broadcast value to an entire band (#366)

* broadcast value to an entire band

* Add tests

Author: jishnub

Project.toml

@@ -1,6 +1,6 @@
 name = "BandedMatrices"
 uuid = "aae01518-5342-5314-be14-df237901396f"
-version = "0.17.24"
+version = "0.17.25"
 
 [deps]
 ArrayLayouts = "4c555306-a7a7-4459-81d9-ec55ddd5c99a"

src/banded/BandedMatrix.jl

@@ -414,6 +414,10 @@ band(V::BandedMatrixBand) = first(parentindices(V)).band.i
 
 Forward a view of a band of a `BandedMatrix` to the parent's data matrix.
 
+!!! warn
+    This will error if the indexing is out-of-bounds for the data matrix, even if it is inbounds
+    for the parent `BandedMatrix`
+
 # Examples
 ```jldoctest
 julia> A = BandedMatrix(0=>1:4, 1=>5:7, -1=>8:10)
@@ -443,6 +447,7 @@ function dataview(V::BandedMatrixBand)
     view(A.data, A.u - b + 1, max(b,0)+1:min(n,m+b))
 end
 
+# B[band(i)]
 function copyto!(v::Vector, B::BandedMatrixBand)
     A = parent(parent(B))
     if -A.l ≤ band(B) ≤ A.u
@@ -454,6 +459,19 @@ function copyto!(v::Vector, B::BandedMatrixBand)
     return v
 end
 
+# B[band(i)] .= x
+function fill!(Bv::BandedMatrixBand, x)
+    b = band(Bv)
+    A = parent(parent(Bv))
+    l, u = bandwidths(A)
+    if -l <= b <= u
+        fill!(dataview(Bv), x)
+    elseif !iszero(x)  # allow setting outside bands to zero
+        throw(BandError(A,b))
+    end
+    Bv
+end
+
 # ~ indexing along a row
 
 

test/test_banded.jl

@@ -479,5 +479,21 @@ Base.similar(::MyMatrix, ::Type{T}, m::Int, n::Int) where T = MyMatrix{T}(undef,
         A[1:2:end,1:2:end] = X1
         @test A == B
     end
+
+    @testset "broadcasting to a band" begin
+        B = brand(Int8, 6, 6, -2, 2)
+        B[band(2)] .= 10
+        @test all(==(10), diag(B, 2))
+        @test all(==(10), B[band(2)])
+
+        B = brand(Int8, 2, 4, 1, 1)
+        B[band(-1)] .= 2
+        B[band(0)] .= 3
+        B[band(1)] .= 4
+        @test B == [3 4 0 0; 2 3 4 0]
+
+        @test_throws BandError B[band(100)] .= 10
+        @test_throws BandError B[band(-100)] .= 10
+    end
 end