[R] Difficult with round() function

[Carl Witthoft]

I believe you've fallen into one of the R FAQs, namely the difference 
between a float and an integer.

There are probably much better ways to set up your 'before' and 'after' 
gridpoint references,  but you could start out by replacing the 
offending line with

positions <- which(round(benchmarks)-benchmarks < 1-e20)

or better yet, replace "1e-20" with, say,  interval/to/100


Carl


<quote>
From: Aaron Polhamus <aaronpolhamus_at_gmail.com>
Date: Mon, 17 Jan 2011 12:02:21 -0800


Dear list,

I'm writing a function to re-grid a data set from finer to coarser 
resolutions in R as follows (I use this function with sapply/apply):

gridResize <- function(startVec = stop("What's your input vector"), to = 
stop("Missing 'to': How long do you want the fnial vector to be?")){ 
from <- length(startVec)
shortVec<-numeric()
tics <- from*to
for(j in 1:to){
interval <- ((j/to)*tics - (1/to)*tics + 1):((j/to)*tics) benchmarks <- 
interval/to
  #FIRST RUN ASSUMES FINAL BENCHMARK/TO IS AN INTEGER... positions <- 
which(round(benchmarks) == benchmarks) indeces <- benchmarks[positions]
fracs <- numeric()
  #SINCE MUCH OF THE TIME THIS WILL NOT BE THE CASE, THIS SCRIPT DEALS 
WITH THE REMAINDER...
for(i in 1:length(positions)){
if(i == 1) fracs[i] <- positions[i]/length(benchmarks) else{ fracs[i] <- 
(positions[i] - sum(positions[1:(i-1)]))/length(benchmarks) }
}
  #AND UPDATES STARTVEC INDECES AND FRACTION MULTIPLIERS 
if(max(positions) != length(benchmarks)) indeces <- c(indeces, 
max(indeces) + 1)
if(sum(fracs) != 1) fracs <- c(fracs, 1 - sum(fracs))  fromVals <- 
startVec[indeces]
  if(any(is.na(fromVals))){
NAindex <- which(is.na(fromVals))
if(sum(Fracs[-NAindex]) >= 0.5) shortVec[j] <- sum(fromVals*fracs, 
na.rm=TRUE) else shortVec[j] <- NA
}else{shortVec[j] <- sum(fromVals*fracs)} }
return(shortVec)
}

for the simple test case test <- gridResize(startVec = 
c(2,4,6,8,10,8,6,4,2), to = 7) the function works fine. For larger 
vectors, however, it breaks down. E.g.: test <- gridResize(startVec = 
rnorm(300, 9, 20), to = 200)

This returns the error:

Error in positions[1:(i - 1)] :
   only 0's may be mixed with negative subscripts

and the problem seems to be in the line positions <- 
which(round(benchmarks) == benchmarks). In this particular example the 
code cracks up at j = 27. When set j = 27 and run the calculation 
manually I discover the following:

 > benchmarks[200]

[1] 40
 > benchmarks[200] == 40

[1] FALSE
 > round(benchmarks[200]) == 40

[1] TRUE Even though my benchmark calculation seems to be returning a 
clean integers to serve as inputs for the creation of the 'positions' 
variable, for whatever reason R doesn't read it that way. I would be 
very grateful for any advice on how I can either alter my approach 
entirely (I am sure there is a far more elegant way to regrid data in R) 
or a simple fix for this rounding error.

Many thanks in advance,
Aaron

-- 
Aaron Polhamus <aaronpolhamus_at_gmail.com>
Statistical consultant, Revolution Analytics
MSc Applied Statistics, The University of Oxford, 2009
838a NW 52nd St, Seattle, WA 98107
Cell: +1 (206) 380.3948