Advent Calendar 2020

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The gift is presented by Laurent Rosenfeld, the first champion of the weekly challenge. Today he is talking about his solution to the task Left Rotation of “The Weekly Challenge - 078”. This is re-produced for Advent Calendar 2020 from the original post by Laurent Rosenfeld.

Left Rotation

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.

Example 1:

Input:
    @A = (10 20 30 40 50)
    @B = (3 4)

Explanation:

a) We left rotate the 3rd index element (40) in the @A to make it 0th index member in the array.
        [40 50 10 20 30]

b) We left rotate the 4th index element (50) in the @A to make it 0th index member in the array.
        [50 10 20 30 40]

Output:
    [40 50 10 20 30]
    [50 10 20 30 40]

Example 2:

Input:
    @A = (7 4 2 6 3)
    @B = (1 3 4)

Explanation:

a) We left rotate the 1st index element (4) in the @A to make it 0th index member in the array.
        [4 2 6 3 7]

b) We left rotate the 3rd index element (6) in the @A to make it 0th index member in the array.
        [6 3 7 4 2]

c) We left rotate the 4th index element (3) in the @A to make it 0th index member in the array.
        [3 7 4 2 6]

Output:
    [4 2 6 3 7]
    [6 3 7 4 2]
    [3 7 4 2 6]

Left Rotation in Raku

We can simply use array slices to get what we need. The only slight difficulty is that we need to flatten the two index slices into a single list.

use v6;

my @a = 10, 20, 30, 40, 50;
my @indices = 3, 4;

say "Input array: ", @a;
for @indices -> $i {
    my @out = @a[($i..@a.end, 0..$i -1).flat];
    say @out;
}

Output:

$ raku left_rotate.raku
Input array: [10 20 30 40 50]
[40 50 10 20 30]
[50 10 20 30 40]

Left Rotation in Perl

Again, we use array slices. Here, the only slight difficulty is the relatively heavy use of nested array references.

use strict;
use warnings;
use feature "say";

my @a = ( [[10, 20, 30, 40, 50],[3, 4]],
          [[7, 4, 2, 6, 3], [1, 3, 4]] );
left_rotate($_) for @a;

sub left_rotate {
    my $inref = shift;
    my ($in, $indices) = @$inref;
    say "\nInput array: @$in - Indices: @$indices";
        for my $i (@$indices){
        my @out = @$in[$i..$#{$in}, 0..$i -1];
        say "@out";
    }
}

Output:

$ perl left_rotate.pl

Input array: 10 20 30 40 50 - Indices: 3 4
40 50 10 20 30
50 10 20 30 40

Input array: 7 4 2 6 3 - Indices: 1 3 4
4 2 6 3 7
6 3 7 4 2
3 7 4 2 6

If you have any suggestion then please do share with us perlweeklychallenge@yahoo.com.

Advent Calendar 2020