问题描述
任务：对于所有 condition == FALSE ，将groupmean设置为所有个数字的均值组。
对于所有 condition == TRUE ，仅在的情况下，将groupmean设置为。 个数字的均值组的condition == TRUE
我想有一个解决方案，它不需要复制整个data.table，而是将所需的列添加到位。我敢打赌，这里有一个简单的解决方案，但是我有点迷路了……
Task: For all condition==FALSE, set groupmean to mean of all numbers by group.For all condition==TRUE set groupmean to mean of numbers only where condition==TRUE by group.I would like to have a solution which does not require copying the whole data.table but adds the desired column in place. I bet there's a plain simple solution, but I got lost a little...
到目前为止我的尝试：
set.seed(42)
require(data.table)
DT <- data.table(condition=sample(c(TRUE,FALSE), 50, replace=T),
group=rep(LETTERS[1:4], times=25),
numbers=1:100)
# modifies the right rows, but wrong value
DT[condition==FALSE, groupmean_1 := mean(numbers), by=group]
# right values, but not only rows where condition=FALSE
DT[, groupmean_2 := mean(numbers), by=group]
head(DT)
condition group numbers groupmean_1 groupmean_2
1: FALSE A 1 42.66667 49
2: FALSE B 2 55.68421 50
3: TRUE C 3 NA 51
4: FALSE D 4 47.78947 52
5: FALSE A 5 42.66667 49
6: FALSE B 6 55.68421 50
推荐答案
您应该颠倒定义 groupmean 。将其计算为所有行的组平均值，然后替换 condition == TRUE 之后的行。
You should reverse the sequence of how you define groupmean. Compute it as the group average for all rows, and substitute the rows where condition == TRUE afterwards.
DT[, groupmean:=mean(numbers), by=group]
DT[condition==TRUE, groupmean:=mean(numbers), by='group,condition']
我希望对您有所帮助
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