[R] Sort matrix by column 1 ascending then by column 2 decending
Duncan Murdoch
murdoch at stats.uwo.ca
Wed May 27 18:48:34 CEST 2009
On 5/27/2009 8:39 AM, Paul Geeleher wrote:
> I've got a matrix with 2 columns and n rows. I need to sort it first
> by the values in column 1 ascending. Then for values which are the
> same in column 1, sort by column 2 decending. For example:
You've seen a few ways. Here are some more:
1. Use the fact that order() uses a stable sort algorithm, so just sort
by the second column then the first:
x <- matrix(c(2,1,1,3,.5,.3,.5,.2), ncol=2)
x1 <- x[order(x[,2], decreasing=TRUE),]
x2 <- x1[order(x1[,1]),]
x2
2. Use the fact that your values are numeric, so negatives sort in the
reverse order of positives:
x[order(x[,1], -x[,2]),]
3. If the values aren't known to be numeric, convert them to numeric
before using them as sort keys:
x[order(xtfrm(x[,1]), -xtfrm(x[,2])),]
In any of these, watch out for NA handling. My methods all put NA
values last, but that might not be what you want.
Duncan Murdoch
>
> 2 .5
> 1 .3
> 1 .5
> 3 .2
>
> Goes to:
>
> 1 .5
> 1 .3
> 2 .5
> 3 .2
>
> This is easy to do in spreadsheet programs but I can't seem to work
> out how to do it in R and haven't been able to find a solution
> anywhere.
>
>
> Thanks!
>
> -Paul.
>
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