[R] "reverse" quantile function

[David Winsemius]

> On Jun 15, 2017, at 12:37 PM, Andras Farkas via R-help <r-help at r-project.org> wrote:
> 
> Dear All,
> 
> we have:
> 
> t<-seq(0,24,1) 
> a<-10*exp(-0.05*t) 
> b<-10*exp(-0.07*t) 
> c<-10*exp(-0.1*t) 
> d<-10*exp(-0.03*t) 
> z<-data.frame(a,b,c,d) 
> 
> res<-t(apply(z, 1, quantile, probs=c(0.3))) 
> 
> 
> 
> my goal is to do a 'reverse" of the function here that produces "res" on a data frame, ie: to get the answer 0.3 back for the percentile location when I have "res" available to me... For a single vector this would be done using ecdf something like this:
> 
> x <- rnorm(100) 
> #then I know this value:  
> quantile(x,0.33) 
> #so do this step
> ecdf(x)(quantile(x,0.33)) 
> #to get 0.33 back...
> 
> any suggestions on how I could to that for a data frame?

Can't you just used ecdf and quantile ecdf?

# See ?ecdf page for both functions

> lapply( lapply(z, ecdf), quantile, 0.33)
$a
     33% 
4.475758 

$b
     33% 
3.245151 

$c
     33% 
2.003595 

$d
     33% 
6.173204 
-- 

David Winsemius
Alameda, CA, USA