[英]How can I access the default value of a function argument?
I want to use a function default_arg()
to access the default value of arg
in the context of a function func
.我想使用 function
default_arg()
在 function func
的上下文中访问arg
的默认值。 However, I'd also like this function to be clever enough to guess which function and which argument I want in a couple of cases:但是,我也希望这个 function 足够聪明,能够在某些情况下猜出哪个 function 和我想要的参数:
tester_1 <- function(x = 1, y = 2, z = 3) {
x * y + z
}
default_arg(x, tester_1) # this would evaluate to 1
tester_2 <- function(x = 1, y = 2, z = 3) {
x_default <- default_arg(x) # this would evaluate to 1
if(x < x_default) {
stop(paste("x should be greater or equal to default value:", x_default))
}
x * y + z
}
tester_1(
x = 2,
y = 1,
z = default_arg() # This would evaluate to 3
)
I'm able to create something that works in cases 1 and 2, but I'm unsure how to approach Case 3. Here is the approach I'm using currently, using some functions from rlang
:我能够创建适用于案例 1 和案例 2 的内容,但我不确定如何处理案例 3。这是我目前使用的方法,使用
rlang
中的一些函数:
default_arg <- function(arg, func = sys.function(sys.parent())) {
formals(func)[[as_name(enquo(arg))]]
}
In my proposed solution, I also eliminated the rlang dependency (please see below):在我提出的解决方案中,我还消除了 rlang 依赖项(请参见下文):
default_arg <- function(arg = NULL, func = NULL) {
if(deparse(substitute(arg)) == "NULL" & is.null(func))
{
arg = sys.calls()[[1]]
val = as.character(arg); names(val) = names(arg)
func = val[1]
variable = names(which(val == "default_arg()"))
return(formals(func)[[variable]])
}
if(is.null(func))
{
func = as.character(sys.call(sys.parent()))[1]
}
return(formals(func)[[deparse(substitute(arg))]])
}
default_arg(x, tester_1) # 1
default_arg(y, tester_1) # 2
default_arg(z, tester_1) # 3
tester_2() # 5
tester_1(x = 1, y = 2, z = default_arg()) # 5
tester_1(x = 1, y = default_arg(), z = 3) # 5
tester_1(x = default_arg(), y = 2, z = 3) # 5
Basically, it is just parsing the call and extracting the relevant information.基本上,它只是解析调用并提取相关信息。 Though this works, I am sure you can still make it neater by avoiding the conditionals.
虽然这行得通,但我相信您仍然可以通过避免条件来使它更整洁。 Good luck!
祝你好运!
Best, Ventrilocus.最好的,Ventrilocus。
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.