数据条件表的最大分组条件 [英] Max by Group with Condition for a data.table

问题描述

我有这样的数据:

library(data.table)
group <- c("a","a","a","b","b","b")
cond <- c("N","Y","N","Y","Y","N")
value <- c(2,1,3,4,2,5)

dt <- data.table(group, cond, value)

group cond value
a     N    2
a     Y    1
a     N    3
b     Y    4
b     Y    2
b     N    5

当整个组的cond为Y时，我想返回最大值.像这样:

I would like to return max value when the cond is Y for the entire group. Something like this:

group cond value max
a     N    2     1
a     Y    1     1
a     N    3     1
b     Y    4     4
b     Y    2     4
b     N    5     4

我尝试将ifelse条件添加到分组的max中，但是，当行不符合条件时，我最终只是返回NA的no条件:

I've tried adding an ifelse condition to a grouped max, however, I end up just returning the no condition of NA when the row doesn't meet the condition:

dt[, max := ifelse(cond=="Y", max(value), NA), by = group]

推荐答案

假定对于每个组"，我们需要获取值"的 max ，其中"cond"为"Y"，按组"分组后，将值"与逻辑条件( cond =='Y')进行子集并获得 max 值

Assuming that for each 'group' we need to get the max of 'value' where the 'cond' is "Y", after grouping by 'group', subset the 'value' with the logical condition (cond == 'Y') and get the max value

dt[, max := max(value[cond == 'Y']), by = group]
dt
#   group cond value max
#1:     a    N     2   1
#2:     a    Y     1   1
#3:     a    N     3   1
#4:     b    Y     4   4
#5:     b    Y     2   4
#6:     b    N     5   4

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