Perl | Implementing a Stack

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:

  #!/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:

  #!/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:

  #!/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:

  #!/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";
  Output:

  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:

  #!/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:

  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");

  Runtime Error:

  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.