从 data.table 的“j”参数中没有得到相同的结果答案

【问题标题】：not getting the same result from within the "j" parameter of a data.table从 data.table 的“j”参数中没有得到相同的结果
【发布时间】：2020-03-21 12:14:43
【问题描述】：

我正在从 data.table 的“j”参数中调用一个函数，但我得到的结果与我直接调用它的结果不同。这似乎与my function called in data.table j not returning expected results有关

get.lower.bound <- function (x) {
  rex <-
    regexpr (pattern = "((?<lower>[\\-+\\d*\\.,]*)%\\s*<\\s*)?X(\\s<\\s(?<uppper>[\\-+\\d\\.,]*)%)?",
             text = x,
             perl = TRUE)
  lower_bound <-
    substring(
      text = x,
      first = attr(rex, "capture.start")[2],
      last = attr(rex, "capture.start")[2] + attr(rex, "capture.length")[2] -1
    )
  lower_bound
}

dat <- data.table(
  A = c('1% < X < 2%', '4% < X',      'X < 8%' ),
  B = c('2% < X < 3%', '5% < X < 6%', '8% < X < 9%' ),
  C = c('3% < X < 4%', '6% < X < 7%', 'X < 10%' )
)

get.lower.bound(dat[1,'A'])                   # this returns 1; as I expect
get.lower.bound(dat[2,'A'])                   # this returns 4; as I expect
get.lower.bound(dat[3,'A'])                   # this returns a blank string; as I expect
dat[i = 1, j = .(lb1 = get.lower.bound(A))]   # this returns a data table of just one cell with 1 in it; as I expect
dat[i = 1:3, j = A]                           # this returns a character vector with the original strings in it; as I expect
dat[i = 1:3, j = .(lb1 = c(A))]               # this returns a data table with the original strings in it; as I expect
dat[i = 1:3, j = .(lb1 = get.lower.bound(A))] # this returns "1% <", "4% <", "X < 8"; but I expect a "1", "4" & ""

为什么最后一行没有给出我所期望的？我需要做什么才能得到我想要的东西？

【问题讨论】：

标签： r data.table

【解决方案1】：

我认为这是因为您的函数 get.lower.bound 没有矢量化。如果按每一行对它进行分组然后传递函数，它会给出预期的输出。

library(data.table)
dat[, j = .(lb1 = get.lower.bound(A)),  by = 1:nrow(dat)]

#   nrow lb1
#1:    1   1
#2:    2   4
#3:    3

【讨论】：