Why can't my Perl code implement the reverse function? - perl

Question

Here is my code named reverse.pl
#!usr/bin/perl -w
use 5.016;
use strict;
while(my $line=<>)
{
my #array=();
push (#array,$line);
#array=reverse#array;
say #array;
}
Test file named a.txt
A B C D
E F G H
I J K L
M N O P
Q R S T
My command is perl reverse.pl a.txt
Why it can't implement the reverse function?
I want to show the result is:
D C B A
H G F E
and so on.

Answer 1

Reverse in a scalar context reverses a scalar.
Reverse in a list context reverses the list, but not each scalar within the list.
You explicitly turn your scalar $line into a list with one item and then reverse the order of the items.
Try this:
#!/usr/bin/perl
use 5.016;
use strict;
while (my $line=<>) {
chomp($line);
say scalar reverse $line;
}
If you have an array and want to reverse each element (but not the elements), use map:
my #array = qw(Alpha Beta Gamma);
#array = map { scalar reverse $_ } #array;
print "#array\n";
If you want to do both (reverse each element and the elements themselves), do:
#array = map { scalar reverse $_ } reverse #array;
or:
#array = reverse map { scalar reverse $_ } #array;

Answer 2

When you say:
push #array, $line;
You're creating an array of one value that's equal to the line.
$array[0] = "A B C D";
When you say:
#array = reverse #array;
You are reversing that single member array. The first element becomes the last, and the last element becomes the first, etc.. However, you only have one element, so there's nothing to reverse.
What you want to do is create an array with your line:
my #array = split /\s+/, $line;
This will create an array with each character being a separate element of the array. For example, your first line:
$array[0] = "A";
$array[1] = "B";
$array[2] = "C";
$array[3] = "D";
Now, if you use reverse on this array, you'll get:
$array[0] = "D";
$array[1] = "C";
$array[2] = "B";
$array[3] = "A";
Here's the program:
use strict;
use warnings;
use feature qw(say);
while ( my $line = <> ) {
chomp $line;
my #array = split /\s+/, $line;
say join " ", reverse $line;
}
The join function takes an array, and joins each element into a single line -- thus rebuilding your line.
By the way, I could have done this:
#array = reverse #array;
say "#array"; #Quotes are important!
This is because Perl will automatically join an array with whatever character is in $". This is a Perl variable that is used for joining arrays when that array is placed in quotation marks, and the default value is a single space.
Personally, I rather prefer the say join " ", reverse $line;. It's more obvious what is going on, and doesn't depend upon the value of rarely used variables.