Stack Lab

Learning Goals

Implement a Stack using a list as the underlying data structure.

Key Vocab

Sequence: a data structure in which data is stored and accessed in a specific order.
Stack is a linear data structure that follows the principle of Last In First Out (LIFO)
Index: the location, represented by an integer, of an element in a sequence.
Iterable: able to be broken down into smaller parts of equal size that can be processed in turn. You can loop through any iterable object.
Slice: a group of neighboring elements in a sequence.
List: a mutable data type in Python that can store many types of data. The most common data structure in Python.
Tuple: an immutable data type in Python that can store many types of data.
Range: a data type in Python that stores integers in a fixed pattern.
String: an immutable data type in Python that stores unicode characters in a fixed pattern. Iterable and indexed, just like other sequences.

Instructions

In the previous lesson, you learned what a Stack is and what methods they commonly include. In this lab, you will be building out an implementation of a Stack. You will be using a list as the underlying data structure, and calling on some built-in Python list methods to build your Stack class's functionality.

Start by forking and cloning this lab. You'll be writing your code in the lib/stack.py file. You can run the tests at any point using pytest -x to check your work.

Bonus

If you'd like an extra challenge, try implementing the additional functionality below. There are tests for these in the testing/stack_test.py file; uncomment the bonus methods section in the test file to try these out.

Modify your Stack __init__() method to take an optional limit value and set that as an attribute.
Update the Stack push() value to only push the passed-in value if there's still room in the Stack. If the Stack is full, the method should throw an error.
Implement the following additional methods:

Stack size(): returns the number of elements contained in the Stack
Stack empty(): returns true if the Stack is empty; false otherwise
Stack full(): returns true if the Stack is full; false otherwise
Stack search(value): returns the distance between the top of the stack and the target element if it's present; -1 otherwise

After you've made these changes, you might want to take another look through your code and see if there's any refactoring you can do.

Conclusion

In this lesson, we got some practice building a data structure from scratch by implementing a Stack class. Recall that the runtime of our data structure will depend on what data structure it uses under the hood. For this lab, we used an list as the underlying data structure, which means the runtime for the search() method is O(n), and the runtime for all of the other methods is O(1).

While our implementation is efficient in terms of time complexity, we have to consider space complexity as well. One of the characteristics of an list is that each of the elements can be accessed directly using the [] operator. In order for this to work, Python stores all the elements that need to be stored in a continuous block of memory. If we're trying to add an element and we're out of memory where the list is located, Python will resize and relocate the continuous block to a bigger continuous block of memory. This is expensive in terms of memory, which means using a list as our underlying data structure is not optimal from the perspective of space complexity.

Given that a Stack only uses push and pop methods, we don't need to use an underlying data structure that allows direct access to all of the elements. A better choice is a LinkedList, because it uses a Dictionary as its underlying data structure and Dictionaries do not need to be stored in a continuous block of memory. The LinkedList is the next data structure we'll learn about. Before we get to that, however, let's get a little practice using Stacks.

Resources

Wikipedia: Stack (abstract data type)

ddeleon267 / python-p3-dsa-stack-lab