我的数据看起来像:
require(data.table)
DT <- data.table(x=c(19,19,19,21,21,19,19,22,22,22),
y=c(53,54,55,32,44,45,49,56,57,58))
我想沿x搜索,并计算y的均值。 但是,使用时。
DT[, .(my=mean(y)), by=.(x)]
我得到了x的重合值的总体方法。 我想沿x搜索,每次x改变,我想计算一个新的均值。对于提供的示例,输出将为:
DTans <- data.table(x=c(19,21,19,22),
my=c(54,38,47,57))
答案 0 :(得分:10)
我们可以使用rleid
创建另一个分组变量,获取&#39; y&#39;的mean
,然后分配&#39; indx&#39;为NULL
library(data.table) # v 1.9.5+
DT[, .(my = mean(y)), by = .(indx = rleid(x), x)][, indx := NULL]
# x my
#1: 19 54
#2: 21 38
#3: 19 47
#4: 22 57
set.seed(24)
foo <- function(x) sample(x, 1e7L, replace = TRUE)
DT <- data.table(x = foo(100L), y = foo(10000L))
josilber <- function() {
new.group <- c(1, diff(DT$x) != 0)
res <- data.table(x = DT$x[new.group == 1],
my = tapply(DT$y, cumsum(new.group), mean))
}
Roland <- function() {
DT[, .(my = mean(y), x = x[1]), by = cumsum(c(1, diff(x) != 0))]
}
akrun <- function() {
DT[, .(my = mean(y)), by = .(indx = rleid(x), x)][,indx := NULL]
}
bgoldst <- function() {
with(rle(DT$x), data.frame(x = values,
my = tapply(DT$y, rep(1:length(lengths), lengths), mean)))
}
system.time(josilber())
# user system elapsed
#159.405 1.759 161.110
system.time(bgoldst())
# user system elapsed
#162.628 0.782 163.380
system.time(Roland())
# user system elapsed
# 18.633 0.052 18.678
system.time(akrun())
# user system elapsed
# 1.242 0.003 1.246
答案 1 :(得分:3)
您可以识别连续元素组,然后确定每个元素的平均值和值:
(new.group <- c(1, diff(DT$x) != 0))
# [1] 1 0 0 1 0 1 0 1 0 0
DT[, list(x = x[1L], my = mean(y)), by = list(indx = cumsum(new.group))]
# indx x my
# 1: 1 19 54
# 2: 2 21 38
# 3: 3 19 47
# 4: 4 22 57
答案 2 :(得分:3)
with(rle(DT$x),data.frame(x=values,my=tapply(DT$y,rep(1:length(lengths),lengths),mean)));
## x my
## 1 19 54
## 2 21 38
## 3 19 47
## 4 22 57