[R] configure ddply() to avoid reordering of '.variables'

[Liviu Andronic]

Hello,
I'm using ddply() in plyr and I notice that it has the habit of
re-ordering the levels of the '.variables' by which the splitting is
done. I'm concerned about correctly retrieving the original ordering.

Consider:
require(plyr)
x <- iris[ order(iris$Species, decreasing=T), ]
head(x)
#    Sepal.Length Sepal.Width Petal.Length Petal.Width   Species
#101          6.3         3.3          6.0         2.5 virginica
#102          5.8         2.7          5.1         1.9 virginica
#103          7.1         3.0          5.9         2.1 virginica
#104          6.3         2.9          5.6         1.8 virginica
#105          6.5         3.0          5.8         2.2 virginica
#106          7.6         3.0          6.6         2.1 virginica
xa <- ddply(x, .(Species), function(x)
{data.frame(Sepal.Length=x$Sepal.Length, mean.adj=(x$Sepal.Length -
mean(x$Sepal.Length)))})
#  |==============================================================================================|
100%
##notice how the ordering of Species is different
##from that in the input data frame
head(xa)
#  Species Sepal.Length mean.adj
#1  setosa          5.1    0.094
#2  setosa          4.9   -0.106
#3  setosa          4.7   -0.306
#4  setosa          4.6   -0.406
#5  setosa          5.0   -0.006
#6  setosa          5.4    0.394
all.equal(xa$Species, x$Species)
#[1] "100 string mismatches"
all.equal(xa[ order(xa$Species, decreasing=T), ]$Species, x$Species)
#[1] TRUE
all.equal(xa$Sepal.Length, x$Sepal.Length)
#[1] "Mean relative difference: 0.2785"
all.equal(xa[ order(xa$Species, decreasing=T), ]$Sepal.Length, x$Sepal.Length)
#[1] TRUE

In my real data, should I be concerned that simply reordering by the
'.variables' variable wouldn't necessarily restore the original
ordering as in the input data frame? Is it possible to instruct
ddply() to avoid re-ordering the supplied '.variables' variable?

Regards,
Liviu


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