问题描述
我正在为 R 中的 par()
选项开发一些通用访问器函数。
getPar
getPar = function(key)
{
par()[[key]];
}
这按预期工作。
getPar("mar");
setPar
# save memory ... restoreState ... pdf
setPar = function(key,val)
{
# par(mar=c(0.25,0.25,0.25)
# R.O. indicates read-only arguments: These may only be used in queries and cannot be set. ("cin","cra","csi","cxy","din" and "page" are always read-only.)
# https://www.rdocumentation.org/packages/graphics/versions/3.6.2/topics/par
pnames = names( par(no.readonly = TRUE) );
if(is.element(key,pnames))
{
par()[[key]] = val;
}
}
这个不行:
mar = c(0.25,0.25);
setPar("mar",mar);
并抛出错误:
Error in par()[[key]] = val : invalid (NULL) left side of assignment
关于如何编写概述的 setter
函数的任何想法?
是的,我明白了,我可以直接通过 par(mar = c(0.25,0.25))
来“设置”该值。我特别在寻找可以在这个简单的 setter
函数中工作的解决方案。可变参数方法。
解决方法
对列表(即 [<-
和 [[<-
)的赋值需要在赋值的 LHS 上有一个 object,而不是生成列表的表达式。同样,
A <- list(a=1)
A$b <- 2
A
# $a
# [1] 1
# $b
# [1] 2
list(a=1)$b <- 3
# Error in list(a = 1)$b <- 3 :
# target of assignment expands to non-language object
我建议你把setPar
改为实际使用par
设置值,而不是尝试对列表进行操作。
setPar = function(key,val)
{
# par(mar=c(0.25,0.25,0.25)
# R.O. indicates read-only arguments: These may only be used in queries and cannot be set. ("cin","cra","csi","cxy","din" and "page" are always read-only.)
# https://www.rdocumentation.org/packages/graphics/versions/3.6.2/topics/par
pnames = names( par(no.readonly = TRUE) );
if(is.element(key,pnames))
{
par(setNames(list(val),key))
}
}
par("mar")
# [1] 5.1 4.1 4.1 2.1
setPar("mar",1:4)
par("mar")
# [1] 1 2 3 4