Perl reverse Function

Perl reverse Function

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This short Perl reverse Tutorial will show you how to use this function in your scripts, in either a scalar or list context. The reverse function doesn’t change the argument. If you want to keep the result returned by the function, you need to assign it back to a variable.

The syntax forms of the Perl reverse function are as follows:

@reversedArray = reverse @array;
$str = reverse @array;
$newStr = reverse $str;
$str = reverse;

As you can see, there are at least 4 syntax forms for this function.

The first syntax form is used in a list context. In a list context, this function is used to reverse the order of an array, returning a resulting array with the elements in an opposite order.

The next three syntax forms are for the scalar context:

you can reverse an array into a string: it concatenates the elements of the array and returns a string value with all the characters in an opposite order
you can use reverse to get a string with the characters in the opposite order
used without argument, it reverses the $_

See the following example for an exemplification of all these syntax forms below.

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Simple examples

#!/usr/local/bin/perl

use strict;
use warnings;

my @array = qw(one two three four);
# list context

my @reversedArray = reverse @array;
print "@reversedArray\n"; 
# it displays: four three two one

# scalar contexts

my $str = reverse @array;
print "$str\n";
# it displays: ruofeerhtowteno

$str = reverse $str;
print "$str\n";
# it displays: onetwothreefour

# the next two examples use reverse with $_

print scalar reverse, " " foreach (@array);
# it displays: eno owt eerht ruof 

print "\n";

$_ = "1234";
$_ = reverse;
print;
# it displays: 4321

print "\n";

Sort an array in a reversed order

You can use both the sort and reverse function to sort an array in a descending alphabetical order. By default the sort function compares the ASCII values of the array elements, so if you have an array of numbers you need to use the <=> operator explicitly to get a correct ordered list.

Here’s an example:

#!/usr/local/bin/perl

use strict;
use warnings;

my @array = qw(13 21 15 175 1 );
@array = reverse sort ({$a <=> $b} @array);

print "@array\n";
# it displays: 175 21 15 13 1

You can get the same output without using the Perl reverse function by swapping the $a and $b special variables:

@array = sort ({$b <=> $a} @array);

If your array contains strings as elements, you can omit the cmp operator and the above line of code will look like this:

@array = reverse sort @array;

Using reverse to invert a hash

The following example shows you how to invert a hash, by swapping the keys with the values. However, at first ensure yourself that your hash has unique values, otherwise the resulting hash will have fewer elements that the initial one.

Here’s an example:

#!/usr/local/bin/perl

use strict;
use warnings;

my %hash = (one   => 1, two  => 2, 
            three => 3, four => 4);
%hash = reverse %hash;

foreach my $key (sort {$a <=> $b} keys %hash) {
    print "$key=>$hash{$key}, ";
}
print "\n";

# it displays: 1=>one, 2=>two, 3=>three, 4=>four

The Perl reverse function inverts the %hash by swapping the values with the keys. The foreach statement prints the elements of the hash. The keys function returns the keys of the hash unordered, so if you want to print the hash keys in an ascending or descending order, you need to use the sort function.

Because after inverting the hash the keys are numbers, the <=> operator is used. In our example the inverted hash is printed with the keys in an ascending order. If you want a descending order you need to swap $a with $b in the sort function: $b <=> @a. If you want to sort the hash keys alphabetically, you need to use instead the <=> operator, the cmp operator.

Check my new How To Tutorial eBooks (PDF format):
"Perl Statements - How To Tutorial"
"Perl Scalar and String Functions - How To Tutorial"

to see a lot of fully commented examples that help you use the Perl statements and the Perl buit-in functions in your scripts.

Get a sentence with the words in an inverted order

You can use the Perl reverse function to reverse the words of a sentence.

Here’s an example:

#!/usr/local/bin/perl

use strict;
use warnings;

my $sentence = 'This is about the Perl reverse function';
my @words = split /\s+/, $sentence;
@words = reverse @words;
$sentence = join ' ', @words;
print "$sentence\n";
#it displays: function reverse Perl the about is This

The split function will return an array from the $sentence string and the array will be inverted by the Perl reverse function. Next the join function will concatenate the elements of the array into the $sentence variable, using the space delimiter. Finally, the content of the $sentence variable will be printed.

You can write a shorthand version of this code using of $_ special variable:

#!/usr/local/bin/perl

use strict;
use warnings;

$_ = 'This is about the Perl reverse function';
print join(' ', reverse (split)), "\n";
#it displays: function reverse Perl the about is This

Here the split function is used without any argument, therefore $_ will be used as expression and whitespace as separator.

Reverse the rows of a matrix hold in an array

You can build a matrix with three rows and three columns and populate a bi-dimensional array with the matrix elements. You can emulate a bi-dimensional array from a usual array that has as elements references to other arrays. The Perl reverse function will be used to reverse the order of the rows.

See the code:

#!/usr/local/bin/perl

use strict;
use warnings;

my @matrix = ( [1, 2, 3],
               [4, 5, 6],
	       [7, 8, 9]
	     );		 
			 
foreach my $i (@matrix) { 
  foreach my $j (reverse @$i) {
    print "$j ";
  }
  print "\n";
}

To traverse the bi-dimensional array, a nested foreach is used. The @$i is used to dereference the references of the @matrix array.

The output is as follows:

3 2 1
6 5 4
9 8 7

An alternative to populate the @matrix array is as follows:

my @rows1 = (1, 2, 3);
my @rows2 = (4, 5, 6);
my @rows3 = (7, 8, 9);

my @matrix = (\@rows1, \@rows2, \@rows3);

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Table of Contents:

A Perl Script
Install Perl
Running Perl
Perl Data Types
Perl Variables
Perl Operators
Perl Lists
Perl Arrays
    Array Size
    Array Length
Perl Hashes
Perl Statements
    Perl if
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    Perl switch
    Perl while
    Perl do-while
    Perl until
    Perl do-until
    Perl for
    Perl foreach
Built-in Perl Functions
    Functions by Category
        String Functions
        Array Functions
    Functions in alphabetical order
        chomp
        chop
        chr
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        defined
        grep
        hex
        index
        join
        lc
        lcfirst
        length
        map
        oct
        ord
        pack
        pop
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        qw
        reverse
        rindex
        scalar
        shift
        sort
        splice
        split
        substr
        uc
        undef
        unshift

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