TL;DR

Here we are with TASK #1 from The Weekly Challenge #184. Enjoy!

The challenge

You are given list of strings in the format aa9999 i.e. first 2 characters can be anything 'a-z' followed by 4 digits '0-9'.

Write a script to replace the first two characters with sequence starting with '00', '01', '02' etc.

Example 1

Input: @list = ( 'ab1234', 'cd5678', 'ef1342')
Output: ('001234', '015678', '021342')

Example 2

Input: @list = ( 'pq1122', 'rs3334')
Output: ('001122', '013334')

The questions

Well, a couple…

• can we really assume that items are compliant to the specification? In case, what to do with the non-compliant ones?
• what if our array has more than 100 items? Do we start back from '00' or should we add a digit? Should we eat a digit from the next section?

The solution

We will use a substring function to substitute the first two characters with the sequence number, obtained using sprintf with a suitable at-least-to-digits template.

#!/usr/bin/env raku
use v6;
sub MAIN {
   my @l = < ab1234 cd5678 ef1342 >;
   sequence-number(@l);
   .put for @l;
}

sub sequence-number (@list) {
   my $n = 0;
   @list.map({.substr-rw(0, 2) = '%02d'.sprintf($n++)});
   @list;
}

We can do the same with Perl of course:

#!/usr/bin/env perl
use v5.24;
use warnings;
use experimental 'signatures';
no warnings 'experimental::signatures';

my @list = qw< ab1234 cd5678 ef1342 >;
sequence_number(\@list);
say for @list;

sub sequence_number ($list) {
   my $n = 0;
   substr $_, 0, 2, sprintf '%02d', $n++ for $list->@*;
   return $list;
}

Both solutions change the array in-place because… well, that’s how I read replace the first two characters…! Should I have asked something in the questions?!?

Stay safe folks!