Modify elements of a nested list in base R without loops

user_3677576 0
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I have a nested list with two or more elements at the top level and want to modify named elements of the sub-lists, e.g. by appending values from a named vector.

Let's assume L is a list of two elements and Nis a vector with the same number of elements:

L <- list(
  foo = list(
    x1=1,
    x2=c(a=1, b=2)
  ),
  bar = list(
    x1=2,
    x2=c(e=2, f=3)
  )
)

N <- c(n=11, n=12)

Then I want to get a result like this:

list(
  foo = list(
    x1=1,
    x2=c(a=1, b=2, n=11)
  ),
  bar = list(
    x1=2,
    x2=c(e=2, f=3, n=12)
  )
)

It is easy to read the elements of the sub-lists:

lapply(L, "[[", "x2")

So I wonder if there is a similarly compact way to change elements.

lapply nested-lists r
1 Answers

We may use Map to loop over the L, and corresponding elements of N, extract the 'x2' component and append the N values and assign back, return the object (x)

L2 <- Map(\(x, y) 
          {
          x$x2 <- c(x$x2, n=y)
          x
          }, L, N)

-checking with OP's output

> all.equal(L2, out)
[1] TRUE

Or a variant is to assign with the name

Map(\(x, y) {x$x2['n'] <- y; x}, L, N)
user_37322710
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