Case When in Julia

case-when functionality via the ternary operator

As far as I can tell, DataFrames.jl doesn’t have a built-in case-when function, but I found this post from Bogumil Kaminski showing how to implement case-when via the ternary operator.

Here’s the basic idea, but it’s also extensible to include multiple variables or to include missing values.

using DataFrames

X = DataFrame(a = 1:10)

10×1 DataFrame

Row	a
	Int64
1	1
2	2
3	3
4	4
5	5
6	6
7	7
8	8
9	9
10	10

transform!(
    X,
    :a => ByRow(a -> a % 2 == 0 ? "foo" : a % 3 == 0 ? "bar" : "baz") => :y
);

first(X, 5)

5×2 DataFrame

Row	a	y
	Int64	String
1	1	baz
2	2	foo
3	3	bar
4	4	foo
5	5	baz