[R] vector vs array

[Adaikalavan Ramasamy]

General Notes :
a) Please try to give a simple example
b) Please avoid the rightwards assignment (i.e. "->"). Eventhough it is
perfectly legal to use it, it is confusing especially when you are
posting to a mailing list.


1) Here is a reproducible example

 set.seed(1)                         # for reproducibility
 v   <- abs( rnorm(1000) )
 thr <- c( 0.5, 1.0, 2.0, 3.0 )


2) If you simply want to count the number of points above a threshold

 sapply( thr, function(x) sum(v > x) )
 [1] 620 326  60   3


3) Or you can cut the data by threshold limits (be careful at the edges
if you have discrete data) followed by breaks

 table( cut( v, breaks=c( -Inf, thr, Inf ) ) ) )

 (-Inf,0.5]    (0.5,1]      (1,2]      (2,3]    (3,Inf]
        380        294        266         57          3

 
4) If you want to turn the problem on its head and ask for which
threshold point would you get 99%, 99.9% and 99.99% of the data below
it, you can use use quantiles

 quantile( v, c(0.99, 0.999, 0.9999) )
      99%    99.9%   99.99%
 2.529139 3.056497 3.734899


Regards, Adai



On Mon, 2005-08-08 at 08:34 -0700, alessandro carletti wrote:
> Hi!
> OK, I'm trying to select some "useful outliers" from
> my dataset: I defined 11 "treshold" values (1 for each
> level of a variable (sampling site) as follows:
> 
> 
> tresholds<-function(x)
> {
> tapply(x,mm$NAME,FUN=mean ,simplify = T, na.rm=T)->med
> 
> 
> tapply(x,mm$NAME,FUN=sd ,simplify = T,
> na.rm=T)->standev 
> 
> standev+med
> 
> }
> tresholds(mm$chl)
> 
> 
> Now I'd like to select those values from vector mm$chl
> that are higher than each "treshold value", but how
> can I compare a vector with 1885 elements with the one
> with 11?
> Sorry for this (probably) stupid question...
> and thanks in advance.
> Alessandro
> 
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