CODEWITHSUNDEEP
rlist
Danny
Danny

Reputation: 488

Efficient iterating over a list in R

What is the most efficient way to iterate over a list in R if I need to access both the names and values of items in the list separately?

If I try something like:

lst <- list(a = 1, b = 2)
for(i in lst) {
  # ...
}

Then I only get the value and can't see a way to access the name.

So currently I do:

lst <- list(a = 1, b = 2)
for(i in names(lst)) {
  # something with i
  # something with lst[[i]]
}

But this seems terribly inefficient to me since I access each item from the list by its name which I assume is slow. To avoid that I could do:

lst <- list(a = 1, b = 2)
for(i in seq_along(lst)) {
  # something with names(lst)[i]
  # something with lst[[i]]
}

But I'm not sure if this is any more or less efficient.

Is there a more efficient way than either of these? And if not, which of these should I be using?

Edit:

The answer to this question and @Gladwell gives another suggestion:

lst <- list(a = 1, b = 2)
names_lst <- names(lst)
for(i in seq_along(lst)) {
  # something with names_lst[i]
  # something with lst[[i]]
}

Is this the fastest option? Can it be improved in any way?

Upvotes: 1

Views: 560

Answers (1)

Gladwell
Gladwell

Reputation: 328

The fastest way is likely to do the two calls separately and store them to use vectorised functions for whatever you want to do.

tmpNames <- names(lst)
tmpValues <- as.vector(unlist(lst))

e.g.

paste0(tmpNames, tmpValues)

is about 2-3 times faster than your current implementation.

Upvotes: 1

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