r - 为什么在inside()中=和<-不相等？

Question

> within( list(a="a",b="b"), c="c" )
Error in eval(expr, envir, enclos) : argument is missing, with no default
> within( list(a="a",b="b"), c<-"c" )
$a
[1] "a"

$b
[1] "b"

$c
[1] "c"

我不确定这两个为什么不相等。似乎 =版本由于 c而被解释为一个名为 ...的参数。有什么办法可以禁用此行为？我试过了，

within( list(a="a",b="b"), `c`="c" )

但这也失败了。

Answer 1

您认为c="c"(或该形式的任何子句)被解释为提供的参数是正确的。不，没有办法禁用它-大概是在R解析器级别进行了处理。
=和<-之间的差异记录在?"<-"中

The operators ‘<-’ and ‘=’ assign into the environment in which they are evaluated. The operator ‘<-’ can be used anywhere, whereas the operator ‘=’ is only allowed at the top level (e.g., in the complete expression typed at the command prompt) or as one of the subexpressions in a braced list of expressions.

“表达式括号列表”的主要示例是函数主体，您可以通过键入以下内容进行验证: is(body(plot.default))， length(body(plot.default))。