[R] Zero Index Origin?
Gabor Grothendieck
ggrothendieck at myway.com
Thu Apr 1 21:17:19 CEST 2004
Peter Wolf wrote:
sort.6<-function(a){
n<-length(a)
adapt<-function(i){i+1} # local function to perform the index correction
a<-c(0,a)
for(i in 2:n){
j<-i-1
a[adapt(0)]<-a[adapt(i)]
while(a[adapt(j)]>a[adapt(0)]){
a[adapt(j+1)]<-a[adapt(j)]
j<-j-1
}
a[adapt(j+1)]<-a[adapt(0)]
}
return(a[-1])
}
and Prof Brian Ripley wrote:
: since you can shift whole blocks at a time rather than use a while loop.
In words the algorithm runs from 2 through length(a) inserting
the current element into the subarray to its left, so to follow up
Prof Riley's suggestion to replace the while loop with code that
copies whole blocks at a time we have the following which does
not seem to suffer from lack of 0-origin and is clearer than
the double loop approach:
sort.6a <- function(a) {
for(i in 2:length(a)){ # insert a[i] into subvector to left of it
left <- a[1:(i-1)]
sel <- left < a[i]
a[1:i] <- c( left[sel], a[i], left[!sel] )
}
a
}
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