# RogerBW's Blog

 The Weekly Challenge 242: One Of Our Matrices Is Missing 12 November 2023 I’ve been doing the Weekly Challenges. The latest involved differentiating lists and processing matrices. (Note that this ends today.) Task 1: Missing Members You are given two arrays of integers. Write a script to find out the missing members in each other arrays. I.e. return [[members of A not in B], [members of B not in A]]. If I'm doing the same thing twice, I usually try to segment it off somewhere. So let's do half the problem, based on a list and a set of entries (here in Perl): ``````sub halfmissing(\$a, \$bh) { return [uniq grep {!exists \$bh->{\$_}} @{\$a}]; } `````` The function "return only the distinct elements in this list" is the sort of thing I'd normally do by flipping them into a set and back (and if we had no repeats it could be a set intersection), but in this case we do have repeats and the implication of the examples is that the returns should be in the original order. Most of the languages I'm using have something like "distinct" or "unique" that'll do this, though in a few cases I ended up writing one. Then I calculate the sets in advance. ``````sub missingmembers(\$a, \$b) { my \$ah = {map {\$_ => 1} @{\$a}}; my \$bh = {map {\$_ => 1} @{\$b}}; return [ halfmissing(\$a, \$bh), halfmissing(\$b, \$ah), ]; } `````` (I'm not sure now why I didn't do this set conversion in the `halfmissing` function, except that I was probably considering extending it to an N-array version.) Task 2: Flip Matrix You are given n x n binary matrix. Write a script to flip the given matrix as below. Which turns out to be: 0 turns to 1, 1 turns to 0,and each row is reversed. I think this is most cleanly done in a functional style, here in Ruby: ``````def flipmatrix(a) return a.collect{|r| r.collect{|m| 1-m}.reverse} end `````` or Kotlin: ``````fun flipmatrix(a: List>): List> { return a.map{row -> row.map{1 - it}.reversed()} } `````` or (my extended) PostScript: ``````/flipmatrix { { { 1 exch sub } map reverse } map } bind def `````` Where I had to deviate from that pattern, it tended to be because the language only provided a reverse-in-place function rather than one that would return a reversed copy of the input—or in the case of Lua no reversing function at all. ``````function flipmatrix(a) local b = {} for _, row in ipairs(a) do local r2 = {} for _b, v in ipairs(row) do table.insert(r2, 1, 1 - v) end table.insert(b, r2) end return b end `````` 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 crystal 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 2021 hugo 2022 hugo 2023 hugo 2024 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