` ([#24279]).
+
   * `isposdef(A::AbstractMatrix, UL::Symbol)` and `isposdef!(A::AbstractMatrix, UL::Symbol)`
     have been deprecated in favor of `isposdef(Hermitian(A, UL))` and `isposdef!(Hermitian(A, UL))`
     respectively ([#22245]).

base/deprecated.jl

@@ -1889,6 +1889,7 @@ end
 # TODO: after 0.7, remove thin keyword argument and associated logic from...
 # (1) base/linalg/svd.jl
 # (2) base/linalg/qr.jl
+# (3) base/linalg/lq.jl
 
 @deprecate find(x::Number)            find(!iszero, x)
 @deprecate findnext(A, v, i::Integer) findnext(equalto(v), A, i)

base/linalg/lq.jl

@@ -33,24 +33,30 @@ lqfact(A::StridedMatrix{<:BlasFloat})  = lqfact!(copy(A))
 lqfact(x::Number) = lqfact(fill(x,1,1))
 
 """
-    lq(A; [thin=true]) -> L, Q
+    lq(A; square = false) -> L, Q
 
-Perform an LQ factorization of `A` such that `A = L*Q`. The default is to compute
-a "thin" factorization. The LQ factorization is the QR factorization of `A.'`.
-`L` is not extendedwith zeros if the explicit, square form of `Q`
-is requested via `thin = false`.
+Perform an LQ factorization of `A` such that `A = L*Q`. The default (`square = false`)
+computes a factorization with possibly-rectangular `L` and `Q`, commonly the "thin"
+factorization. The LQ factorization is the QR factorization of `A.'`. If the explicit,
+square form of `Q` is requested via `square = true`, `L` is not extended with zeros.
 
 !!! note
     While in QR factorization the "thin" factorization is so named due to yielding
-    either a square or "tall"/"thin" factor `Q`, in LQ factorization the "thin"
-    factorization somewhat confusingly produces either a square or "short"/"wide"
-    factor `Q`. "Thin" factorizations more broadly are also (more descriptively)
-    referred to as "truncated" or "reduced" factorizatons.
+    either a square or "tall"/"thin" rectangular factor `Q`, in LQ factorization the
+    "thin" factorization somewhat confusingly produces either a square or "short"/"wide"
+    rectangular factor `Q`. "Thin" factorizations more broadly are also
+    referred to as "reduced" factorizatons.
 """
-function lq(A::Union{Number,AbstractMatrix}; thin::Bool = true)
+function lq(A::Union{Number,AbstractMatrix}; square::Bool = false, thin::Union{Bool,Void} = nothing)
+    # DEPRECATION TODO: remove deprecated thin argument and associated logic after 0.7
+    if thin != nothing
+        Base.depwarn(string("the `thin` keyword argument in `lq(A; thin = $(thin))` has ",
+            "been deprecated in favor of `square`, i.e. `lq(A; square = $(!thin))`."), :lq)
+        square::Bool = !thin
+    end
     F = lqfact(A)
     L, Q = F[:L], F[:Q]
-    return L, thin ? Array(Q) : A_mul_B!(Q, eye(eltype(Q), size(Q.factors, 2)))
+    return L, !square ? Array(Q) : A_mul_B!(Q, eye(eltype(Q), size(Q.factors, 2)))
 end
 
 copy(A::LQ) = LQ(copy(A.factors), copy(A.τ))

test/linalg/lq.jl

@@ -111,35 +111,35 @@ end
 @testset "correct form of Q from lq(...) (#23729)" begin
     # where the original matrix (say A) is square or has more rows than columns,
     # then A's factorization's triangular factor (say L) should have the same shape
-    # as A independent of factorization form (truncated, square), and A's factorization's
+    # as A independent of factorization form (square, rectangular/"thin"), and A's factorization's
     # orthogonal factor (say Q) should be a square matrix of order of A's number of
-    # columns independent of factorization form (truncated, square), and L and Q
+    # columns independent of factorization form (square, rectangular/"thin"), and L and Q
     # should have multiplication-compatible shapes.
     m, n = 4, 2
     A = randn(m, n)
-    for thin in (true, false)
-        L, Q = lq(A, thin = thin)
+    for square in (false, true)
+        L, Q = lq(A, square = square)
         @test size(L) == (m, n)
         @test size(Q) == (n, n)
         @test isapprox(A, L*Q)
     end
     # where the original matrix has strictly fewer rows than columns ...
     m, n = 2, 4
     A = randn(m, n)
-    # ... then, for a truncated factorization of A, L should be a square matrix
+    # ... then, for a rectangular/"thin" factorization of A, L should be a square matrix
     # of order of A's number of rows, Q should have the same shape as A,
     # and L and Q should have multiplication-compatible shapes
-    Lthin, Qthin = lq(A, thin = true)
-    @test size(Lthin) == (m, m)
-    @test size(Qthin) == (m, n)
-    @test isapprox(A, Lthin * Qthin)
-    # ... and, for a square/non-truncated factorization of A, L should have the
+    Lrect, Qrect = lq(A, square = false)
+    @test size(Lrect) == (m, m)
+    @test size(Qrect) == (m, n)
+    @test isapprox(A, Lrect * Qrect)
+    # ... and, for a square factorization of A, L should have the
     # same shape as A, Q should be a square matrix of order of A's number of columns,
     # and L and Q should have multiplication-compatible shape. but instead the L returned
-    # has no zero-padding on the right / is L for the truncated factorization,
+    # has no zero-padding on the right / is L for the rectangular/"thin" factorization,
     # so for L and Q to have multiplication-compatible shapes, L must be zero-padded
     # to have the shape of A.
-    Lsquare, Qsquare = lq(A, thin = false)
+    Lsquare, Qsquare = lq(A, square = true)
     @test size(Lsquare) == (m, m)
     @test size(Qsquare) == (n, n)
     @test isapprox(A, [Lsquare zeros(m, n - m)] * Qsquare)