RogerBW's Blog

The Weekly Challenge 206: Array Time! 05 March 2023

I’ve been doing the Weekly Challenges. The latest involved lots of combinatorial searches. (Note that this closes today.)

Both of these problems are best solved with combinatorics. I could have done task 1 in the languages that don't have a readily available combination generator as a fixed depth task (nested for loops), and task 2 ditto with proper combination and permutation generators, but I've done both of these before, I know I can do them (because I've coded up both of these algorithms for my PostScript utility libraries) and this week I lacked enthusiasm for doing it again. So no JavaScript, Kotlin or Lua this time, though eventually I'll probably write the relevant library functions for them (also cartesian product maybe).

Task 1: Shortest Time

You are given a list of time points, at least 2, in the 24-hour clock format HH:MM.

Write a script to find out the shortest time in minutes between any two time points.

We did full time-date parsing in challenge 178 task 2, but this is just hour-and-minute stuff.

Ruby:

def shortesttime(n)
  dl = 1440

Convert each time to a number of minutes past midnight.

  ni = []
  for x in n do
    ni.push(x[0..1].to_i * 60 + x[3..4].to_i)
  end

For each combination of two times, calculate the time difference in both directions (i.e. A to B and B to A, crossing midnight if necessary).

  o = []
  ni.combination(2) do |p|
    d = (p[0] - p[1]).abs
    o.push(d)
    o.push(dl - d)
  end

And return the smallest value.

  return o.min
end

Task 2: Array Pairings

You are given an array of integers having even number of elements..

Write a script to find the maximum sum of the minimum of each pairs.

This was rather fun to work out. Here it is in Raku:

sub arraypairing(@n) {

Make sure we do have an even number of elements.

    my $nl = @n.elems;
    if ($nl % 2 == 1) {
        return 0;
    }
    my $hl = $nl div 2;
    my @out;

Split the array in half in every possible way (combination generator on the array's indices, so that I can easily generate the half that's excluded from the combination too). This does twice as much work as it needs to: it'll generate [a, b] leaving [c, d] and [c, d] leaving [a, b]. I suspect it would have made sense to generate pa based on the combinations on the range of indices from 1 to nl-1, then add index zero to it.

    for [0 .. $nl-1].combinations($hl) -> @px {
        my @pa = map {@n[$_]},@px;
        my $ps = @px.Set;
        my @pb = map {@n[$_]}, grep {$ps{$_}:!exists}, (0 .. $nl-1);

Now @pa and @pb are disjoint lists each consisting of half the entries. Now we take every possible pairing between the two, by working through the permutations of @pa, and add up the values of the minimum of each pair.

        for @pa.permutations -> @pp {
            my $s = 0;
            for (0 .. $hl-1) -> $i {
                $s += min(@pp[$i], @pb[$i]);
            }
            @out.push($s);
        }
    }

Then return the largest one. (As in task 1, if I were working on large data sets I'd just store a single value, out = max(out, s).)

    return max(@out);
}

Full code on github.

Comments on this post are now closed. If you have particular grounds for adding a late comment, comment on a more recent post quoting the URL of this one.

Search
Archive
Tags 1920s 1930s 1940s 1950s 1960s 1970s 1980s 1990s 2000s 2010s 3d printing action advent of code aeronautics aikakirja anecdote animation anime army astronomy audio audio tech aviation base commerce battletech beer boardgaming book of the week bookmonth chain of command children chris chronicle church of no redeeming virtues cold war comedy computing contemporary cornish smuggler cosmic encounter coup covid-19 crime cthulhu eternal cycling dead of winter doctor who documentary drama driving drone ecchi economics en garde espionage essen 2015 essen 2016 essen 2017 essen 2018 essen 2019 essen 2022 essen 2023 existential risk falklands war fandom fanfic fantasy feminism film firefly first world war flash point flight simulation food garmin drive gazebo genesys geocaching geodata gin gkp gurps gurps 101 gus harpoon historical history horror hugo 2014 hugo 2015 hugo 2016 hugo 2017 hugo 2018 hugo 2019 hugo 2020 hugo 2022 hugo-nebula reread in brief avoid instrumented life javascript julian simpson julie enfield kickstarter kotlin learn to play leaving earth linux liquor lovecraftiana lua mecha men with beards mpd museum music mystery naval noir non-fiction one for the brow opera parody paul temple perl perl weekly challenge photography podcast politics postscript powers prediction privacy project woolsack pyracantha python quantum rail raku ranting raspberry pi reading reading boardgames social real life restaurant reviews romance rpg a day rpgs ruby rust scala science fiction scythe second world war security shipwreck simutrans smartphone south atlantic war squaddies stationery steampunk stuarts suburbia superheroes suspense television the resistance the weekly challenge thirsty meeples thriller tin soldier torg toys trailers travel type 26 type 31 type 45 vietnam war war wargaming weather wives and sweethearts writing about writing x-wing young adult
Special All book reviews, All film reviews
Produced by aikakirja v0.1