# ETOOBUSY ðŸš€ minimal blogging for the impatient

# PWC203 - Special Quadruplets

**TL;DR**

Here we are with TASK #1 from The Weekly Challenge #203. Enjoy!

# The challenge

You are given an array of integers.

Write a script to find out the total special quadruplets for the given array.

`Special Quadruplets are such that satisfies the following 2 rules. 1) nums[a] + nums[b] + nums[c] == nums[d] 2) a < b < c < d`

Example 1`Input: @nums = (1,2,3,6) Output: 1 Since the only special quadruplets found is $nums[0] + $nums[1] + $nums[2] == $nums[3].`

Example 2`Input: @nums = (1,1,1,3,5) Output: 4 $nums[0] + $nums[1] + $nums[2] == $nums[3] $nums[0] + $nums[1] + $nums[3] == $nums[4] $nums[0] + $nums[2] + $nums[3] == $nums[4] $nums[1] + $nums[2] + $nums[3] == $nums[4]`

Example 3`Input: @nums = (3,3,6,4,5) Output: 0`

# The questions

How can manwar find such sweet spots? I was starting to think that this would be sort of a choreâ€¦ but it ended up to allow for a very intuitive solution.

# The solution

Instead of doing a lot of nested iterations and the like, letâ€™s just
think that we can test all possible *combinations* of 4 elements out of
the input array, looking for those where the first three items sum to
the fourth. Weâ€™re lucky that Raku has `combinations`

out of the box!

```
#!/usr/bin/env raku
use v6;
sub MAIN (*@args) { put special-quadruplets(@args) }
sub special-quadruplets (@nums) {
combinations(@nums, 4).grep({$_[0..2].sum == $_[3]}).elems
}
```

In Perl it will take some more effortâ€¦ with the help of copy-and-paste from the Combinations iterator:

```
#!/usr/bin/env perl
use v5.24;
use warnings;
use experimental 'signatures';
no warnings 'experimental::signatures';
use List::Util 'sum';
say special_quadruplets(@ARGV);
sub special_quadruplets (@nums) {
my $it = combinations_iterator(4, @nums);
my $count = 0;
while (my ($c, undef) = $it->()) {
++$count if sum($c->@[0..2]) == $c->[3];
}
return $count;
}
sub combinations_iterator ($k, @items) {
my @indexes = (0 .. ($k - 1));
my $n = @items;
return sub {
return unless @indexes;
my (@combination, @remaining);
my $j = 0;
for my $i (0 .. ($n - 1)) {
if ($j < $k && $i == $indexes[$j]) {
push @combination, $items[$i];
++$j;
}
else {
push @remaining, $items[$i];
}
}
for my $incc (reverse(-1, 0 .. ($k - 1))) {
if ($incc < 0) {
@indexes = (); # finished!
}
elsif ((my $v = $indexes[$incc]) < $incc - $k + $n) {
$indexes[$_] = ++$v for $incc .. ($k - 1);
last;
}
}
return (\@combination, \@remaining);
}
}
```

Stay safe!

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