R函数中的默认参数(由多个实参匹配的正式参数)

Simple question, I hope. I want to write a plotting function that has a default value for the y-axis label if the user doesn't specify. I'd also like to allow the ... argument for other plotting parameters, and allow the user to set ylab manually. But I can't figure out how to do this.

简单的问题,我希望。我想要编写一个绘图函数，如果用户没有指定，它对y轴标签具有默认值。我还想让……为其他绘图参数提供参数，并允许用户手动设置ylab。但我不知道怎么做。

# simple scatterplot function with a default ylab
scatter <- function(x,y, ...) {
    plot(x, y, ylab="Default y-axis label", ...)
}

# generate data
x <- rnorm(100)
y <- x+rnorm(100)

# use the default
scatter(x,y)

# here I want to use my own label, but I get an error!
scatter(x, y, ylab="New y-axis label")

The error I get is:

我得到的错误是:

Error in plot.default(x, y, ylab = "Default y-axis label", ...) : 
  formal argument "ylab" matched by multiple actual arguments

I understand the problem, but I don't know the best way to fix it. Thanks for the help!

我理解这个问题，但我不知道解决它的最好方法。谢谢你的帮助!

EDIT: I realize I can do something like

编辑:我意识到我可以做一些类似的事情。

scatter <- function(x,y,ylab = "Default y-axis label", ...) {
    plot(x, y, ylab= ylab, ...)
}

...but if I'm writing a package to submit to CRAN, and I have lots of default options I'd like to fiddle with, I don't want to have to document all these standard plotting arguments because they're used in my function definition.

…但是如果我要写一个提交给CRAN的包，并且我有很多默认的选项，我想要修改，我不想要记录所有这些标准的绘图参数，因为它们是在我的函数定义中使用的。

3 个解决方案

#1

Try doing this instead:

试试这样做:

scatter <- function(x,y,ylab = "Default y-axis label", ...) {
    plot(x, y, ylab= ylab, ...)
}

Expanding slightly on Arun's answer, this is a sketch of one route to take if you have many arguments:

在阿伦的回答中稍微扩展一下，这是一条路线的草图，如果你有很多论点的话:

def_args <- list(ylab = "Default Label",xlab = "Default Label")

scatter <- function(x,y, ...) {
    cl <- as.list(match.call())[-1L]
    do.call("plot",c(cl,def_args[!names(def_args) %in% names(cl)]))
}

Some thought would be needed to decide how you want to handle partial matching of arguments (if at all). e.g. perhaps something like this:

需要一些想法来决定如何处理参数的部分匹配(如果有的话)。例如:

scatter <- function(x,y, ...) {
    cl <- as.list(match.call())[-1L]
    names(cl) <- match.arg(names(cl),
                           names(formals(plot.default)),several.ok = TRUE)
    do.call("plot",c(cl,def_args[!names(def_args) %in% names(cl)]))
}

would handle partial matching of arguments.

将处理参数的部分匹配。

#2

One way using match.call to check if ylab has been specified as an argument:

使用匹配的一种方式。调用检查ylab是否已被指定为参数:

scatter <- function(x,y, ...) {
    mcall = as.list(match.call())[-1L]
    if (!"ylab" %in% names(mcall))
        plot(x, y, ylab="Default y-axis label", ...)
    else plot(x, y, ...)
}

As mentioned under comment list(...) is a nicer way to get just the dots argument expanded than having to get all the formal arguments with match.call.

正如在注释列表中所提到的(…)是一种更好的方法来得到点参数的扩展，而不是必须得到所有正式的参数。

You might also try using pmatch instead of %in% for partial matching of arguments.

您也可以尝试使用pmatch而不是%in%来进行部分匹配的参数。

#3

I use a function to build an argument list. In my case, I do not care about partially matching argument names, which is good because this won't support it.

我使用一个函数来建立一个参数列表。在我的例子中，我不关心部分匹配的参数名，这很好，因为这不会支持它。

# Create a list of input arguments.
# Allow arguments to be specified multiple times, first definition wins.
# The resulting list is intended to be passed to do.call().

make.args <- function(..., PRE.ARGS=list(), POST.ARGS=list()) {
  a <- list()
  l <- c(PRE.ARGS, list(...), POST.ARGS)
  for (name in unique(names(l))) {
    a[[name]] <- l[[name]] # First occurrence will be found.
  }
  return(a)
}

An example of its use:

它使用的一个例子:

plot.rate <- function(col, cond=NULL, ...) {
    col <- paste(col, collapse=' + ')
    f <- paste(col, '~ Rate')
    if (!is.null(cond)) {
        cond <- paste(cond, collapse=' + ')
        f <- paste(f, cond, sep='|')
    }

    arg.list <- make.args(...
                    , x = as.formula(f)
                    , main=col
                    , grid=TRUE
                    , scales=list(x=list(alternating=1)    # bottom(/left)
                                , y=list(alternating=3)) # both
                                , xlab='x RTM'
    )

    do.call(xyplot, arg.list)
}

#1