r - 如何在 R 中将 NULL 作为参数传递？

Question

我有一个矢量 arguments ，其中一些值为 NA .我想将这些参数按顺序传递给这样的函数:

myFunction(argument = ifelse(!is.na(arguments[i]),
           arguments[i], NULL))

以便它将采用 arguments[i] 中的值只要不是 NA ，并采用默认 NULL除此以外。但这会产生错误。
如果它有任何区别，则有问题的函数是 match_on() ，来自 optmatch包裹。有问题的论点是 caliper ，因为我只想在一个可用的情况下提供一个卡尺(即当卡尺向量中的值不是 NA 时)。错误信息是这样的:

Error in ans[!test & ok] <- rep(no,  length.out = length(ans))[!test &  : 
replacement has length zero
In addition: Warning message:
In rep(no, length.out = length(ans)) :'x' is NULL so the result will be NULL 

Answer 1

您可以使用 ?switch()而不是 ifelse ——

myFunction(argument = switch(is.na(arguments[i]) + 1, arguments[i], NULL))

这是 switch 的帮助文档——

switch(EXPR, ...)

Arguments

EXPR an expression evaluating to a number or a character string.

... the list of alternatives. If it is intended that EXPR has a character-string value these will be named, perhaps except for one alternative to be used as a ‘default’ value.

Details

switch works in two distinct ways depending whether the first argument evaluates to a character string or a number.

If the value of EXPR is not a character string it is coerced to integer. If the integer is between 1 and nargs()-1 then the corresponding element of ... is evaluated and the result returned: thus if the first argument is 3 then the fourth argument is evaluated and returned

基本上，当参数是 NA然后 EXPR计算结果为 2 返回 NULL如果不是 NA然后 EXPR计算结果为 1 并返回 arguments[i] .