Computer ScienceChoosing Arrays, Stacks, or Queues for the Job
Byte stands at a busy dispatch center filled with towering shelves of labeled bins, a spring-loaded stack of cafeteria trays, and a velvet rope queue of waiting customers, pointing at each in turn with a stylus and a knowing grin.
- Explain how an indexed array enables direct access to any element by position.
- Identify the last-in, first-out (LIFO) behavior of a stack and name a real scenario where it applies.
- Identify the first-in, first-out (FIFO) behavior of a queue and name a real scenario where it applies.
- Compare arrays, stacks, and queues by matching each to an access pattern it supports efficiently.
- Predict which data structure is most appropriate given a described problem's access requirements.
Key terms
- Abstract data type
- A model defined by allowed operations rather than its underlying memory layout.
- Stack
- A LIFO collection where the last item pushed is the first popped.
- Queue
- A FIFO collection where the first item enqueued is the first dequeued.
- LIFO
- Last-In, First-Out ordering used by stacks for push and pop.
- FIFO
- First-In, First-Out ordering used by queues for enqueue and dequeue.
Access Pattern Defines the Structure
An abstract data type is defined by the operations it permits, not by how memory is arranged underneath, so two structures storing the same sequence can behave completely differently. A stack permits only push and pop at the top, enforcing last-in, first-out order; a queue permits enqueue at the back and dequeue at the front, enforcing first-in, first-out order; an array permits direct indexed access to any position. Choosing correctly means matching the structure's allowed access pattern to what the problem actually requires, because using the wrong order silently produces wrong behavior.
Where Each Structure Shines
Arrays excel when you know the position you need, like reading pixel [row][col] in an image or jumping to a leaderboard rank in O(1). Stacks model anything that must reverse the most recent action first: browser back-history, undo features, and matching nested brackets in code all push and pop naturally. Queues model fairness and arrival order: printer jobs, support tickets, and task scheduling all process the oldest waiting item first. Recognizing keywords like 'undo' or 'reverse' versus 'serve in order' or 'schedule' quickly points you to the right structure.
Worked examples
Trace a stack used for an undo feature after the actions type-A, type-B, type-C are performed.
- Push type-A onto the stack; stack from bottom to top is [A].
- Push type-B, then type-C; stack is now [A, B, C] with C on top.
- User presses Undo: pop the top, removing C first because it was last in.
- User presses Undo again: pop B next, confirming last-in, first-out order.
Answer: Undo reverses C then B then A, exactly the LIFO order a stack provides.
Activity
Drag each scenario card into the bin labeled with the best data structure for that access pattern.
Practice
Which structure best handles printer jobs that must print in submission order?
Explain which structure fits checking that every opening bracket has a matching close.
Common mistakes to avoid
- Stacks and queues are interchangeableThey remove items in opposite orders, so swapping them breaks features that depend on LIFO or FIFO.
- Any sequence structure suffices for undoUndo needs LIFO removal; a queue would undo the oldest action first, which is incorrect.
Check your understanding
A text editor's Undo feature needs to reverse the most recent action first, then the one before it, and so on. Which data structure models this access pattern correctly?
A student argues that a queue could replace a stack for an undo feature because 'both store a sequence of items.' What is wrong with this reasoning?
A game stores the high-score table so that the program can instantly retrieve the score ranked at any position (1st, 2nd, 10th, etc.) without scanning from the beginning. Which structure fits best?
Recap
Arrays, stacks, and queues are abstract data types distinguished by access pattern: arrays give indexed O(1) access, stacks enforce LIFO for undo and backtracking, and queues enforce FIFO for fair, in-order processing. Match the structure to the required order.
Reflect
Think of an app feature you used today; was it powered by a stack or a queue?