Finding local maxima and minima

Question

I\'m looking for a computationally efficient way to find local maxima/minima for a large list of numbers in R. Hopefully without for loops...

For exampl

Answer 1

There are some good solutions provided, but it depends on what you need.

Just diff(tt) returns the differences.

You want to detect when you go from increasing values to decreasing values. One way to do this is provided by @Ben:

 diff(sign(diff(tt)))==-2

The problem here is that this will only detect changes that go immediately from strictly increasing to strictly decreasing.

A slight change will allow for repeated values at the peak (returning TRUE for last occurence of the peak value):

 diff(diff(x)>=0)<0

Then, you simply need to properly pad the front and back if you want to detect maxima at the beginning or end of

Here's everything wrapped in a function (including finding of valleys):

 which.peaks <- function(x,partial=TRUE,decreasing=FALSE){
     if (decreasing){
         if (partial){
             which(diff(c(FALSE,diff(x)>0,TRUE))>0)
         }else {
             which(diff(diff(x)>0)>0)+1
         }
     }else {
         if (partial){
             which(diff(c(TRUE,diff(x)>=0,FALSE))<0)
         }else {
             which(diff(diff(x)>=0)<0)+1
         }
     }
 }