[R] Condition to factor (easy to remember)

[Peter Dalgaard]

Douglas Bates wrote:
> On Wed, Sep 30, 2009 at 2:42 PM, Douglas Bates <bates at stat.wisc.edu> wrote:
>> 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.
> 
> And besides, Frank Harrell will soon be weighing in to tell you why
> you shouldn't dichotomize in the first place.

And someone might also remind you that it is safest to include 
levels=c(FALSE,TRUE), just in case the condition is always TRUE. (Terry 
Thernau has the scars from the implementation of Surv()...)

-- 
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