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