# Multiplication of 1-element arrays fails

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?

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
``````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

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

2 Likes

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”.

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.