PWC117 - Missing Row


Here we are with TASK #1 from the Perl Weekly Challenge #117. Enjoy!

The challenge

You are given text file with rows numbered 1-15 in random order but there is a catch one row in missing in the file.

11, Line Eleven
1, Line one
9, Line Nine
13, Line Thirteen
2, Line two
6, Line Six
8, Line Eight
10, Line Ten
7, Line Seven
4, Line Four
14, Line Fourteen
3, Line three
15, Line Fifteen
5, Line Five

Write a script to find the missing row number.

The questions

More than questions… assumptions:

  • each line starts with one or two digits
  • the rest of the line might contain whatever, not necessarily the kind of lines in the example

The solution

I started with Raku:

#!/usr/bin/env raku
use v6;

sub missing-row ($file) {
   constant All = set(1 .. 15);
   (All (-) set(${+ S/^ (\d+) .*/$0/}))).keys;

put missing-row(@*ARGS[0]);

The idea is to leverage the Set data type, subtracting the set created from the line numbers found in the file from the overall set of possible line numbers, thanks to the (-) operator.

The solution in Perl is conceptually similar, although at a lower level:

#!/usr/bin/env perl
use 5.024;
use warnings;
use experimental qw< postderef signatures >;
no warnings qw< experimental::postderef experimental::signatures >;

sub missing_row ($file) {
   open my $fh, '<', $file or die "open('$file'): $!\n";
   my %all = map {$_ => 1} 1 .. 15;
   delete $all{s{\A (\d+) .*}{$1}rmxs} while <$fh>;
   return keys %all;

say missing_row($ARGV[0]);

Here, we leverage the mighty hash data type by initializing %all to all line numbers, then progressively removing the lines found in the file.

I have to admit that I like Perl’s ability to confuse integers and strings, although I understand the advantages in telling them apart. To be honest, adding a + is not that overhead to turn a Raku string into an integer… so well, I will not complain (too much) about it.

I hope you enjoyed!

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