本文介绍了data.table drop key rows和summarize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
我正在寻找一种优雅的方式来遍历data.table的键，删除具有该键的行，然后对剩余行进行摘要。例如：
I'm looking for an elegant way to iterate over the key of data.table, drop the rows that have that key, then take a summary over the remaining rows. For example:
mydt <- data.table(cat=c("a","a","b","b","c","c","c"), vals = 1:7)
setkey(mydt,cat)
tmp1 <- mydt[!"a"][,mean(vals)]
tmp2 <- mydt[!"b"][,mean(vals)]
tmp3 <- mydt[!"c"][,mean(vals)]
outdt <- data.table(cat=c("a","b","c"),means=c(tmp1,tmp2,tmp3))
有没有办法循环键，优雅地做？感谢。
Is there a way to loop over the key and do this elegantly? Thanks.
推荐答案
我认为这样做，使用更传统的 data.table code：
I think this does it, using more traditional data.table code:
setkey(mydt,cat)
mydt[, list(means=mean(mydt[!.BY,vals])), by=cat]
# or without needing to key first
mydt[, list(means=mean(mydt[cat != .BY,vals])), by=cat]
# cat means
#1: a 5.0
#2: b 4.2
#3: c 2.5
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