df1[B] = df2[B] where df1 and df2 are DataFrames B is boolean array

code.jl

using DataFrames

df1 = DataFrame(a = repeat([1], 100), b = "a")
df2 = DataFrame(a = repeat([2], 100), b = "b")

B = Array{Bool, 2}(undef, 100, 2)

df1[B] # doesn't work

# Let's overload get index get index

function Base.getindex(df::AbstractDataFrame, B::AbstractArray{Bool, 2})
    @assert size(B) == size(df)

    res = []

    for (colnumber, Bcol) in enumerate(eachcol(B))
        res = vcat(res, df[Bcol, colnumber])
    end

    res
end

df1[B]

findall(B)

# How do I make assignment work? df1[B] = df2[B]
# Overload the Base.setindex! method
function Base.setindex!(df::AbstractDataFrame, vals_to_assign, B::AbstractArray{Bool, 2})
    @assert size(B) == size(df)

    idx_to_assign = findall(B)

    @assert length(idx_to_assign) == length(vals_to_assign)

    for (idx, val) in zip(idx_to_assign, vals_to_assign)
        df[idx] = val
    end

    vals_to_assign
end

# check the vaules should be 1 and "a"
df1[B]

# check the vaules should be 2 and "b"
df2[B]

# assign
df1[B] = df2[B]

# viola!
df1[B]

df1