[R] Unexpected/undocumented behavior of 'within': dropping variable names that start with '.'

[Brian]

Dear List,

Somewhere I missed something, and now I'm really missing something!
 
> d.f <- data.frame(.id = c(TRUE, FALSE, TRUE), dummy = c(1, 2, 3), a =
c(1, 2, 3), b = c(1, 2, 3) + 1)
 > within(d.f, {d = a + b})
   dummy a b d
 1     1 1 2 3
 2     2 2 3 5
 3     3 3 4 7
 > d.f <- data.frame(.id = c(TRUE, FALSE, TRUE), .dummy = c(1, 2, 3), a
= c(1, 2, 3), b = c(1, 2, 3) + 1)
 > within(d.f, {d = a + b})
   a b d
 1 1 2 3
 2 2 3 5
 3 3 4 7

Could somebody please explain to me why this does this? I think could be
considered a feature (for lots of calculations within a data frame you
don't have to remove all extra variables at the end).  I just wish it
was documented.

Cheers,
Brian


sessionInfo()
 R version 3.1.0 (2014-04-10)
 Platform: x86_64-pc-linux-gnu (64-bit)

 locale:
  [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C
  [3] LC_TIME=en_US.UTF-8        LC_COLLATE=en_US.UTF-8
  [5] LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8
  [7] LC_PAPER=en_US.UTF-8       LC_NAME=C
  [9] LC_ADDRESS=C               LC_TELEPHONE=C
 [11] LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C

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

 other attached packages:
  [1] scales_0.2.4       plyr_1.8.3         reshape2_1.4      
ccchDataProc_0.7
  [5] ccchTools_0.6      xtable_1.7-4       tables_0.7.79      Hmisc_3.14-5
  [9] Formula_1.1-2      survival_2.37-7    ggplot2_1.0.1     
IDPmisc_1.1.17
 [13] lattice_0.20-29    myRplots_1.1       myRtools_1.2       meteoconv_0.1
 [17] pixmap_0.4-11      RColorBrewer_1.0-5 maptools_0.8-30    sp_1.1-1
 [21] mapdata_2.2-3      mapproj_1.2-2      maps_2.3-9         chron_2.3-45
 [25] MASS_7.3-35

 loaded via a namespace (and not attached):
  [1] acepack_1.3-3.3     cluster_1.15.2      colorspace_1.2-4
  [4] compiler_3.1.0      data.table_1.9.4    digest_0.6.4
  [7] foreign_0.8-61      gtable_0.1.2        labeling_0.3
 [10] latticeExtra_0.6-26 munsell_0.4.2       nnet_7.3-8
 [13] proto_0.3-10        Rcpp_0.12.0         rpart_4.1-8
 [16] stringr_0.6.2       tools_3.1.0
 > within
 function (data, expr, ...)
 UseMethod("within")
 <bytecode: 0x26d32c8>
 <environment: namespace:base>