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Answer: How do I make a stack?

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Q&A > Data Structures > How do I make a stack? contributed by GrandFather

Perl arrays are a really simple way of implementing stacks and related data structures. The key is to realise that you can access the ends of the array using [0] for the left/bottom/start of the array and [-1] for the right/top/end of the array.

With that in mind you can either use unshift/shift to add/remove items at the start, or push/pop to add/remove things at the end of the array and implement a stack. The following pushes an item onto a stack, accesses it (at the top of the stack), then pops it off again:

my @stack; push @stack, "An item to push on the stack"; print "The top stack item is: $stack[-1]\n"; pop @stack;

Using unshift/shift/[0] gives stack semantics as does push/pop/[-1].

Using push/shift/[0] or unshift/pop/[-1] gives queue semantics.

push/pop/shift/unshift will return undef if the stack/queue was empty. If you may be intentionally placing undef values in the stack, you can still check if it is empty: scalar(@stack) returns the number of elements on the stack.

By combining these and adding splice to the mix you can do some really nifty stuff that combine stack, list and queue sematics in one data structure.

For info on splice see How do I insert an element into an arbitrary (in range) array index? and How do I completely remove an element from an array?.

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Re: Answer: How do I make a stack?
by TedPride (Priest) on Jun 17, 2005 at 07:52 UTC
    Is it significantly more efficient to say $stack[-1] than $stack[$#stack]? I know the former is more concise, but it isn't as immediately intuitive. I wish you could do something like $stack[+] instead to denote top.
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