r - 在R中找到整数中最低有效位的最快/最有效方法是什么？

Question

我有类似 What's the fastest method to return the position of the least significant bit set in an integer in Python 3? 的问题, finding the first set bit in a binary number , 和 Position of least significant bit that is set对于R。我需要找到R中整数中最低有效位的位置。Spätzle提出的解决方案是以下内容:

unlist(lapply(x, function(z) min(which(as.integer(intToBits(z)) == 1))-1))

有没有更有效的方法？

Answer 1

下面的要快得多:

f <- function(x){
  log2(bitwAnd(x,-x))
}

比较:

g <- function(x){
  unlist(lapply(x, function(z) min(which(as.integer(intToBits(z)) == 1))-1))
}

快速测试:

> library(microbenchmark)
> tests <- floor(runif(1000,1,2^31))
> sum(f(tests) == g(tests)) #just to check
[1] 1000
> microbenchmark(f(tests),g(tests))
Unit: microseconds
     expr      min        lq       mean   median        uq       max neval
 f(tests)   38.435   40.5515   45.82673   42.667   45.1355   146.337   100
 g(tests) 1985.940 2083.9680 2530.79036 2131.218 2287.4280 11749.204   100