[R] Second largest element from each matrix row

David Winsemius dwinsemius at comcast.net
Tue Apr 26 14:36:11 CEST 2011

On Apr 26, 2011, at 8:01 AM, Lars Bishop wrote:

> Hi,
> I need to extract the second largest element from each row of a
> matrix. Below is my solution, but I think there should be a more  
> efficient
> way to accomplish the same, or not?
> set.seed(1)
> a <- matrix(rnorm(9), 3 ,3)
> sec.large <- as.vector(apply(a, 1, order, decreasing=T)[2,])
> ans <- sapply(1:length(sec.large), function(i) a[i, sec.large[i]])
> ans

There are probably many but this one is reasonably compact, one-step,  
and readable:

 > ans2 <- apply(a, 1, function(i) sort(i)[ dim(a)[2]-1])
 > ans2

Refreshing my mail client proves I was right about many solutions, but  
this is the first (so far) to use the dim attribute.

David Winsemius, MD
West Hartford, CT

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