R help - Oct 2007 - having problems re-ordering a dataframe

Dear R users,

I need to reorder a dataframe using 3 variables for the determine the  
sorting order.

When I create a simple dataframe to test the method, things work as I  
expected:

a1 <- rep(1:10, each=8)
a2 <- rep(rep(1:2, each=4), 10)
a3 <- rep(c(1:4),20)
(a <- data.frame(a1, a2, a3))

for each combination of a1 and a2, a3 is increasing

t <- order(a$a1, a$a2, rev(a$a3))
b <- a[t,]

In this new dataframe, the 3rd variable is in decreasing order within  
each combination of a1 and a2, which is the desired result.
As expected, this still works if the second variable never changes:

e1 <- rep(1:10, each=8)
e2 <- rep(2, 80)
e3 <- rep(c(1:8),10)
e <- data.frame(e1, e2, e3)
t <- order(e$e1, e$e2, rev(e$e3))
f <- e[t,]

With my real data, I do not get the 3rd variable in inverse order for  
each combination of the first 2. Here I recreate the beginning of my  
dataframe for you to see what I mean:

c1 <- c(rep(813,102), rep(826,48))
c2 <- rep(2,150)
c3 <- c(seq(1:102), seq(1,48))
c <- data.frame(c1, c2, c3)
t <- order(c$c1, c$c2, rev(c$c3))
d <- c[t,]

The dataframe d is properly ordered for c1==826, but not for c1==813,  
where c3 goes from 48 down to 1, and then from 102 down to 49,  
instead of going from 102 down to 1. All the variables are numeric,  
not factors.

Any help/explanation will be greatly appreciated.

Denis
sessionInfo()
R version 2.6.0 (2007-10-03)
powerpc-apple-darwin8.10.1

locale:
fr_CA.UTF-8/fr_CA.UTF-8/fr_CA.UTF-8/C/fr_CA.UTF-8/fr_CA.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

Chabot Denis wrote:
> Dear R users,
>
> I need to reorder a dataframe using 3 variables for the determine the  
> sorting order.
>
> When I create a simple dataframe to test the method, things work as I  
> expected:
>
> a1 <- rep(1:10, each=8)
> a2 <- rep(rep(1:2, each=4), 10)
> a3 <- rep(c(1:4),20)
> (a <- data.frame(a1, a2, a3))
>
> for each combination of a1 and a2, a3 is increasing
>
> t <- order(a$a1, a$a2, rev(a$a3))
> b <- a[t,]
>   
Using "rev(a$a3)" messes things up when the values of a3 aren't 
identical in every (a1, a2) group.  If you know a3 is numeric with no NA 
or NaN values, you could use

t <- order(a$a1, a$a2, -a$a3)

to get the sort order reversed.  If not, you need to do two sorts, 
relying on the fact that order uses a stable sort:

t1 <- order(a$a3, decreasing=TRUE)
t2 <- with(a[t1,], order(a1, a2))
t <- t1[t2]
b <- a[t,]

It would be nice if the decreasing arg to order was allowed to be a 
vector, with entries applied in the obvious way, so that

t <- with(a, order(a1, a2,  a3, decreasing = c(FALSE, FALSE, TRUE)))

would work, but alas, it doesn't.

Duncan Murdoch
> In this new dataframe, the 3rd variable is in decreasing order within  
> each combination of a1 and a2, which is the desired result.
> As expected, this still works if the second variable never changes:
>
> e1 <- rep(1:10, each=8)
> e2 <- rep(2, 80)
> e3 <- rep(c(1:8),10)
> e <- data.frame(e1, e2, e3)
> t <- order(e$e1, e$e2, rev(e$e3))
> f <- e[t,]
>
> With my real data, I do not get the 3rd variable in inverse order for  
> each combination of the first 2. Here I recreate the beginning of my  
> dataframe for you to see what I mean:
>
> c1 <- c(rep(813,102), rep(826,48))
> c2 <- rep(2,150)
> c3 <- c(seq(1:102), seq(1,48))
> c <- data.frame(c1, c2, c3)
> t <- order(c$c1, c$c2, rev(c$c3))
> d <- c[t,]
>
> The dataframe d is properly ordered for c1==826, but not for c1==813,  
> where c3 goes from 48 down to 1, and then from 102 down to 49,  
> instead of going from 102 down to 1. All the variables are numeric,  
> not factors.
>
> Any help/explanation will be greatly appreciated.
>
> Denis
> sessionInfo()
> R version 2.6.0 (2007-10-03)
> powerpc-apple-darwin8.10.1
>
> locale:
> fr_CA.UTF-8/fr_CA.UTF-8/fr_CA.UTF-8/C/fr_CA.UTF-8/fr_CA.UTF-8
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
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