提取组中R的行 [英] extracting rows of group medians n R
本文介绍了提取组中R的行的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
如果我有一个如下的数据框架：
If I have a data frame like the following:
v2 <- c(4.5, 2.5, 3.5, 5.5, 7.5, 6.5, 2.5, 1.5, 3.5)
v1 <- c(2.2, 3.2, 1.2, 4.2, 2.2, 3.2, 2.2, 1.2, 5.2)
lvl <- c("a","a","a","b","b","b","c","c","c")
d <- data.frame(v1,v2,lvl)
> d
v1 v2 lvl
1 2.2 4.5 a
2 3.2 2.5 a
3 1.2 3.5 a
4 4.2 5.5 b
5 2.2 7.5 b
6 3.2 6.5 b
7 2.2 2.5 c
8 1.2 1.5 c
9 5.2 3.5 c
在$ code> d $ lvl 的每个级别中，我想提取值为 d $ v1 为中位数（最简单的情况是，每个级别的 d $ lvl 有三行）。所以我想得到：
Within each level of d$lvl, I want to extract the row with value of d$v1 being median (for the simplest case, each level of d$lvl has three rows). So I want to get:
v1 v2 l
1 2.2 4.5 a
6 3.2 6.5 b
7 2.2 2.5 c
推荐答案
这行的行数。您需要考虑如何处理具有偶数行数的组。例如，您可能希望在一个或另一个方向上舍入中位数，请参阅？round 。
For groups with odd number of rows this works. You need to think of how to handle groups with even number of rows. For example you may wish to round the median in one or the other direction, see ?round.
library(plyr)
d2 <- ddply(.data = d, .variables = .(lvl), function(x)
x[which(x$v1 == median(x$v1)), ])
# v1 v2 lvl
# 1 2.2 4.5 a
# 2 3.2 6.5 b
# 3 2.2 2.5 c
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