[R] Condition to factor (easy to remember)

[Douglas Bates]

On Wed, Sep 30, 2009 at 2:43 AM, Dieter Menne
<dieter.menne at menne-biomed.de> wrote:

> Dear List,

> creating factors in a given non-default orders is notoriously difficult to
> explain in a course. Students love the ifelse construct given below most,
> but I remember some comment from Martin Mächler (?) that ifelse should be
> banned from courses.

> Any better idea? Not necessarily short, easy to remember is important.

> Dieter

> data = c(1,7,10,50,70)
> levs = c("Pre","Post")
>
> # Typical C-Programmer style
> factor(levs[as.integer(data >10)+1], levels=levs)
>
> # Easiest to understand
> factor(ifelse(data <=10, levs[1], levs[2]), levels=levs)

Why not

> factor(data > 10, labels = c("Pre", "Post"))
[1] Pre  Pre  Pre  Post Post
Levels: Pre Post

All you have to remember is that FALSE comes before TRUE.