#!/usr/bin/env perl
use strict;
use warnings;

=head1

Lesson 1

This stuff contained between =head and =cut is called POD (Plain Old Documentation) and it's used for extended documentation inside of a script.
It is ignored during compile/run time.

The first thing you need to learn is about strings, lists, arrays, and scalars.

=cut

my $string = 'Michael Fontanesi';

=head2

This is a simple string. $string is a variable that contains Michael Fontanesi. You can print this variable.

=cut

print $string;
# outputs Michael Fontanesi

=head3

You can change the value of a string very easily.

=cut

$string = 'Connor Yates';
print $string;
# outputs Connor Yates. Rememeber, it originally was Michael Fontanesi.

=head4

You can also combine strings very easily. This is called "concatentation"

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my $first = 'Michael';
my $last  = 'Fontanesi';
my $full  = "$first $last";
print $full;
# outputs Michael Fontanesi

$full = $first . $last;
# periods are frequnetly used in concatentation. It combines $first and $last with no space.
# outputs MichaelFontanesi

=head5

The next thing you need to learn about is lists. It's exactly how it sounds, it's a list of things.
The important part of lists is that they ARE NOT NAMED.

=cut

# ( 'Michael', 'Fontanesi', 'Connor', 'Yates', );
# this would cause an error if it wasn't commented out. You will see how lists are used later.

=head6

The way that lists are normally used is in array context. This is so we have a named variable to we can call.

=cut

my @array = ('Michael', 'Fontanesi', 'Connor', 'Yates');

=head7

The order of the values in the array is preserved. Arrays have "indexes" so you can refer to specific values within
the array. This is called an array slice. Since array slices refer to a single value, it is a string. In computer languages,
the first value is 0 instead of 1

=cut

my $array_slice = $array[0];
# outputs Michael
$array_slice    = $array[1];
# outputs Fontanesi
$array_slice    = $array[2];
# outputs Connor
$array_slice    = $array[3];
# outputs Yates

=head8

Arrays are important. Let's do stuff with them. To iterate through each value in an array (OR LIST),
you use a foreach (or for) loop.

=cut

my @names = ('Michael, 'Connor', 'Joey', 'Barton');
foreach my $name (@names) {
    print "Fuck you $name\n";
}
# outputs Fuck you Michael
#         Fuck you Connor
#         Fuck you Joey
#         Fuck you Barton

# what if we only want to say Fuck you to Barton?

my @names = ('Michael, 'Connor', 'Joey', 'Barton');
foreach my $name (@names) {
    print "Fuck you $name\n" if $name eq 'Barton';
}
# outputs Fuck you Barton

=head9

So that's great and all. In older languages like C, programs are absolutely riddled with foreach and if loops and it gets
very hard to read. Modern languages make working with lists and arrays easier with "list operators". The most important ones
are map, grep, and sort.

map iterates through each value of an array or list and does whatever operation you tell it to.
grep is the comparison operator and will filter a list or array with your given parameters.
sort does exactly what you think. You can specify how you want things sorted (i.e. ascending, descending).

Let's use these

=cut

my @animals = ('DOG', 'CAT', 'BAT', 'BUFFALO', 'ELEPHANT', 'TIGER');
# let's make all of these lowercase

my @lowercase = map { lc($_) } @animals;
# @lowercase now contains dog, cat, bat, buffalo, elephant, tiger
# the $_ is a special variable in perl meaning "the current value" so we don't have to make a separate string variable
# like we did with foreach loops

# You also don't need to make a new array variable! You can modify the values in the original array

@animals = map { lc($_) } @animals;

# Now, what if we don't want all the values in this array? Let's filter it with grep.
@animals = ('DOG', 'CAT', 'BAT', 'BUFFALO', 'ELEPHANT', 'TIGER');
@animals = grep { /BAT/ } @animals;
# @animals only consists of the value BAT now. We can also do
@animals = grep { !/BAT/ } @animals;
# ! is negation, or "not" like in "not equal to", so now @animals contains everything except BAT

# What if we want these in alphabetical order? We use sort. sort has special variables $a and $b which you use to specify
# the order you want

@animals= sort { $a <=> $b } @animals;
# sorts it in alphabetical order

@animals= sort { $b <=> $a } @animals;
# sorts it in reverse alphabetical order

=head9

Imagine doing all of these operations with foreach and if loops. It would get very, very messy. List operators are very
powerful tools in modern programming languages and you can accomplish complicated tasks by using them in smart ways.

Let's combine these list operators to do all of the work we just ^ did in a single line.

=cut

@animals = ('DOG', 'CAT', 'BAT', 'BUFFALO', 'ELEPHANT', 'TIGER');
@animals = sort { $a <=> $b } map { lc($_) } grep { !/BAT/ } @animals;
# you read list operators from right to left. This first filters out BAT from the array with grep
# then converts all values to lowercase with map
# and then sorts all of the values in alphabetical order

=head10

map, grep, and sort are extremely important. Your first practice assignment is to write me some code that uses map, grep, and
sort. Master them. Start thinking in terms of list operators instead foreach loops.

END OF LESSON 1

=cut