基于变量名称R的名称列表元素 [英] Name list elements based on variable names R

问题描述

我想将元素动态添加到一个空列表中.列表中的每个元素应在一组变量会自动命名，这些变量的值会有所不同.

I want to add elements to an empty list on the fly. Each element in the list should be named automatically after a set of variables which value will vary.

但是，我似乎无法找到一种在没有错误的情况下即时命名列表元素的方法.考虑下面的示例:

However, I cannot seem to find a way to name list elements on the fly without getting errors. Consider the example below:

L <- list()

var1 <- "wood"
var2 <- 1.0
var3 <- "z4"

varname <- paste(var1, as.character(var2), var3, sep="_")

# This works fine:
L$"wood_1_z4" <- c(0,1)
L$"wood_1_z4"
0 1

# This doesn't!!
L$paste(var1, as.character(var2), var3, sep="_") <- c(0,1)
Error in L$paste(var1, as.character(var2), var3, sep = "_") <- c(0, 1) : 
  invalid function in complex assignment

# Ths doesn't either ... 
L$eval(parse(text = "varname")) <- c(0,1)
Error in L$eval(parse(text = "varname")) <- c(0, 1) : 
target of assignment expands to non-language object

有没有办法做到这一点?

Is there a way to do this?

推荐答案

您不能使用<-运算符将其分配给paste()(我相信eval()函数也是如此).相反，您需要使用[[运算符或使用names()函数.您可以这样操作:

You cannot assign to paste() using the <- operator (and I believe this is true for the eval() function as well). Instead, you need to either use the [[ operator or use the names() function. You can do this like so:

L <- list()
var1 <- "wood"
var2 <- 1.0
var3 <- "z4"
varname <- paste(var1, as.character(var2), var3, sep="_")

# Using [[
L[[varname]] <- c(0,1)

> L
$wood_1_z4
[1] 0 1

# Using names()
L[[1]] <- c(0,1)
names(L)[1] <- varname

> L
$wood_1_z4
[1] 0 1

一种更有效的方法可能是使用一个既创建值又命名列表元素的函数，或者甚至是一个仅创建值的函数-如果您随后使用sapply，则可以使用以下命令命名每个列表元素调用函数和USE.NAMES选项的参数.

A more effective way to do this might be to use a function that both creates the value and names the list element, or even one that just creates the value - if you then use sapply you can name each list element using arguments from the call to your function and the USE.NAMES options.

通常来说，当列表的最终大小和结构不为人所知时，R对于随时间增长的列表并没有得到很好的优化.虽然可以做到，但更"R-ish"的做法是提前弄清楚结构.

In a general sense, R isn't really well-optimized for growing lists over time when the final size and structure of the list aren't well-known. While it can be done, a more "R-ish" way to do it would be to figure out the structure ahead of time.

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