[R] Warning: as.numeric reorders factor data

[Bud Gibson]

Recently, I was using aggregate() to develop averages by trial for an 
experiment I was running.  Trials were indicated as ordinal numbers for 
each subject.  aggregate() turned trial into factors during the 
aggregation process.  I then wanted to create a scatter plot of subject 
performance by trial, so I applied as.numeric to the (now) factor 
variable trial.  as.numeric reordered the trial indicator creating some 
(at first) incomprehensible results.

Investigation revealed that aggregate must first be interpreting trial 
as a character and then turning it into a factor.  The behavior I 
observed is reproducible from the following transcript using R1.6.1 on 
RH linux 7.3.

 > test <- as.factor(as.character(c(1,2,3,4,5,6,7,8,9,10,11)))
 > test
  [1] 1  2  3  4  5  6  7  8  9  10 11
Levels: 1 10 11 2 3 4 5 6 7 8 9
 > as.numeric(test)
  [1]  1  4  5  6  7  8  9 10 11  2  3

It strikes me that as.numeric should *never* reorder the vector it is 
working on.  There is this workaround for the problem:

 > as.numeric(as.character(test))
  [1]  1  2  3  4  5  6  7  8  9 10 11

However, I should not have to know about the internals of aggregate to 
be able to use its results.

Bud Gibson