Perl | Implementing a Stack
Last Updated :
21 Feb, 2019
Stack in
Perl is a linear data structure that follows the
LIFO (Last In First Out) or
FILO (First In Last Out) order.
In simpler terms, a stack is an array in which insertion and deletion takes place at only one end called the top of the stack.
Pushing is the process of insertion of elements into a stack.
Popping is the process of removal of topmost element of a stack.
Making a stack
Creating a stack in Perl is rather simple. All we need to do is declare an array.
The stack could be empty, as follows:
@stack;
Or it could be initialized:
@stack = (1, 2, 3);
Pushing items to a stack
Pushing can be done using either the
push() function or the
splice() function.
-
Pushing using push():
Syntax: push(@stack, list);
Parameters:
- @stack - The stack on which push is to be performed.
- list - The elements to be pushed into a stack. These elements might be scalar, array, hash or any combination of these.
Example:
Perl
#!/usr/bin/perl
# Intitialising the Stack
@stack = (1..3);
# Original stack
print "Original Stack: @stack";
# Scalar to be pushed
$scalar = "scalar";
# Array to be pushed
@array = ("a", "r", "r", "a", "y");
# Hash to be pushed
%hash = ("Geeks" => 10,
"for Geeks" => 20);
# scalars, arrays and hashes can be
# inserted at the same time
push(@stack, ($scalar, @array, %hash));
# Updated Stack after
# Push operations
print("\nUpdated Stack: @stack");
Output:
Original Stack: 1 2 3
Updated Stack: 1 2 3 scalar a r r a y Geeks 10 for Geeks 20
-
Pushing using splice():
Syntax: splice(@stack, scalar(@stack), 0, list);
Parameters:
- splice() function appends the 'list' at the end of @stack.
- THe 'list' could be a scalar, an array or a hash.
Example:
perl
#!/usr/bin/perl
# Intitialising the Stack
@stack = (1..3);
# Original stack
print "Original Stack: @stack";
# Scalar to be pushed
$scalar = "scalar";
# Array to be pushed
@array = ("a", "r", "r", "a", "y");
# Hash to be pushed
%hash = ("Geeks" => 10,
"for Geeks" => 20);
# scalars, arrays and hashes can be
# inserted at the same time
splice(@stack, scalar(@stack), 0,
($scalar, @array, %hash));
# Updated Stack after
# Push operations
print("\nUpdated Stack: @stack");
Output:
Original Stack: 1 2 3
Updated Stack: 1 2 3 scalar a r r a y Geeks 10 for Geeks 20
Popping elements from a Stack
Popping can be done using either the pop() function or the splice() function.
-
Popping using pop():
Syntax: $popped_element = pop(@stack);
Parameters:
- pop() function returns the popped element.
- $popped_element contains the element popped from the stack.
Example:
perl
#!/usr/bin/perl
# Intitialising the Stack
@stack = (1..3);
# Original stack
print "Original Stack: @stack";
# Topmost element i.e. 3 is
# removed and returned
$popped_element = pop(@stack);
# Printing popped element
print "\nPopped element: $popped_element";
# Updated Stack after
# Pop operation
print("\nUpdated Stack: @stack");
Output:
Original Stack: 1 2 3
Popped element: 3
Updated Stack: 1 2
- If the stack is empty, undef is returned. undef is analogous to NULL in Java and None in Python. However, no error is raised.
Example:
perl
#!/usr/bin/perl
# Creating a Stack
@stack;
# undef is returned since the
# stack is empty.
# No error is raised.
$popped_element = pop(@stack);
# Printing popped element
# Since it contains no value,
# hence a blank space is returned
print "Popped element: $popped_element";
-
Popping using splice()::
Syntax: $popped_element=splice(@stack, -1);
Parameters:
- splice() function removes the last element of the stack and returns it.
- $popped_element stores the returned value.
Example:
perl
#!/usr/bin/perl
# Intitialising the Stack
@stack = (1..3);
# Original stack
print "Original Stack: @stack";
# popping using splice()
$popped_element = splice(@stack, -1);
# Printing popped element
print "\nPopped element: $popped_element";
# Updated Stack after
# Pop operation
print("\nUpdated Stack: @stack");
Output:
Original Stack: 1 2 3
Popped element: 3
Updated Stack: 1 2
- An error is raised, if the stack is empty. The following code raises an error:
perl
use warnings;
#!/usr/bin/perl
use warnings;
# Creating a Stack
@stack;
# popping using splice()
# An error is raised here
$popped_element = splice(@stack, -1);
# Printing popped element
print "\nPopped element: $popped_element";
# Updated Stack after
# Pop operation
print("\nStack: @stack");
Useless use of a variable in void context at /home/59c7c19979aa9e46564cd145d5fe5601.pl line 6.
Modification of non-creatable array value attempted, subscript -1 at /home/59c7c19979aa9e46564cd145d5fe5601.pl line 10.