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find closest value in a vector based on another vector values

William Dunlap 
That method could be written as the following function
f0 <- function (a, b, unique = TRUE) 
{
    ret <- a[sapply(b, function(x) which.min(abs(x - a)))]
    if (unique) { 
        ret <- unique(ret)
    }
    ret
}

If 'a' is in sorted order then I think the following, based on findInterval,
does the same thing in less time, especially when 'b' is longish.
If 'a' may not be sorted then add

f1 <- function (a, b, unique = TRUE) 
{
    leftI <- findInterval(b, a)
    rightI <- leftI + 1
    leftI[leftI == 0] <- 1
    rightI[rightI > length(a)] <- length(a)
    ret <- ifelse(abs(b - a[leftI]) < abs(b - a[rightI]), a[leftI],  a[rightI])
    if (unique) { 
        ret <- unique(ret)
    }
    ret
}

E.g.,

R> a <- sort(rnorm(1e6))
R> b <- sort(rnorm(1000))
R> system.time(r0 <- f0(a, b))
   user  system elapsed 
   4.88    3.48    8.36 
R> system.time(r1 <- f1(a, b))
   user  system elapsed 
      0       0       0 
R> identical(r0, r1)
[1] TRUE

If 'a' might be unsorted then add
    if (is.unsorted(a))  a <- sort(a)
at the beginning.  If the output must be in the same order as the original
'a' then use order(a) and subscript 'a' and 'ret' with its output.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

Thread (9 messages)

Andras Farkas find closest value in a vector based on another vector values Jun 18 Jorge I Velez find closest value in a vector based on another vector values Jun 18 arun find closest value in a vector based on another vector values Jun 18 Bert Gunter find closest value in a vector based on another vector values Jun 18 Jorge I Velez find closest value in a vector based on another vector values Jun 18 Andras Farkas find closest value in a vector based on another vector values Jun 18 Bert Gunter find closest value in a vector based on another vector values Jun 18 Andras Farkas find closest value in a vector based on another vector values Jun 18 William Dunlap find closest value in a vector based on another vector values Jun 18