Conditional statements with Dataframes [Julia v1.0] - julia

Question

I am porting over custom functions from R. I would like to use Julia Dataframes to store my data. I like to reference by column name instead of, say, array indices hence I am using the Dataframes package.
I simplified the follow to illustrate:
if( DataFrame(x=1).x .>1) end
The error is:
ERROR: TypeError: non-boolean (BitArray{1}) used in boolean context
Is there a simple workaround that would allow me to continue using DataFrames?

Answer 1

The expression:
DataFrame(x=1).x .> 1
Does the following things:
Creates a DataFrame
Extracts a column x from it
Compares all elements of this column to 1 using vectorized operation .> (broadcasting in Julia parlance)
In effect you get the following one element array:
julia> DataFrame(x=1).x .> 1
1-element BitArray{1}:
false
As opposed to R, Julia distinguishes between vectors and scalars so it is not the same as simply writing false. Moreover if statement expects a scalar not a vector, so something like this works:
if 2 > 1
println("2 is greater than 1")
end
but not something like this:
if DataFrame(x=2).x .> 1
println("success!")
end
However, for instance this would work:
if (DataFrame(x=2).x .> 1)[1]
println("success!")
end
as you extract the first (and only in this case) element from the array.
Notice that in R if you passed more than one-element vector to a conditional expression you get a warning like this:
> if (c(T,F)) {
+ print("aaa") } else {print("bbb")}
[1] "aaa"
Warning message:
In
the condition has length > 1 and only the first element will be used
Simply Julia is stricter than R in checking the types in this case. In R you do not have a distinction between scalars and vectors, but in Julia you have.
EDIT:
length(df) returns you the number of columns of a DataFrame (not number of rows). If you are coming from R it is easier to remember nrow and ncol functions.
Now regarding your question you can write either:
for i in 1:nrow(df)
if df.x[i] > 3
df.y[i] = df.x[i] + 1
end
end
or
bigx = df.x .> 3
df.y[bigx] = df.x[bigx] .+ 1
or
df.y .= ifelse.(df.x .> 3, df.x .+ 1, df.y)
or using DataFramesMeta to shorten the notation:
using DataFramesMeta
#with df begin
df.y .= ifelse.(:x .> 3, :x .+ 1, :y)
end
or
using DataFramesMeta
#byrow! df begin
if :x > 3
:y = :x + 1
end
end