PWC - Left Rotation


The solution to task #2 of the Perl Weekly Challenge for this week, and additional reflections.

In last post Leader element I shared a trivial thought about how the Perl Weekly Challenge tasks can be a good excercise for thinking about the problems beyond providing a quick answer for them.

This is also the case for the task #2 in this week’s challenge:

You are given array @A containing positive numbers and @B containing one or more indices from the array @A.

Write a script to left rotate @A so that the number at the first index of @B becomes the first element in the array. Similary, left rotate @A again so that the number at the second index of @B becomes the first element in the array.

My solution is pretty straightforward (most is scaffolding for testing):

 1 #!/usr/bin/env perl
 2 use 5.024;
 3 use warnings;
 4 use experimental qw< postderef signatures >;
 5 no warnings qw< experimental::postderef experimental::signatures >;
 7 sub shift_left_by ($n, @A) { (@A[$n..$#A], @A[0..($n-1)]) }
 8 sub shift_left ($A, $B) { map { [shift_left_by($_, $A->@*)] } $B->@* }
10 for my $test (
11    [
12       'first test',
13       [qw< 10 20 30 40 50 >],
14       [qw< 3 4 >],
15    ],
16    [
17       'second test',
18       [qw< 7 4 2 6 3 >],
19       [qw< 1 3 4 >]
20    ],
21 ) {
22    my ($title, $A, $B) = $test->@*;
23    say {*STDERR} $title;
24    say {*STDOUT} '[', join(', ', $_->@*), ']' for shift_left($A, $B);
25 }

Function shift_left iterates over the indexes in $B (which is a reference to @B) and calls the actual left-shifting function shift_left_by. This, in turn, takes the relevant parts out of the array and arranges them as requested.

There’s a lot that might be discussed, e.g.:

  • this solution assumes that it’s OK to create copies of the @A array around, which should be double checked;
  • it might be interesting to understand whether a solution that “starts from the previous iteration” might go better (e.g. speed-wise) with respect to this, especially if we’re not allowed to make copies of @A but have to do the shifting in-place (in this case, we would have to do some modulo scalar(@A) arithmetic over the indices in @B);
  • what should the functions/script do in case of wrong inputs (e.g. an index in @B that is not present in @A).


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