[R] Second largest element from each matrix row

[Peter Ehlers]

On 2011-04-26 05:26, Dimitris Rizopoulos wrote:
> one way is the following:
>
> a<- matrix(rnorm(9), 3 ,3)
>
> aa<- a[order(row(a), -a)]
> matrix(aa, nrow(a), byrow = TRUE)[, 2]

That's clever, Dmitris. And very fast!

Lars: my apologies; I didn't read your request
carefully. You asked for more _efficient_ and
I just thought 'shorter code'. I should know
that whenver I think 'apply', I should think
'matrix'.

Peter Ehlers

>
>
> I hope it helps.
>
> Best,
> Dimitris
>
>
> On 4/26/2011 2:01 PM, 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
>>
>> Thanks in advance for your help,
>> Lars.
>>
>> 	[[alternative HTML version deleted]]
>>
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>