# RogerBW's Blog

 Perl Weekly Challenge 68: zero matrix and list reordering 10 July 2020 I’ve been doing the Perl Weekly Challenges. The latest involved zeroing parts of a matrix and reordering a list. You are given a matrix of size `M x N` having only `0s` and `1s`. Write a script to set the entire row and column to `0` if an element is `0`. Back to test-driven development. ``````use Test::More tests => 2; is_deeply(zm([[1,0,1],[1,1,1],[1,1,1]]), [[0,0,0],[1,0,1],[1,0,1]], 'example 1', ); is_deeply(zm([[1,0,1],[1,1,1],[1,0,1]]), [[0,0,0],[1,0,1],[0,0,0]], 'example 2', ); `````` Then the actual function. ``````sub zm { my \$in=shift; `````` Get the M and N sizes (assume N is consistent). `````` my \$a=scalar @{\$in}-1; my \$b=scalar @{\$in->[0]}-1; my %seta; my %setb; `````` For each zero we find, flag the rows and columns in the list. `````` foreach my \$ai (0..\$a) { foreach my \$bi (0..\$b) { if (\$in->[\$ai][\$bi]==0) { \$seta{\$ai}=1; \$setb{\$bi}=1; } } } `````` Then, using those flags, zero the relevant cells. We may set a cell more than once if it's in both a zeroed row and a zeroed column (or indeed if it's the zero cell that triggered the setting in the first place), but I suspect it's quicker to do that than to keep another level of edit lists. `````` foreach my \$aa (keys %seta) { foreach my \$bi (0..\$b) { \$in->[\$aa][\$bi]=0; } } foreach my \$bb (keys %setb) { foreach my \$ai (0..\$a) { \$in->[\$ai][\$bb]=0; } } return \$in; } `````` You are given a singly linked list \$L as below: `L0 → L1 → … → Ln-1 → Ln` Write a script to reorder list as below: `L0 → Ln → L1 → Ln-1 → L2 → Ln-2 →` Perl really doesn't do linked lists; there's rarely a need for them. But I'll try to solve this in the spirit of the problem. ``````use Test::More tests => 2; is_deeply(rl([1,2,3,4]), [1,4,2,3], 'example 1', ); is_deeply(rl([1,2,3,4,5]), [1,5,2,4,3], 'example 2', ); sub rl { my \$list=shift; `````` I'll ignore the contents for now, and simply build a list of indices in the required order. `````` my \$n=scalar @{\$list}; my \$nx=\$n-1; my @i; foreach my \$ni (0..int(\$nx/2)) { push @i,\$ni,\$nx-\$ni; } if (\$i[-1] == \$i[-2]) { pop @i; } `````` Now we have a list of indices, and we can rearrange the list of actual data in a single operation with array slicing: `````` @{\$list}=@{\$list}[@i]; return \$list; } `````` 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
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