Perl Basic Examples

Let us learn Perl the practical way with some examples that will help to understand the usage of different commands and constructs in Perl.

Example 1: Given an array of numbers, write a Perl code to remove the elements that are divisible by 2 and print the array.

Logic: 

1. Traverse each element of the array using foreach (for loop)
2. If the element is not divisible by 2, push to a new array
3. Copy the new array to the old one and print it. 

Perl Code:

my $element;
my @array = (1,2,3,4,5,6);
foreach $element (@array)
{
 if ($element % 2 != 0) {push(@result, $element);}
}
@array = @result;
print "@array\n";

Example 2: Perform the above example without the use of an additional array variable.

Logic:

1. Get the length of the array using $#array + 1 ($# is a special variable)
2. Have an index variable i going from 0 to length-1 to traverse the array
3. Get the first element of array at index 0.
4. If the element is not divisible by 2, push the element to the end of the same array.
5. Shift the array one step to the left, so that the next element falls at index 0.
6. Increment i. Goto step 3 until while loop completes (when i goes beyond original array length).
7. When loop completes, all our original elements would have been shifted out and we are left with only the newly pushed elements.
8. Print the array.

Perl Code:

my $i;
my $element;
my @array = (1,2,3,4,5,6);
my $len = $#array + 1;
while ($i < $len)
{
 $element = $array[0];
 if ($element % 2 != 0) {push(@array, $element);}
 shift (@array);
 $i++;
}
print "@array\n";

Example 3: Consider a paragraph with many round brackets ( ). Write a Perl code to do the following: 

If any line contains either an open or closed bracket, remove it.

If the line contains both the brackets, do nothing.

Logic:

1. Split the paragraph into lines by using split at '. '
2. Use while loop to traverse each line.
3. Use variable curline to store the current line.
4. If curline contains either '(' or ')' , basically xor, then we remove the bracket by performing substitution with nothing. Note the use of backslash for special characters like brackets.
5. Then we use join function to join each line and get back the paragraph.
6. Print the modified paragraph.

Perl Code:

my $para = "This is the para (here) line 1. This is (line 2.";
my $i = 0;
my @line = split (/\. /, $para);
$lines = $#line + 1;
while ($i < $lines)
{
 $curline = $line[$i];
 if ($curline =~ m/\(/ xor $curline =~ m/\)/) {$curline =~ s/\(//g; $curline =~ s/\)//g;}
 if ($i > 0) {$para = join (". ", $para, $curline);} else {$para = $curline;}
 $i++;
}
print "$para\n";

Example 4: Perform the above example with these modifications:

Get the paragraph from user instead of having it in code.

If the line contains a closed bracket followed by an open bracket, invert them to make it open followed by closed.  
If the line contains an open bracket followed by a closed bracket, do nothing.

Assume that no line contains more than one set of open and closed brackets.

Logic:

This will be similar to the above code, changes have been added in comments.

Note the trick in the last substitution. 

Since g is omitted, substitution will be done only at the first occurence of ) to ensure that the previous substitution is not overwritten.

Perl Code:

print "Enter the paragraph: ";
my $para = <STDIN>;	#accept input
chomp($para);		#remove newline
my $i = 0;
my @line = split (/\. /, $para);
$lines = $#line + 1;
while ($i < $lines)
{
 $curline = $line[$i];
 if ($curline =~ m/\(/ xor $curline =~ m/\)/) {$curline =~ s/\(//g; $curline =~ s/\)//g;}
 if ($curline =~ m/\(/ and $curline =~ m/\)/) #condition to invert brackets if both present
 {
  $index1 = index($curline,'('); $index2 = index($curline,')');
  if ($index1 > $index2) {$curline =~ s/\(/\)/g; $curline =~ s/\)/\(/;} #no g for second substitution
 }
 if ($i > 0) {$para = join (". ", $para, $curline);} else {$para = $curline;}
 $i++;
}
print "$para\n";