Questions? Feel free to head to CS50 on Reddit, CS50 on StackExchange, the #cs50ap channel on CS50x Slack (after signing up), or the CS50 Facebook group.

Objectives

  • Deepen your understanding of stacks and queues

Academic Honesty

This course’s philosophy on academic honesty is best stated as "be reasonable." The course recognizes that interactions with classmates and others can facilitate mastery of the course’s material. However, there remains a line between enlisting the help of another and submitting the work of another. This policy characterizes both sides of that line.

The essence of all work that you submit to this course must be your own. Collaboration on problems is not permitted (unless explicitly stated otherwise) except to the extent that you may ask classmates and others for help so long as that help does not reduce to another doing your work for you. Generally speaking, when asking for help, you may show your code or writing to others, but you may not view theirs, so long as you and they respect this policy’s other constraints. Collaboration on quizzes and tests is not permitted at all. Collaboration on the final project is permitted to the extent prescribed by its specification.

Below are rules of thumb that (inexhaustively) characterize acts that the course considers reasonable and not reasonable. If in doubt as to whether some act is reasonable, do not commit it until you solicit and receive approval in writing from your instructor. If a violation of this policy is suspected and confirmed, your instructor reserves the right to impose local sanctions on top of any disciplinary outcome that may include an unsatisfactory or failing grade for work submitted or for the course itself.

Reasonable

  • Communicating with classmates about problems in English (or some other spoken language).

  • Discussing the course’s material with others in order to understand it better.

  • Helping a classmate identify a bug in his or her code, such as by viewing, compiling, or running his or her code, even on your own computer.

  • Incorporating snippets of code that you find online or elsewhere into your own code, provided that those snippets are not themselves solutions to assigned problems and that you cite the snippets' origins.

  • Reviewing past years' quizzes, tests, and solutions thereto.

  • Sending or showing code that you’ve written to someone, possibly a classmate, so that he or she might help you identify and fix a bug.

  • Sharing snippets of your own solutions to problems online so that others might help you identify and fix a bug or other issue.

  • Turning to the web or elsewhere for instruction beyond the course’s own, for references, and for solutions to technical difficulties, but not for outright solutions to problems or your own final project.

  • Whiteboarding solutions to problems with others using diagrams or pseudocode but not actual code.

  • Working with (and even paying) a tutor to help you with the course, provided the tutor does not do your work for you.

Not Reasonable

  • Accessing a solution to some problem prior to (re-)submitting your own.

  • Asking a classmate to see his or her solution to a problem before (re-)submitting your own.

  • Decompiling, deobfuscating, or disassembling the staff’s solutions to problems.

  • Failing to cite (as with comments) the origins of code, writing, or techniques that you discover outside of the course’s own lessons and integrate into your own work, even while respecting this policy’s other constraints.

  • Giving or showing to a classmate a solution to a problem when it is he or she, and not you, who is struggling to solve it.

  • Looking at another individual’s work during a quiz or test.

  • Paying or offering to pay an individual for work that you may submit as (part of) your own.

  • Providing or making available solutions to problems to individuals who might take this course in the future.

  • Searching for, soliciting, or viewing a quiz’s questions or answers prior to taking the quiz.

  • Searching for or soliciting outright solutions to problems online or elsewhere.

  • Splitting a problem’s workload with another individual and combining your work (unless explicitly authorized by the problem itself).

  • Submitting (after possibly modifying) the work of another individual beyond allowed snippets.

  • Submitting the same or similar work to this course that you have submitted or will submit to another.

  • Using resources during a quiz beyond those explicitly allowed in the quiz’s instructions.

  • Viewing another’s solution to a problem and basing your own solution on it.

Assessment

Your work on this writing problem will be evaluated along three axes primarily.

Scope

To what extent does your submission align with the requirements of the specification?

Correctness

To what extent is your submission correct and free of factual errors?

Style

To what extent is your submission readable (i.e., thoughtfully organized, coherent, words properly spelled)?

To obtain a passing grade in this course, all students must ordinarily submit all assigned problems unless granted an exception in writing by the instructor.

Getting Started

No need to open up the IDE for this problem, just get out some good old-fashioned paper and pencil[1].

Warm-Ups

  1. Queues are known as a FIFO structure while stacks are a LIFO structure. Describe what is meant by LIFO and FIFO.

  2. When implementing a stack, one must keep track of the top of the stack, but not the bottom. Describe why this is different than the implementation of a queue, where one must keep track of both the head and tail.

  3. Describe what must be done during pop() and push() when implementing a stack with an array.

  4. Describe what must be done during enqueue() and dequeue() when implementing a queue with an array.

  5. Describe a real-world problem where one would want to implement or make use of a stack.

  6. Describe a real-world problem where one would want to implement or make use of a queue.

Stack Attack

Alright! Now on to some tougher ones.

Consider the (global) declaration and initialization of a stack for integers, below, wherein CAPACITY is some constant.

struct
{
    int data[CAPACITY];
    int size;
}
stack;


stack.size = 0;

Note the absence of typedef. With the above, we’re simply declaring one global struct called stack, whose size is initially 0.

  1. In pseudocode, complete the implementation of push, below, in such a way that the function pushes datum on top of stack if not already at the defined maximum capacity. Upon success, the function should return true. Upon failure (e.g., stack is at capacity), the function should return false. A function declaration for push appears below.

    bool push(int datum);

  2. In pseudocode, complete the implementation of pop, below, in such a way that the function pops off the topmost integer from stack. Upon success, the function should remove and return the integer on the top of stack. (To remove the int, it suffices to "forget" it; you needn’t overwrite its bits.) Upon failure (e.g., the stack is empty), the function should return INT_MAX, a constant value[2]. A function declaration for pop appears below.

    int pop(void);

Queue Guide

Suppose that a queue for (non-negative) integers is defined per the below, wherein CAPACITY (a global constant) is the maximum number of integers that can be in the queue, size is the number of integers currently in the queue, and front is the index of the front of (i.e., first integer in) the queue.

typedef struct
{
    int numbers[CAPACITY];
    int front;
    int size;
}
queue;

Assume that a queue has been declared globally with

queue q;

and that q has been initialized for you (e.g., in the main function of this program) per the below.

q.front = 0;
q.size = 0;

  1. In pseudocode, complete the implementation of enqueue below in such a way that the function adds datum to (the end of) q and then returns true. If q is full or datum is negative, the function should instead return false. A function declaration for enqueue appears below.

    bool enqueue(int datum);

  2. In pseudocode, complete the implementation of dequeue, below, in such a way that the function dequeues the integer at the front of q. Upon success, the function should return the integer at the front of q. Upon failure (e.g., the queue is empty), the function should return INT_MAX. A function declaration for dequeue appears below.

    int dequeue(void);

Extra Credit?

If feeling confident with this material, try re-writing your answers to questions 6, 7, 8, and 9, but this time in C instead of pseudocode! To be clear, this isn’t required for this problem, but is only intended to give you the chance to flex some programming muscles, if you’re so inclined!

This was Writing Problem 5-0.

1. Okay, or open up your IDE and answer these questions in a file called, say, questions.txt.
2. This, of course, requires us to assume that INT_MAX would not otherwise appear in stack, but that’s a risk we’re willing to take!