小编Jan*_* Lu的帖子

我在R中关注了DT（data.table）。

dt <- fread("
id| rowids | charge | payment | balance
a |   1    |  7.1   |   0     |     
a |   2    |  1.2   |   3     |   
a |   3    |  1.7   |   1     |   
b |   1    |  8.1   |   0     |   
b |   2    |  2.5   |   4     |   
b |   3    |  2.3   |   2     |   
b |   4    |  3.2   |   1     |   
            ", 
            sep = "|",
            colClasses = c("character", "numeric", "numeric", "numeric", 
"numeric"))

应该在每个id组中将“余额”计算为“余额<-previous.row.balance …