[R] How to optimize this loop ?

[Chuck Cleland]

Nicolas Prune wrote:
> Dear R Users,
> 
> I request your help to optimize a loop.
> 
> Given a series of observations, I want to know how many consecutive past
> observations are below the last one.
> 
> e.g :
> my_series <- c(3, 4, 10,14,8,3,4,6,9)
> 
> As the last number (9)  is higher than the four preceding numbers (6, 4, 3, 8),
> this function should return 4.
> 
> my_series <- c(3, 4, 10,14,8,3,4,11,9)
> Here, it should return 0, as 9 is immediately preceeded by a higher number.
> 
> So far, I do this awful loop :
> 
> result <- 0
> for (i in 1:length(my_series-1))
> {
>  if (my_series[length(my_series)-i]>end(my_series)[1])
> { result <- i-1 ; break }
> }
> 
> I thing there's a better way...

  Just in terms of finding out which values are bigger than the
preceding value and avoiding the explicit loop, you could do something
like this:

seriesA <- c(3,4,10,14,8,3,4,6,9)
seriesB <- c(3,4,10,14,8,3,4,11,9)

sign(diff(seriesA)) == 1
[1]  TRUE  TRUE  TRUE FALSE FALSE  TRUE  TRUE  TRUE

sign(diff(seriesB)) == 1
[1]  TRUE  TRUE  TRUE FALSE FALSE  TRUE  TRUE FALSE

> my_series > my_series[end][1] returns :
> TRUE TRUE FALSE FALSE TRUE TRUE TRUE TRUE FALSE
> , which seems more appealing (once the last "FALSE" is removed), but now, how to
> know the size of the last consecutive series of "TRUE" ?
> 
> Can you see a better way ?
> 
> Thanks.
> 
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-- 
Chuck Cleland, Ph.D.
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