creation of BitArray using comprehension

[diegozea]

Now is necessary create a Bool Array using list comprehension an after that make the conversion to BitArray:

julia> BitArray[ ab_index_array[res]!=0 for res in seq ]
ERROR: no method convert(Type{BitArray{N}},Bool)
 in anonymous at no file

julia> Bool[ab_index_array[res]!=0 for res in seq ]
100000000-element Bool Array:
 false
  true
      ⋮
 false
 false

julia> convert(BitArray,Bool[ab_index_array[res]!=0 for res in seq ])
100000000-element BitArray:
 false
  true
     ⋮
 false
 false

[PythonNut]

On another note, can we also make a Nullable{T} comprehension into an NullableArray{T,1} (discussed in #8423)?

[StefanKarpinski]

This seem to call for some more general mechanism. The same thing would be handy for using array literals that contain null to construct NullableArray objects.

[PythonNut]

Is there any way we can do something like C++ template specializations, and make the in-memory representation of a Array{Nullable{T},1} identical to a NullableArray{T,1}?

[johnmyleswhite]

I think there's two separate issues here:

• The array-of-structs to struct-of-arrays transform needed to turn Array{Nullable{T}} into NullableArray{T}
• The ability to use something like list comprehensions that generates something other than an array as output.

Both are interesting, although the second point seems to cause more trouble for beginners (who don't seem to think about the first point) since they keep failing to understand why comprehensions don't produce DataArray objects.

[stevengj]

This should be do-able with the new generalized comprehension syntax, but it looks like we need to add a constructor method:

julia> BitVector(rand(Bool) for i = 1:10)
ERROR: MethodError: Cannot `convert` an object of type Base.Generator{UnitRange{Int64},##5#6} to an object of type BitArray{1}
This may have arisen from a call to the constructor BitArray{1}(...),
since type constructors fall back to convert methods.
 in BitArray{1}(::Base.Generator{UnitRange{Int64},##5#6}) at ./sysimg.jl:53

[StefanKarpinski]

Since we can use generators now, I'm removing the breaking label and accepting this.