Matching of array elements in Julia

Question

With x = Any[[1,2],[2,3],[3,4],[4,5]], I tried the following line with Julia0.4.0

x[ x .== [3,4] ]

but it resulted in an error

ERROR: DimensionMismatch("arrays could not be broadcast to a common size")

I expected it to give something like Any[ [3,4] ] because

x[3] == [3,4]   # => true

is no problem. Although this operation itself may not be useful, I would like to know what the error message means. So I would appreciate any hints why this error occurs.

Answer 1

To make an element-by-element comparison, Julia requires that both arrays have the same number of elements. This can be achieved in this case with a comprehension:

julia> x = Any[[1,2],[2,3],[3,4],[4,5]]
4-element Array{Any,1}:
 [1,2]
 [2,3]
 [3,4]
 [4,5]

julia> x[x.==[[3,4] for i in 1:length(x)]]
1-element Array{Any,1}:
 [3,4]

So the question in my mind was "why doesn't Julia broadcast the [3,4] into this shape automatically?". The following example is broadcast correctly:

julia> y = [1,2,3,4]
4-element Array{Int64,1}:
 1
 2
 3
 4

julia> y.==3
4-element BitArray{1}:
 false
 false
  true
 false

julia> y[y.==3]
1-element Array{Int64,1}:
 3

It appears that Julia's broadcast mechanism is unable to infer that we want [3,4] to be broadcast into [[3,4],[3,4],[3,4],[3,4]] rather than some other shape of array.

Answer 2

You can help Julia a little in figuring out how to broadcast the second variable by writing the comparison like this:

julia> x .== Any[[3, 4]]

You get a BitArray out as expected:

4-element BitArray{1}:
 false
 false
  true
 false

So indexing with the result of the comparison works too:

julia> x[ x .== Any[[3,4]] ]
1-element Array{Any,1}:
 [3,4]