在j
中的data.table
参数中,是否有语法允许我在同一j
语句中引用先前创建的变量?我在考虑类似Lisp的let*
构造。
library(data.table)
set.seed(22)
DT <- data.table(a = rep(1:5, each = 10),
b = sample(c(0,1), 50, rep = TRUE))
DT[ ,
list(attempts = .N,
successes = sum(b),
rate = successes / attempts),
by = a]
这导致
# Error in `[.data.table`(DT, , list(attempts = .N, successes = sum(b), :
# object 'successes' not found
我理解为什么,但在同一j
中有不同的方法来实现这一目标吗?
答案 0 :(得分:7)
这样可以解决问题:
DT[ , {
list(attempts = attempts <- .N,
successes = successes <- sum(b),
rate = successes/attempts)
}, by = a]
# a attempts successes rate
# 1: 1 10 5 0.5
# 2: 2 10 6 0.6
# 3: 3 10 3 0.3
# 4: 4 10 5 0.5
# 5: 5 10 5 0.5
FWIW,this closely related data.table feature request可以使你的问题中使用+/-语法成为可能。从链接页面引用:
<强>要点:强>
:=
(和`:=`(...)
)的迭代RHS,以及:=
语法中的多个j = {...}
详细说明
e.g。
DT[, `:=`( m1 = mean(a), m2 = sd(a), s = m1/m2 ), by = group]
其中s可以使用先前的lhs名称(使用'iterative'一词尝试传达它)。
答案 1 :(得分:4)
请改为尝试:
DT[,
{successes = sum(b);
attempts = .N;
list(attempts = attempts,
successes = successes,
rate = successes / attempts)
},
by = a]
或
DT[,
list(attempts = .N,
successes = sum(b)),
by = a][, rate := successes / attempts]