# RogerBW's Blog

 The Weekly Challenge 249: Equal Match 31 December 2023 I’ve been doing the Weekly Challenges. The latest involved breaking lists into groups and building a numerical sequence. (Note that this ends today.) Task 1: Equal Pairs You are given an array of integers with even number of elements. Write a script to divide the given array into equal pairs such that: a) Each element belongs to exactly one pair. b) The elements present in a pair are equal. The simple solution seems to me to take the input list, sort it, then consume it two at a time and make sure each pair of values fits. (If not, bail out with an empty list.) Most languages have some sort of "take two items off the list at a time" functionality, usually called something like "chunk", and the ones that don't have `for` loops with a step. Kotlin: ``````fun shortestdistance(a0: List): List> { if (a0.size % 2 != 0) { return emptyList>() } var a = ArrayList(a0) a.sort() var out = ArrayList>() for (t in a.chunked(2)) { if (t[0] != t[1]) { return emptyList>() } out.add(listOf(t[0], t[0])) } return out.toList() } `````` Python: ``````def equalpairs(a0): a = a0 a.sort() out = [] for i in range(0, len(a), 2): if a[i] != a[i+1]: return [] out.append([a[i], a[i]]) return out `````` Task 2: DI String Match You are given a string s, consisting of only the characters "D" and "I". Find a permutation of the integers [0 .. length(s)] such that for each character s[i] in the string: s[i] == 'I' ⇒ perm[i] < perm[i + 1] s[i] == 'D' ⇒ perm[i] > perm[i + 1] In other words: arrange `0..len(s)` such that each entry after the first is higher ("I") or lower ("D") than the previous one. The most enjoyable part of this was working out an algorithm. I ended up using powers of two; say my first value is 16, then the next can be 16+8 or 16-8, then ±4, etc. Then I map the sorted order of those values to `0..len(s)`, and use that to create the output. Perl: ``````sub distringmatch(\$a) { my \$v = 1 << (length(\$a) - 1); my \$wv = \$v << 1; my @out = (\$wv); `````` My first value is always in the mid-point of the potential range. `````` foreach my \$c (split '', \$a) { `````` For each character, add or subtract the current delta value. `````` if (\$c eq 'I') { \$wv += \$v } else { \$wv -= \$v; } `````` Then halve the delta value and store the current working value. `````` \$v >>= 1; push @out, \$wv; } `````` Build a separate list that's all those values in order. `````` my @r = sort{\$::a <=> \$::b} @out; `````` Create a hash to map list values to index values. `````` my %c = map {\$r[\$_] => \$_} (0..\$#r); `````` Return the index values in that order. `````` return [map{\$c{\$_}} @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 10 per page 20 per page 50 per page 100 per page 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
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