Using previous row values to create values for a new column

[jling]

looks like you have order dependency thus the task is not columnar. you can do it in a loop:

julia> b = similar(df.a, Int); b[1] = 1;

julia> for i in 2:10
           b[i] = b[i-1] + (df.a[i] >= 0.5)
       end

julia> df.b = b