adding scalar to an array in julia

Question

Trying to get a linear equation y = m*x + c. I've the following lines of code, trying to add a scalar to an array.

m = 1.1; c = 0.11;
x = rand(1,2)
  1×2 Array{Float64,2}:
  0.920045  0.660015

y = m*x + c
ERROR: MethodError: no method matching +(::Array{Float64,2}, ::Float64)
Closest candidates are:
  +(::Any, ::Any, ::Any, ::Any...) at operators.jl:502
  +(::Bool, ::T<:AbstractFloat) where T<:AbstractFloat at bool.jl:112
  +(::Float64, ::Float64) at float.jl:395
  ...
Stacktrace:
 [1] top-level scope at none:0

Currently using Julia 1.0. Directly adding a scalar to an array didn't work. In previous versions this used to work I suppose.

Scalar multiplication works

m*x
 1×2 Array{Float64,2}:
 1.01205  0.726016

But I've to define another array and then perform addition as shown.

c = [0.11 0.11]
y = m*x + c
1×2 Array{Float64,2}:
 1.12205  0.836016

Isn't this an overhead? What difference does it make when I can perform scalar multiplication m*x on arrays but not addition?

score 16 · Accepted Answer · answered Sep 14 '18 at 15:23

16

I guess this has become stricter. In Julia syntax (like in MATLAB), + and * operate on congruent arrays.

For element-wise operation, you should use .+ and .*. Strangely, this does not seem to matter for * but it does for +.

Anyway, both the following works:

y = m * x .+ c

and:

y = m .* x .+ c

answered Sep 14 '18 at 15:23

norok2

25,683
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6

It does not matter for `*` because multiplication of a matrix by scalar is a well defined operation (in mathematical sense), but you can also broadcast it to get the same result. On the other hand addition of a scalar and a matrix is not a standard operation so you have to broadcast it. There is a difference though if you wanted to use broadcast fusion as then you would need to use `.` everywhere. – Bogumił Kamiński Sep 14 '18 at 15:27

Julia Learner · Answer 2 · 2018-09-25T04:42:26.057

This code illustrates Bogumił Kamiński's helpful comment:

julia> m = 1.1
1.1
julia> c = 0.11
0.11
julia> x = rand(1,2)
1×2 Array{Float64,2}:
 0.77683  0.510671

# .+ operator IS used for the addition of the constant.    
julia> y = m*x .+ c
1×2 Array{Float64,2}:
 0.964514  0.671738

# .+ operator is NOT used for the addition of the constant, get Error.    
julia> y = m*x + c
ERROR: MethodError: no method matching +(::Array{Float64,2}, ::Float64)

You are not alone in wanting to do a matrix + a scalar operation, see here.

However, whether it is allowed depends on how you define your math and how you define your software. I have written data matrix software where adding a scalar to a matrix is a standard operation.

However, the rules for that software were quite different from the rules for matrices in standard linear algebra. For example, in the data matrix algebra employed by that software, AB = BA, thus commutative for matrix multiplication because the matrix multiplication operator was defined as element-by-element multiplication.

That is totally different from standard matrix algebra, but was what I wanted.

adding scalar to an array in julia

2 Answers2