Replacing nested list using a vector of names of depths as an index

问题

Take a simple nested list L:

L <- list(lev1 = list(lev2 = c("bit1","bit2")), other=list(yep=1))
L
#$lev1
#$lev1$lev2
#[1] "bit1" "bit2"
#
# 
#$other
#$other$yep
#[1] 1

And a vector giving a series of depths for each part I want to select from L:

sel <- c("lev1","lev2")

The result I want when indexing is:

L[["lev1"]][["lev2"]]
#[1] "bit1" "bit2"

Which I can generalise using Reduce like so:

Reduce(`[[`, sel, init=L)
#[1] "bit1" "bit2"

Now, I want to extend this logic to do a replacement, like so:

L[["lev1"]][["lev2"]] <- "new val"

, but I am genuinely stumped as to how to generate the recursive [[ selection in a way that will allow me to then assign to it as well.

回答1:

Why cant you just do

L[[sel]] <- "new val"

well if you want to do the long way then You could still use Reduce with modifyList or you could use [[<-. Here is an example with modifyList:

modifyList(L,Reduce(function(x,y)setNames(list(x),y),rev(sel),init = "new val"))
$lev1
$lev1$lev2
[1] "new val"


$other
$other$yep
[1] 1

回答2:

You could eval() and parse() by concatenating everything. I am not sure how generalized you could make it:

``` r
L <- list(lev1 = list(lev2 = c("bit1","bit2")), other=list(yep=1))
L
#> $lev1
#> $lev1$lev2
#> [1] "bit1" "bit2"
#> 
#> 
#> $other
#> $other$yep
#> [1] 1

sel <- c("lev1","lev2")

eval(parse(text = paste0('L', paste0('[["', sel, '"]]', collapse = ''), '<- "new val"')))

L
#> $lev1
#> $lev1$lev2
#> [1] "new val"
#> 
#> 
#> $other
#> $other$yep
#> [1] 1

^{Created on 2019-11-25 by the reprex package (v0.3.0)}

来源：https://stackoverflow.com/questions/59042112/replacing-nested-list-using-a-vector-of-names-of-depths-as-an-index

标签

list

nested

assign