可能有更好的方法，但这是我的想法。这使用 roll = TRUE 参数。想法是首先设置 energy = 0.0 其中前景< 0.27 ：

 字段[prospects< 0.27，energy：= 0.0] 
 

然后，如果我们从字段，我们可以使用 roll = TRUE 通过与所有组合进行联接，如下所示：

 字段[！is.na（energy）] [CJ（c（a，b），1：3） = field $ prospects] [] 
＃玩家时间前景能量
＃1：a 1 0.81 0.63 
＃2：a 2 0.25 0.00 
＃3：a 3 2.05 0.00 
＃4：b 1 1.63 -0.28 
＃5：b 2 2.20 -0.28 
＃6：b 3 0.49 -0.28 
 

我们必须重置潜在客户，因为 roll 更改它也。

这是一个变体，所以只能在 energy 列：

 字段[！is.na（energy） CJ（c（a，b），1:3），list（energy），
 roll = TRUE] [，prospects：= field $ prospects] [] 
 

或者使用 na.locf 从包动物园：

  = 1，energy：= round（rnorm（2），2）] 
 field [prospects& 0.27，energy：= 0.0] 
 require（zoo）
 field [，energy：= na.locf（energy，na.rm = FALSE）] 
 / pre> 

如果每个组的第一行保证为非NA，这是按照构造工作的。但是如果没有，你也可以按组运行na.locf：
  field [，energy：= na.locf ，na.rm = FALSE），by = player] 
 
 
I would like to update the value in a table with values of the previous row, within groups, (and probably stop the updates on a given condition)

Here is an example:
set.seed(12345)

field <- data.table(time=1:3, player = letters[1:2], prospects = round(rnorm(6),2))
setkey(field, player, time)
field[time == 1, energy := round(rnorm(2),2)] #initial level - this is what I want to propagate down the table
#let 'prospects < 0.27' be the condition that stops the process, and sets 'energy = 0'
#player defines the groups within which the updates are made
Here is the table I have. 
> field
time player prospects energy
1:    1      a      0.81  -0.32
2:    2      a      0.25     NA
3:    3      a      2.05     NA
4:    1      b      1.63  -1.66
5:    2      b      2.20     NA
6:    3      b      0.49     NA
Here is the table I want.
> field
time player prospects energy
1:    1      a      0.81  -0.32
2:    2      a      0.25  0
3:    3      a      2.05  0
4:    1      b      1.63  -1.66
5:    2      b      2.20  -1.66
6:    3      b      0.49  -1.66
Thanks in advance
Probably there are better ways, but this is what came to my mind. This makes use of roll=TRUE argument. The idea is to first set energy=0.0 where prospects < 0.27:
field[prospects < 0.27, energy := 0.0]
Then, if we remove the NA values from field, we can use roll=TRUE by doing a join with all combinations as follows:
field[!is.na(energy)][CJ(c("a", "b"), 1:3), roll=TRUE][, prospects := field$prospects][]
#    player time prospects energy
# 1:      a    1      0.81   0.63
# 2:      a    2      0.25   0.00
# 3:      a    3      2.05   0.00
# 4:      b    1      1.63  -0.28
# 5:      b    2      2.20  -0.28
# 6:      b    3      0.49  -0.28
We've to reset prospects because the roll changes it too. You could do it better, but you get the idea.

A variation, so that the roll is performed only on energy column:
field[!is.na(energy)][CJ(c("a", "b"), 1:3), list(energy), 
           roll=TRUE][, prospects := field$prospects][]


Or it may be simpler to use na.locf from package zoo :
field[time == 1, energy := round(rnorm(2),2)]
field[prospects < 0.27, energy := 0.0]
require(zoo)
field[, energy := na.locf(energy, na.rm=FALSE)]
which works if the first row of each group is guaranteed to be non-NA, which it is here by construction.  But if not, you can run na.locf by group, too :
field[, energy := na.locf(energy, na.rm=FALSE), by=player]