linalg: add diagm(v) as shorthand for diagm(0 => v), fixes #31111 (#3…

…1125)

NEWS.md

@@ -52,6 +52,7 @@ Standard library changes
 * Eigenvalues λ of general matrices are now sorted lexicographically by (Re λ, Im λ) ([#21598]).
 * `one` for structured matrices (`Diagonal`, `Bidiagonal`, `Tridiagonal`, `Symtridiagonal`) now preserves
   structure and type. ([#29777])
+* `diagm(v)` is now a shorthand for `diagm(0 => v)`. ([#31125]).
 
 #### SparseArrays
 

stdlib/LinearAlgebra/src/dense.jl

@@ -291,6 +291,21 @@ function diagm_container(kv::Pair{<:Integer,<:BitVector}...)
     return falses(n, n)
 end
 
+"""
+    diagm(v::AbstractVector)
+
+Construct a square matrix with elements of the vector as diagonal elements.
+
+# Examples
+```jldoctest
+julia> diagm([1,2,3])
+3×3 Array{Int64,2}:
+ 1  0  0
+ 0  2  0
+ 0  0  3
+```
+"""
+diagm(v::AbstractVector) = diagm(0 => v)
 
 function tr(A::Matrix{T}) where T
     n = checksquare(A)

stdlib/LinearAlgebra/test/dense.jl

@@ -84,6 +84,13 @@ bimg  = randn(n,2)/2
         end
     end # for eltyb
 
+@testset "Test diagm for vectors" begin
+    @test diagm(zeros(50)) == diagm(0 => zeros(50))
+    @test diagm(ones(50)) == diagm(0 => ones(50))
+    v = randn(500)
+    @test diagm(v) == diagm(0 => v)
+end
+
 @testset "Test pinv (rtol, atol)" begin
     M = [1 0 0; 0 1 0; 0 0 0]
     @test pinv(M,atol=1)== zeros(3,3)