我认为这个问题与非标准评估有关。 请帮助我理解为什么第一种方法有效，第二种方法没有。 谢谢！
> sessionInfo()
R version 3.3.0 (2016-05-03)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 7 x64 (build 7601) Service Pack 1
locale:
[1] LC_COLLATE=English_United States.1252 LC_CTYPE=English_United States.1252 LC_MONETARY=English_United States.1252
[4] LC_NUMERIC=C LC_TIME=English_United States.1252
attached base packages:
[1] stats graphics grDevices utils datasets methods base
loaded via a namespace (and not attached):
[1] MASS_7.3-45 lazyeval_0.2.0 magrittr_1.5 R6_2.2.0 assertthat_0.1 DBI_0.5-1 tools_3.3.0 dplyr_0.5.0
[9] tibble_1.2 Rcpp_0.12.9
会话信息
{{1}}