## Perl Weekly Challenge 284: Lucky Integer

These are some answers to the Week 284, Task 1, of the Perl Weekly Challenge organized by Mohammad S. Anwar.

Spoiler Alert: This weekly challenge deadline is due in a few days from now (on September 1, 2024, at 23:59). This blog post provides some solutions to this challenge. Please don’t read on if you intend to complete the challenge on your own.

## Task 1: Lucky Integer

You are given an array of integers, `@ints`.

Write a script to find the lucky integer if found otherwise return -1. If there are more than one then return the largest.

A lucky integer is an integer that has a frequency in the array equal to its value.

Example 1

``````Input: @ints = (2, 2, 3, 4)
Output: 2
``````

Example 2

``````Input: @ints = (1, 2, 2, 3, 3, 3)
Output: 3
``````

Example 3

``````Input: @ints = (1, 1, 1, 3)
Output: -1
``````

We will assume that the input array contains only strictly positive integers, as in the examples provided. Negative numbers would not make sense in the context, as a negative frequency is impossible.

### Lucky Integer in Raku

Once again, we use a Bag to create the `\$count` histogram of frequencies. Then, we create an array of items whose count is equal to the key value. And we return -1 if none was found, and the max value otherwise.

``````sub lucky-integer (@in) {
my \$count = @in.Bag;
my @lucky = grep { \$_ == \$count{\$_} }, \$count.keys;
return -1 if @lucky.elems == 0;
return @lucky.max;
}

my @tests = <2 2 3 4>, <1 2 2 3 3 3>, <1 1 1 3>;
for @tests -> @test {
printf "%-12s => ", "@test[]";
say lucky-integer @test;
}
``````

This program displays the following output:

``````\$ raku ./lucky-integer.raku
2 2 3 4      => 2
1 2 2 3 3 3  => 3
1 1 1 3      => -1
``````

### Lucky Integer in Perl

This is a port to Perl of the above Raku program. Note that we use a hash (`%count`) instead of a `Bag` to host the histogram of frequencies. We also need a loop to find maximum value in the `@lucky` array.

``````sub lucky_integer {
my %count;
\$count{\$_}++ for @_;
my @lucky = grep { \$_ == \$count{\$_} } keys %count;
return -1 if \$#lucky == 0;
my \$max = shift @lucky;
for my \$i (@lucky) {
\$max = \$i if \$i > \$max;
}
return \$max;
}

my @tests = ( [<2 2 3 4>], [<1 2 2 3 3 3>], [<1 1 1 3>] );
for my \$test (@tests) {
printf "%-12s => ", "@\$test";
say lucky_integer @\$test;
}
``````

This program displays the following output:

``````\$ raku ./lucky-integer.raku
2 2 3 4      => 2
1 2 2 3 3 3  => 3
1 1 1 3      => -1
``````

## Wrapping up

The next week Perl Weekly Challenge will start soon. If you want to participate in this challenge, please check https://perlweeklychallenge.org/ and make sure you answer the challenge before 23:59 BST (British summer time) on September 9, 2024. And, please, also spread the word about the Perl Weekly Challenge if you can.