[R] Sorting based a custom sorting function

[Martin Morgan]

In the spirit of 'advent of code', maybe it is better to exploit the features of the particular language you've chosen? Then the use of factors seems very relevant.

value_levels <- c("Small", "Medium", "Large")
df <- data.frame(
    person = c("Alice", "Bob", "Bob", "Charlie"),
    value = factor(
        c("Medium", "Large", "Small", "Large"),
        levels = value_levels
    )
)
df[with(df, order(person, value)),]

Likely this is more efficient than the hints of your existing solution, because it will act on vectors rather than iterating through individual elements of the 'person' and 'value' vectors.

For a more general solution, I don't think I'd follow the low-level approach Duncan suggests (maybe see also ?Math for S3 generics), but rather define a class (e.g., that requires vectors person and value) and implement a corresponding `xtfrm()` method.

Have fun with the remainder of the advent!

Another Martin

From: R-help <r-help-bounces using r-project.org> on behalf of Martin Møller Skarbiniks Pedersen <traxplayer using gmail.com>
Date: Thursday, December 14, 2023 at 6:42 AM
To: R mailing list <r-help using r-project.org>
Subject: Re: [R] Sorting based a custom sorting function
On Thu, 14 Dec 2023 at 12:02, Duncan Murdoch <murdoch.duncan using gmail.com> wrote:
>

> class(df$value) <- "sizeclass"
>
> `>.sizeclass` <- function(left, right) custom_sort(unclass(left),
> unclass(right)) == 1
>
> `==.sizeclass` <- function(left, right) custom_sort(unclass(left),
> unclass(right)) == 0
>
> `[.sizeclass` <- function(x, i) structure(unclass(x)[i], class="sizeclass")
>
> df[order(df$value),]
>
> All the "unclass()" calls are needed to avoid infinite recursion.  For a
> more complex kind of object where you are extracting attributes to
> compare, you probably wouldn't need so many of those.

Great! Just what I need. I will create a class and overwrite > and ==.
I didn't know that order() used these exact methods.

My best solution was something like this:

quicksort <- function(arr, compare_func) {
  if (length(arr) <= 1) {
    return(arr)
  } else {
    pivot <- arr[[1]]
    less <- arr[-1][compare_func(arr[-1], pivot) <= 0]
    greater <- arr[-1][compare_func(arr[-1], pivot) > 0]
    return(c(quicksort(less, compare_func), pivot, quicksort(greater,
compare_func)))
  }
}

persons <- c("alfa", "bravo", "charlie", "delta", "echo", "foxtrot", "golf",
             "hotel", "india", "juliett", "kilo", "lima", "mike", "november",
             "oscar", "papa", "quebec", "romeo", "sierra", "tango", "uniform",
             "victor", "whiskey", "x-ray", "yankee", "zulu")

quicksort(persons, function(left, right) {
  nchar(left) - nchar(right)
})

Regards
Martin

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