Multiplication of 1-element arrays fails

New to Julia
1

Julia v1.0.0: I don’t know if anyone else has encountered this yet, but it seems that the native * operator, while defined for two array objects with proper dimensions, is undefined for multiplication of 1x1 elements.

For example,

julia> a = [1]
1-element Array{Int64,1}:
 1

julia> b = [2]
1-element Array{Int64,1}:
 2

julia> a*b
ERROR: MethodError: no method matching *(::Array{Int64,1}, ::Array{Int64,1})
Closest candidates are:
  *(::Any, ::Any, ::Any, ::Any...) at operators.jl:502
  *(::LinearAlgebra.Adjoint{#s549,#s548} where #s548<:Union{DenseArray{T<:Union{Complex{Float32}, Complex{Float64}, Float32, Float64},2}, ReinterpretArray{T<:Union{Complex{Float32}, Complex{Float64}, Float32, Float64},2,S,A} where S where A<:Union{SubArray{T,N,A,I,true} where I<:Tuple{AbstractUnitRange,Vararg{Any,N} where N} where A<:DenseArray where N where T, DenseArray}, ReshapedArray{T<:Union{Complex{Float32}, Complex{Float64}, Float32, Float64},2,A,MI} where MI<:Tuple{Vararg{SignedMultiplicativeInverse{Int64},N} where N} where A<:Union{ReinterpretArray{T,N,S,A} where S where A<:Union{SubArray{T,N,A,I,true} where I<:Tuple{AbstractUnitRange,Vararg{Any,N} where N} where A<:DenseArray where N where T, DenseArray} where N where T, SubArray{T,N,A,I,true} where I<:Tuple{AbstractUnitRange,Vararg{Any,N} where N} where A<:DenseArray where N where T, DenseArray}, SubArray{T<:Union{Complex{Float32}, Complex{Float64}, Float32, Float64},2,A,I,L} where L where I<:Tuple{Vararg{Union{Int64, AbstractRange{Int64}, AbstractCartesianIndex},N} where N} where A<:Union{ReinterpretArray{T,N,S,A} where S where A<:Union{SubArray{T,N,A,I,true} where I<:Tuple{AbstractUnitRange,Vararg{Any,N} where N} where A<:DenseArray where N where T, DenseArray} where N where T, ReshapedArray{T,N,A,MI} where MI<:Tuple{Vararg{SignedMultiplicativeInverse{Int64},N} where N} where A<:Union{ReinterpretArray{T,N,S,A} where S where A<:Union{SubArray{T,N,A,I,true} where I<:Tuple{AbstractUnitRange,Vararg{Any,N} where N} where A<:DenseArray where N where T, DenseArray} where N where T, SubArray{T,N,A,I,true} where I<:Tuple{AbstractUnitRange,Vararg{Any,N} where N} where A<:DenseArray where N where T, DenseArray} where N where T, DenseArray}} where #s549, ::Union{DenseArray{S,1}, ReinterpretArray{S,1,S,A} where S where A<:Union{SubArray{T,N,A,I,true} where I<:Tuple{AbstractUnitRange,Vararg{Any,N} where N} where A<:DenseArray where N where T, DenseArray}, ReshapedArray{S,1,A,MI} where MI<:Tuple{Vararg{SignedMultiplicativeInverse{Int64},N} where N} where A<:Union{ReinterpretArray{T,N,S,A} where S where A<:Union{SubArray{T,N,A,I,true} where I<:Tuple{AbstractUnitRange,Vararg{Any,N} where N} where A<:DenseArray where N where T, DenseArray} where N where T, SubArray{T,N,A,I,true} where I<:Tuple{AbstractUnitRange,Vararg{Any,N} where N} where A<:DenseArray where N where T, DenseArray}, SubArray{S,1,A,I,L} where L where I<:Tuple{Vararg{Union{Int64, AbstractRange{Int64}, AbstractCartesianIndex},N} where N} where A<:Union{ReinterpretArray{T,N,S,A} where S where A<:Union{SubArray{T,N,A,I,true} where I<:Tuple{AbstractUnitRange,Vararg{Any,N} where N} where A<:DenseArray where N where T, DenseArray} where N where T, ReshapedArray{T,N,A,MI} where MI<:Tuple{Vararg{SignedMultiplicativeInverse{Int64},N} where N} where A<:Union{ReinterpretArray{T,N,S,A} where S where A<:Union{SubArray{T,N,A,I,true} where I<:Tuple{AbstractUnitRange,Vararg{Any,N} where N} where A<:DenseArray where N where T, DenseArray} where N where T, SubArray{T,N,A,I,true} where I<:Tuple{AbstractUnitRange,Vararg{Any,N} where N} where A<:DenseArray where N where T, DenseArray} where N where T, DenseArray}}) where {T<:Union{Complex{Float32}, Complex{Float64}, Float32, Float64}, S} at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/LinearAlgebra/src/matmul.jl:97
  *(::LinearAlgebra.Adjoint{#s549,#s548} where #s548<:LinearAlgebra.AbstractTriangular where #s549, ::AbstractArray{T,1} where T) at /Users/osx/buildbot/slave/package_osx64/build/usr/share/julia/stdlib/v1.0/LinearAlgebra/src/triangular.jl:1805
  ...
Stacktrace:
 [1] top-level scope at none:0

julia> VERSION
v"1.0.0"

julia> ones(3,3) * ones(3,1)
3×1 Array{Float64,2}:
 3.0
 3.0
 3.0

Any thoughts on why this may be failing?

2

Because a is a column vector (it has only one dimension). For a*b the first dimension of b is contracted with the second dimension of a, but there is none.

1 Like
3 
julia> ones(1,1) * ones(1,1) # matrix times matrix = matrix
1×1 Array{Float64,2}:
 1.0

julia> ones(1,1) * ones(1) # matrix times vector = vector
1-element Array{Float64,1}:
 1.0

julia> ones(1) * ones(1) # vector times vector, error
ERROR: MethodError: no method matching *(::Array{Float64,1}, ::Array{Float64,1})
4 Likes
4

Maybe we should define that method and have it print a more informative error message.

2 Likes
5

OTOH, for one coming from Matlab, R, or similar, and did yet not learn about broadcasting, there are going to be very many methods that just “don’t work”.

6

Unlike Matlab, a 1-element column vector (1d array) is not the same thing as a 1x1 matrix (2d array), and neither is the same thing as a scalar.