r - 我可以在同一个 `j` 参数中使用在 `j` 中新创建的变量吗？

Question

在 j参数在 data.table , 是否有语法允许我在相同的 j 中引用以前创建的变量陈述？我在想类似 Lisp 的东西 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

我明白为什么，但在同一个 j 中是否有不同的方法来实现这一点?

Answer 1

这将解决问题:

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将使 +/- 在您的问题中使用的语法成为可能。从链接页面引用:

Summary:

Iterative RHS of := (and `:=`(...)), and multiple := inside j = {...} syntax

Detailed description

e.g. DT[, `:=`( m1 = mean(a), m2 = sd(a), s = m1/m2 ), by = group]

where s can use previous lhs names ( using the word 'iterative' tries to convey that ).