Quickest way to find closest elements in an array in R

Question

I would like find the fastes way in R to indentify indexes of elements in Ytimes array which are closest to given Xtimes values.

So far I have been using a simple for-lo

Answer 1

R is vectorized, so skip the for loop. This saves time in scripting and computation. Simply replace the for loop with an apply function. Since we're returning a 1D vector, we use sapply.

YmatchIndex <- sapply(Xtimes, function(x){which.min(abs(Ytimes - x))})

---

Proof that apply is faster:

library(microbenchmark)
library(ggplot2)

# set up data
Xtimes <- c(1,5,8,10,15,19,23,34,45,51,55,57,78,120)
Ytimes <- seq(0,120,length.out = 1000)

# time it
mbm <- microbenchmark(
  for_loop = for (i in 1:length(Xtimes)) {
    YmatchIndex[i] = which.min(abs(Ytimes - Xtimes[i]))
  },
  apply    = sapply(Xtimes, function(x){which.min(abs(Ytimes - x))}),
  times = 100
)

# plot
autoplot(mbm)

See ?apply for more.