How to sort list in R?

Question

I have list of lists similar to this:

a <- list(
  list(day = 5, text = "foo"),
  list(text = "bar", day = 1),
  list(text = "baz", day = 3),
  list(day = 2, text = "quux")
)

with unknown number of fields and the fields my be out of order.

how can I sort this list based on day? I need the list to be sorted ascending. I've search but I only found how to sort vectors. Is it possible to sort a list?

Answer 1

In order to sort that given "list of lists" a you can try to use sapply()with the extraction operator [[ to retrieve data from the list. These are used in the call to order():

a[order(sapply(a, `[[`, i = "day"))]
#[[1]]
#[[1]]$day
#[1] 1
#
#[[1]]$text
#[1] "bar"
#
#
#[[2]]
#[[2]]$day
#[1] 2
#
#[[2]]$text
#[1] "quux"
# ...

As suggested in this comment, this can also be achieved by using an anonymous function in sapply():

a[order(sapply(a, function(x) x$day))]

This also works when used in a function definition as the OP did:

sortBy <- function(a, field) a[order(sapply(a, "[[", i = field))]
sortBy(a, "day")

Note that we need to enclose the extraction operator [[ either in backquotes or quotes.