Equivalent of the dot prefix for an arbitrary function?

Question

Julia allows me to do elementwise binary operators on arrays/matrices with .:

[1 2 3] .+ [3 2 1]
[1 2 3] .> [3 2 1]

Is there a standard way to apply an arbitrary function f elementwise to a pair of arrays?

Answer 1

If you just want to apply a binary function to each pair of values from two lists, you can use map:

f(x,y) = 2x - y^2

julia> map(f, [1, 2, 3], [3, 2, 1])
3-element Array{Int64,1}:
 -7
  0
  5

The .+ and .> operators also have the additional behavior that singleton dimensions are broadcast, like so:

julia> [1, 2, 3] .+ [-1 -2 -3]
3x3 Array{Int64,2}:
 0  -1  -2
 1   0  -1
 2   1   0

julia> rand(3,4) .+ [1, 2, 3]
3x4 Array{Float64,2}:
 1.73798  1.84132  1.12923  1.30192
 2.10961  2.17835  2.52779  2.3028
 3.16457  3.04659  3.67604  3.08869

julia> rand(3,4) .+ [1 2 3 4]
3x4 Array{Float64,2}:
 1.40294  2.31384  3.34001  4.60027
 1.13289  2.99275  3.50606  4.51049
 1.31486  2.7585   3.64655  4.59647

If you also want this kind of behavior, you can use the broadcast function:

julia> broadcast(f, [1 2 3], [3, 2, 1])
3x3 Array{Int64,2}:
 -7  -5  -3
 -2   0   2
  1   3   5