我有一个长向量，需要根据阈值将其分成若干段。一段是连续的超过阈值的值。当值低于阈值时，该段结束并且下一段从值再次超过阈值的地方开始。我需要记录每个段的起始和结束索引。
以下是一个效率低下的实现。如何编写最快和最合适的方式？这很丑陋，我不得不假设有一个更简洁的实现方式。
set.seed(10)
test.vec <- rnorm(100, 8, 10)
threshold <- 0
segments <- list()
in.segment <- FALSE
for(i in 1:length(test.vec)){
# If we're in a segment
if(in.segment){
if(test.vec[i] > threshold){
next
}else{
end.ind <- i - 1
in.segment <- FALSE
segments[[length(segments) + 1]] <- c(start.ind, end.ind)
}
}
# if not in segment
else{
if(test.vec[i] > threshold){
start.ind <- i
in.segment <- TRUE
}
}
}
编辑：所有解决方案的运行时间
感谢所有回复，这对我很有帮助，也很有教育意义。下面是五种解决方案的小测试（包括提供的四种和原始示例）。正如您所看到的，所有四种解决方案都比原始解决方案要快得多，但Khashaa的解决方案是迄今为止最快的。
set.seed(1)
test.vec <- rnorm(1e6, 8, 10);threshold <- 0
originalFunction <- function(x, threshold){
segments <- list()
in.segment <- FALSE
for(i in 1:length(test.vec)){
# If we're in a segment
if(in.segment){
if(test.vec[i] > threshold){
next
}else{
end.ind <- i - 1
in.segment <- FALSE
segments[[length(segments) + 1]] <- c(start.ind, end.ind)
}
}
# if not in segment
else{
if(test.vec[i] > threshold){
start.ind <- i
in.segment <- TRUE
}
}
}
segments
}
SimonG <- function(x, threshold){
hit <- which(x > threshold)
n <- length(hit)
ind <- which(hit[-1] - hit[-n] > 1)
starts <- c(hit[1], hit[ ind+1 ])
ends <- c(hit[ ind ], hit[n])
cbind(starts,ends)
}
Rcpp::cppFunction('DataFrame Khashaa(NumericVector x, double threshold) {
x.push_back(-1);
int n = x.size(), startind, endind;
std::vector<int> startinds, endinds;
bool insegment = false;
for(int i=0; i<n; i++){
if(!insegment){
if(x[i] > threshold){
startind = i + 1;
insegment = true; }
}else{
if(x[i] < threshold){
endind = i;
insegment = false;
startinds.push_back(startind);
endinds.push_back(endind);
}
}
}
return DataFrame::create(_["start"]= startinds, _["end"]= endinds);
}')
bgoldst <- function(x, threshold){
with(rle(x>threshold),
t(matrix(c(0L,rep(cumsum(lengths),2L)[-length(lengths)]),2L,byrow=T)+1:0)[values,])
}
ClausWilke <- function(x, threshold){
suppressMessages(require(dplyr, quietly = TRUE))
in.segment <- (x > threshold)
start <- which(c(FALSE, in.segment) == TRUE & lag(c(FALSE, in.segment) == FALSE)) - 1
end <- which(c(in.segment, FALSE) == TRUE & lead(c(in.segment, FALSE) == FALSE))
data.frame(start, end)
}
system.time({ originalFunction(test.vec, threshold); })
## user system elapsed
## 66.539 1.232 67.770
system.time({ SimonG(test.vec, threshold); })
## user system elapsed
## 0.028 0.008 0.036
system.time({ Khashaa(test.vec, threshold); })
## user system elapsed
## 0.008 0.000 0.008
system.time({ bgoldst(test.vec, threshold); })
## user system elapsed
## 0.065 0.000 0.065
system.time({ ClausWilke(test.vec, threshold); })
## user system elapsed
## 0.274 0.012 0.285