The Formal Definition of Stacks and Queues A stack is LIFO FIFO and LIFO accounting are methods used in managing inventory and financial matters involving the amount of money a company has tied up within inventory of produced goods, raw materials, parts, components, or feed stocks. They are used to manage assumptions of cost flows related to inventory, stock repurchases (if purchased at different prices), and various other accounting purposes.FIFO and LIFO accounting
Can a stack exhibit FIFO or LIFO behaviour?
But are there cases where a stack can exhibit FIFO behaviour, or a queue exhibit LIFO behaviour? The answer is yes: For stacks, this occurs when the sequence of items to be added and then removed is only of size one.
What is a stack?
Let’s get started! Clone the accompanying repository for this article here. A Stack is a structure that is responsible for gathering data dynamically following the LIFO principle (last in, first out).
Is a queue FIFO or not?
A queue is FIFO. Those definitions have always bothered me, and I think I've finally articulated why. To explain myself, I'll start with a typical model of stacks and queues that are presented in CS classes:
What are the advantages of using stacks?
Because the order of a stack is fixed, reverse order can be achieved very easily by popping elements off one stack and immediately onto another. No messing around with swapping indexes! Another good use case for stacks is testing symmetry.
What are the acronyms for stacks?
The following acronyms from the above list correctly describe the behaviour of stacks: LIFO, FOLI, FILO, LOFI.
Why is ordering better than stacks?
It is also better because it focuses on the operation being performed on the data, which is mutually exclusive for stacks vs. queues, instead of the properties of the output which are not mutually exclusive when n = 1. Furthermore, the inclusion of the word 'ordering' forces the reader to visualize a sequence of items when attempting to understanding the behaviour of a base case instead of potentially considering the edge case with one item where both properties apply.
Is SSOU the same as queue?
The pronunciation of ' SSOU ' is the same as the name 'Sue' is ( it rhymes with queue). The pronunciation of ' STOR ' sounds exactly like the pronunciation of the word 'store'. The spoken language to be used when interpreting these pronunciations is Canadian English as spoken in the province of Ontario in the year 2017. All other pronunciations are strictly prohibited.
Can a stack exhibit FIFO?
But are there cases where a stack can exhibit FIFO behaviour, or a queue exhibit LIFO behaviour? The answer is yes : For stacks, this occurs when the sequence of items to be added and then removed is only of size one. For queues, the case is quite similar, but with an added restriction that the queue must be empty before the insertion (although this depends on your definition of 'first' discussed later). In both cases, we can say this happens when the first item is equal to the last item:
What is stack in data?
Stack A stack is a linear data structure in which elements can be inserted and deleted only from one side of the list, called the top. A stack follows the LIFO (Last In First Out) principle, i.e., the element inserted at the last is the first element to come out.
What is a queue in a list?
Queue: A queue is a linear data structure in which elements can be inserted only from one side of the list called rear, and the elements can be deleted only from the other side called the front.
What is stack data?
A Stack is a structure that is responsible for gathering data dynamically following the LIFO principle (last in, first out). As an analogy, you could imagine a stack of cafeteria trays: When you want to add a new tray, it gets introduced to the top of the stack (instead of being inserted somewhere within).
How to reverse order a stack?
Because the order of a stack is fixed, reverse order can be achieved very easily by popping elements off one stack and immediately onto another . No messing around with swapping indexes!
How does LIFO differ from array?
If we compare this structure to a typical array, some distinct differences become clear. While memory allocation for arrays is taken care of in an intelligent fashion by Swift, in foundational languages array capacity is set during initialization and is static. Stacks on the other hand, allocate and deallocate memory for individual nodes as required, making them very flexible. Additionally, arrays allow access to elements at any index while stacks only interact with the end node. While this might seem limiting (it is…) it actually makes data access very lightweight and fast. Additionally, LIFO behaviour can be very useful for addressing specific situations, which we will look at later in this article.
Why is the last tray at the top of the stack?
Because the last tray is at the top of the stack, it will also be the first to come off when an individual tray is required. Stacks provide two primary functions: ‘pushing’ or introducing a new element, and ‘popping’ or removing the last element.
How to push an element to a stack?
Next let’s take a look at how to push a new element onto our stack. Step 1: Create a new node with the value that we want. Step 2: Provided endNode is pointing to a node, we can assign the next property in our end node to the new node, and the previous property in our new node back to our end node. Remember that the previous property is weak so that we avoid a retain cycle. Step 3: Finally, we can move our endNode pointer to our newNode by accessing its next property. Notice that the memory required for our list grows with the allocation and initialization of the newNode.
The Formal Definition of Stacks and Queues
The First/Last of What exactly?
- The terms 'FIFO' and 'LIFO' make reference to a 'first one' and a 'last one'. But an important question is 'first/last one among what set of elements?' The only reasonable choices are the following: 1. 1) The first/last one among the set of items that you are about to add to the data structure. 2. 2) The first/last one among the set of items that you are about to add to the data str…
FIFO, Fofi, LIFO, Foli, Filo and Lofi
- The interchangeability of 'First' vs. 'Last' and 'Input' vs. 'Output' also opens the door for us to question whether there are also other 'easy to remember' acronyms that can come out of the following regex: For example, assigning F=0, L=1, I=0, O=1: As a summary of the above table: 1. The following acronyms from the above list correctly describe the behaviour of queues: FIFO, FO…
An Alternative Definition
- I hereby decree, with universal authority, that the terms 'FIFO' and 'LIFO' shall no longer be used. It is therefore mandated that the following alternative terms be implemented in their place effective immediately: 1. SSOU - Subsequence Order Unchanging 2. STOR - Substring Order Reversing Furthermore, 1. Queues are SSOU. 2. Stacks are STOR. Queues...
Substring vs. Subsequence
- Please note: If you mix up the definition of substring with subsequenceyou're going to have a bad time. Pay special attention to the difference between these words in this article.
Is This Definition Actually Better?
- I claim that it is, primarily because it is less ambiguous about what operations are performed on your data. This is contrasted with the concept of 'first' and 'last' in the conventional definition because of the question mentioned earlier: "First or last among the items being inserted, or all items?" It is also better because it focuses on the operation being performed on the data, which i…
Conclusion
- As we've seen above, there are a number of confusing aspects to the terms 'FIFO' and 'LIFO'. These confusing aspects stem from the lack of a formal definition for queues and stacks, but also from the ease with which you can create variations of the input/output, first/last acronyms. Therefore, if you've recently commissioned a textbook printing to use the phrases 'FIFO' or 'LIFO', …