I’m not even going to try to come up with something clever to tie this to music; just thinking about the first task makes my brain swim. So I’m tackling the second task first.

Perl Weekly Challenge 248

Task 2: Submatrix Sum

You are given a NxM matrix A of integers.

Write a script to construct a (N-1)x(M-1) matrix B having elements that are the sum over the 2x2 submatrices of A,

b[i,k] = a[i,k] + a[i,k+1] + a[i+1,k] + a[i+1,k+1]

Example 1

Input: $a = [
              [1,  2,  3,  4],
              [5,  6,  7,  8],
              [9, 10, 11, 12]
            ]

Output: $b = [
               [14, 18, 22],
               [30, 34, 38]
             ]

Example 2

Input: $a = [
              [1, 0, 0, 0],
              [0, 1, 0, 0],
              [0, 0, 1, 0],
              [0, 0, 0, 1]
            ]

Output: $b = [
               [2, 1, 0],
               [1, 2, 1],
               [0, 1, 2]
             ]

Approach

The approach here is fairly straightforward: accept the input matrix, determine the size M and N, then run through the calculations to determine the elements for the sums of the 2×2 submatricies. Jorg Sommrey was even kind enough to give us the formula.

Raku

sub submatrixSum(@a) {
  # subtract 1 because we're 0-indexed
  my $M = @a.elems - 1;    # rows
  my $N = @a[0].elems - 1; # columns
  # we are ASSUMING the matrix is consistent with
  # each row having the same number of columns
  my @b;
  for 0 .. $M - 1 -> $i {
    for 0 .. $N - 1 -> $k {
      @b[$i;$k] = @a[$i;  $k] + @a[$i;  $k+1]
                + @a[$i+1;$k] + @a[$i+1;$k+1];
    }
  }
  return @b;
}

View the entire Raku script for this task on GitHub.

Perl

sub submatrixSum(@a) {
  my $M = $#a;       # rows
  my $N = $#{$a[0]}; # columns
  # we are ASSUMING the matrix is consistent with
  # each row having the same number of columns
  my @b;
  foreach my $i ( 0 .. $M - 1 ) {
    push @b, [];
    foreach my $k ( 0 .. $N - 1 ) {
      $b[$i]->[$k] = $a[$i]->[$k]   + $a[$i]->[$k+1]
                   + $a[$i+1]->[$k] + $a[$i+1]->[$k+1];
    }
  }
  return @b;
}

View the entire Perl script for this task on GitHub.

Python

def submatrixSum(a):
    # subtract 1 because we're 0-indexed
    M = len(a) - 1    # rows
    N = len(a[0]) - 1 # columns
    # we are ASSUMING the matrix is consistent with
    # each row having the same number of columns
    b = []
    for i in range(M): # range is 0 .. M-1
        row = []
        for k in range(N):
            row.append( a[i  ][k] + a[i  ][k+1] +
                        a[i+1][k] + a[i+1][k+1] )
        b.append(row)
    return b

View the entire Python script for this task on GitHub.

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Task 1: Shortest Distance

You are given a string and a character in the given string.

Write a script to return an array of integers of size same as length of the given string such that:

distance[i] is the distance from index i to the closest occurrence
of the given character in the given string.

The distance between two indices i and j is abs(i - j).

Example 1

Input: $str = "loveleetcode", $char = "e"
Output: (3,2,1,0,1,0,0,1,2,2,1,0)

The character 'e' appears at indices 3, 5, 6, and 11 (0-indexed).
The closest occurrence of 'e' for index 0 is at index 3, so the distance is abs(0 - 3) = 3.
The closest occurrence of 'e' for index 1 is at index 3, so the distance is abs(1 - 3) = 2.
For index 4, there is a tie between the 'e' at index 3 and the 'e' at index 5,
but the distance is still the same: abs(4 - 3) == abs(4 - 5) = 1.
The closest occurrence of 'e' for index 8 is at index 6, so the distance is abs(8 - 6) = 2.

Example 2

Input: $str = "aaab", $char = "b"
Output: (3,2,1,0)

Approach

The approach we should take is pretty much outlined in the description of the example: first generate a list of what indices the target character occurs at, then calculate the closest occurrence based on abs(x - y) where x and y are the positions of the current character and an occurrence of the target character.

Raku

sub shortestDistance($str, $char) {
  # split the string into an array of characters
  my @strchar = $str.split('', :skip-empty);
  # find the positions of the target $char
  my @pos = (0 .. @strchar.end).grep: { @strchar[$_] eq $char };

  my @output;
  for 0 .. @strchar.end -> $i {
    # find the distances
    my @distance = @pos.map: { abs($i - $_) };
    # find the minimum distance
    @output.push( @distance.min );
  }
  return @output;
}

View the entire Raku script for this task on GitHub.

Perl

sub shortestDistance($str, $char) {
  # split the string into an array of characters
  my @strchar = split(//, $str);
  # find the positions of the target $char
  my @pos = grep { $strchar[$_] eq $char } 0 .. $#strchar;
  
  my @output;
  foreach my $i ( 0 .. $#strchar ) {
    # find the distances
    my @distance = map { abs($i - $_) } @pos;
    # find the minimum distance
    push @output, min(@distance);
  }
  return @output;
}

View the entire Perl script for this task on GitHub.

Python

def shortestDistance(s, c):
    # split the string into an array of characters
    strchar = list(s)
    # find the positions of the target char
    pos = [ x for x in range(len(s)) if strchar[x] == c ]

    output = []
    for i in range(len(s)):
        # find the distances
        distance = [ abs(i - p) for p in pos ]
        # find the minimum distance
        output.append(  min(distance) )
    return output

View the entire Python script for this task on GitHub.

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Here’s all my solutions in GItHub: https://github.com/packy/perlweeklychallenge-club/tree/master/challenge-248/packy-anderson