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:

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    #!/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");

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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:
    

    

    

    

    

    
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    #!/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"); 
    

    
    		

    
    

    

    

    

    

    

    
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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:
    

    

    

    

    

    
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    #!/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"); 
    

    
    		

    
    

    

    

    

    

    

    
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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:
      

      

      

      

      

      
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      #!/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"; 
      

      
      		

      
      

      

      

      

      

      

      
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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:
    

    

    

    

    

    
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    #!/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"); 
    

    
    		

    
    

    

    

    

    

    

    
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Output:
    

    Original Stack: 1 2 3
Popped element: 3
Updated Stack: 1 2