R help - Sep 2010 - approxfun returning value higher than I would expect

Below is the code that I am using in a much larger function.  I would
expect a bankfull measure at zero to be between 0.6 and 0.8 approxfun
is returning 0.8136986.  I am sure that I am missing something.

measure_bkf <- (structure(list(measurment_num = c(0, 0.2, 0.4, 0.6,
0.8, 1, 1.2,
1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4), bankfull_depths_m
= c(-0.48,
-0.48, -0.42, -0.26, 0.58, 0.48, 0.47, 0.54, 0.5, 0.52, 0.52,
0.56, 0.58, 0.61, 0.68, 0.62, 0.67, 0.66)), .Names =
c("measurment_num",
"bankfull_depths_m"), row.names = c("6124", "612",
"613", "614",
"615", "616", "617", "618",
"619", "620", "621", "622",
"623",
"624", "625", "626", "627",
"628"), class = "data.frame"))


measure_bkf_not_zero <- subset(measure_bkf, measure_bkf$bankfull_depths_m!=0)

bkf_min <-
which.max(measure_bkf_not_zero[,"bankfull_depths_m"]<0)

bkf_max <- which.max(measure_bkf_not_zero[,"bankfull_depths_m"])

#bkf_min <- ifelse(length(bkf_min)>1, bkf_min[1], bkf_min)
#bkf_max <- ifelse(length(bkf_max)>1, bkf_max[1], bkf_max)

#s <- with(measure_bkf_not_zero, approx(measurment_num,
bankfull_depths_m,
xout=seq(measure_bkf_not_zero[bkf_min,"measurment_num"],
measure_bkf_not_zero[bkf_max,"measurment_num"], length=2000)))
#int_bkf <- with(s, x[which.min(y[y>0])])

s <- with(measure_bkf_not_zero[bkf_min:bkf_max,],
approxfun(bankfull_depths_m, measurment_num), ties=mean)

int_bkf <- s(0)

-- 
Stephen Sefick
____________________________________
| Auburn University? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? |
| Department of Biological Sciences? ? ? ? ?? |
| 331 Funchess Hall? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? |
| Auburn, Alabama? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?? |
| 36849? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? |
|___________________________________|
| sas0025 at auburn.edu? ? ? ? ? ? ? ? ? ? ? ? ? ?? |
| http://www.auburn.edu/~sas0025? ? ? ? ? ?? |
|___________________________________|

Let's not spend our time and resources thinking about things that are
so little or so large that all they really do for us is puff us up and
make us feel like gods.? We are mammals, and have not exhausted the
annoying little problems of being mammals.

? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? -K. Mullis

"A big computer, a complex algorithm and a long time does not equal
science."

? ? ? ? ? ? ? ? ? ? ? ? ? ? ? -Robert Gentleman

On 15/09/2010 3:48 PM, stephen sefick wrote:
> Below is the code that I am using in a much larger function.  I would
> expect a bankfull measure at zero to be between 0.6 and 0.8 approxfun
> is returning 0.8136986.  I am sure that I am missing something.
>
> measure_bkf<- (structure(list(measurment_num = c(0, 0.2, 0.4, 0.6,
> 0.8, 1, 1.2,
> 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4), bankfull_depths_m
> = c(-0.48,
> -0.48, -0.42, -0.26, 0.58, 0.48, 0.47, 0.54, 0.5, 0.52, 0.52,
> 0.56, 0.58, 0.61, 0.68, 0.62, 0.67, 0.66)), .Names =
c("measurment_num",
> "bankfull_depths_m"), row.names = c("6124",
"612", "613", "614",
> "615", "616", "617", "618",
"619", "620", "621", "622",
"623",
> "624", "625", "626", "627",
"628"), class = "data.frame"))
>
>
> measure_bkf_not_zero<- subset(measure_bkf,
measure_bkf$bankfull_depths_m!=0)
>
> bkf_min<-
which.max(measure_bkf_not_zero[,"bankfull_depths_m"]<0)
>
> bkf_max<-
which.max(measure_bkf_not_zero[,"bankfull_depths_m"])
>
> #bkf_min<- ifelse(length(bkf_min)>1, bkf_min[1], bkf_min)
> #bkf_max<- ifelse(length(bkf_max)>1, bkf_max[1], bkf_max)
>
> #s<- with(measure_bkf_not_zero, approx(measurment_num,
> bankfull_depths_m,
> xout=seq(measure_bkf_not_zero[bkf_min,"measurment_num"],
> measure_bkf_not_zero[bkf_max,"measurment_num"], length=2000)))
> #int_bkf<- with(s, x[which.min(y[y>0])])
>
> s<- with(measure_bkf_not_zero[bkf_min:bkf_max,],
> approxfun(bankfull_depths_m, measurment_num), ties=mean)
>
> int_bkf<- s(0)
>
It is easier to see the problem if you don't leave all the complications 
in the beginning.  Just define some variables and
show the interpolation on a plot:

x <- 
c(-0.48,-0.48,-0.42,-0.26,0.58,0.48,0.47,0.54,0.5,0.52,0.52,0.56,0.58,0.61,0.68)
y <- c(0,0.2,0.4,0.6,0.8,1,1.2,1.4,1.6,1.8,2,2.2,2.4,2.6,2.8)
plot(x, y)
s <- approxfun(x, y, ties=mean)
curve(s, add=TRUE)

On my system, this looks okay in 2.11.1, but not in R-patched or R-devel 
(soon to be 2.12.0).  It is fixed if the x values are ordered, but it's 
not supposed to need that.  I'll take a look.

Duncan Murdoch