Data.table: 代入演算子 := はラッパー関数の本体を変更します

こんにちは！

最小限の再現可能な例:

library(data.table)

f = function(flag = FALSE) {
  dt1 = data.table(a = 1)
  dt2 = data.table(a = 1)
  dt3 = dt1[, .(a, b = 0)]

  print(body(f))
  if (flag) dt3[dt2, b := 999, on = "a"]
  print(body(f))

  return(invisible(dt3))
}

f(flag = FALSE)
f(flag = TRUE)

> f(flag = FALSE)
{
    dt1 = data.table(a = 1)
    dt2 = data.table(a = 1)
    dt3 = dt1[, .(a, b = 0)]
    print(body(f))
    if (flag) 
        dt3[dt2, `:=`(b, 999), on = "a"]
    print(body(f))
    return(invisible(dt3))
}
{
    dt1 = data.table(a = 1)
    dt2 = data.table(a = 1)
    dt3 = dt1[, .(a, b = 0)]
    print(body(f))
    if (flag) 
        dt3[dt2, `:=`(b, 999), on = "a"]
    print(body(f))
    return(invisible(dt3))
}

> f(flag = TRUE)
{
    dt1 = data.table(a = 1)
    dt2 = data.table(a = 1)
    dt3 = dt1[, .(a, b = 0)]
    print(body(f))
    if (flag) 
        dt3[dt2, `:=`(b, 999), on = "a"]
    print(body(f))
    return(invisible(dt3))
}
{
    dt1 = data.table(a = 1)
    dt2 = data.table(a = 1)
    dt3 = dt1[, .(a, b = 999)]
    print(body(f))
    if (flag) 
        dt3[dt2, `:=`(b, 999), on = "a"]
    print(body(f))
    return(invisible(dt3))
}

ご覧のとおり、列「b」に新しい値を割り当てた後 (フラグが TRUE の場合)、「f」関数の本体は行「dt3 = ...」(「b = 0」) で変更されます。 " は "b = 999" に置き換えられます)。 FALSE フラグ値を指定して実行すると、関数の実行結果は異なります。

sessionInfo() の出力:

R version 3.5.1 (2018-07-02)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 7 x64 (build 7600)

Matrix products: default

locale:
[1] LC_COLLATE=Russian_Russia.1251  LC_CTYPE=Russian_Russia.1251    LC_MONETARY=Russian_Russia.1251
[4] LC_NUMERIC=C                    LC_TIME=Russian_Russia.1251    

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] data.table_1.12.3

loaded via a namespace (and not attached):
[1] compiler_3.5.1 tools_3.5.1    yaml_2.2.0