[英]R data.table selecting the previous row within group blocks
I have the following example data frame. 我有以下示例数据框。
id value
a 3
a 4
a 8
b 9
b 8
I want to convert it so that I can calculate differences in the column "value" between successive rows. 我想转换它,以便我可以计算连续行之间的“值”列的差异。 So the expected result is
所以预期的结果是
id value prevValue
a 3 0
a 4 3
a 8 4
b 9 0
b 8 9
Notice within each group I want the sequence of values to start with a 0 and successive values are from the one prior. 请注意,在每个组中,我希望值序列以0开头,连续值来自前一个值。 I tried the following
我尝试了以下内容
x = x[,list(
prevValue = c(0,value[1:(.N-1)])
),by=id]
but no luck. 但没有运气。 Thanks in advance.
提前致谢。
使用负索引,例如:
x[,prev.value := c(0,value[-.N]) ,by=id]
Without data.table
: 没有
data.table
:
with(dat,ave(value,id,FUN=function(x) c(0,head(x,-1))))
[1] 0 3 4 0 9
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