Name list elements based on variable names R

Question

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.

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

Is there a way to do this?

Tongue

Use [[: L[[paste(var1, as.character(var2), var3, sep="_")]]<-c(0,1) – Flexure

What about if you assign values to names(L)? – Rollick

$ doesn't do any evaluation on the right hand side, hence the errors. – Wisdom

Answer 1

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

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.

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.

Paint

While it's true that you can't assign to paste and eval, the reason why it isn't working is because the primitive $ doesn't evaluate on the right hand side. Per the comments in the C code for $: "We need to be sure to only evaluate the first argument. The second will be a symbol that needs to be matched, not evaluated." – Wisdom

Answer 2

An alternative would be to use the wrapper setNames() from the stats package, like so:

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

L <- setNames(list(c(0, 1)), varname)

> L
$wood_1_z4
[1] 0 1 

Saintsimon

Answer 3

Use the `names<-` infix operator as a normal function to return a named list. Then use c() to concatenate this with the list L. This one-liner can be used to continue to add elements to L 'on the fly' as requested.

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

L <- c(L, `names<-`(list(c(0, 1)), varname))

L
#> $wood_1_z4
#> [1] 0 1

Knick