This notebook was prepared by Donne Martin. Source and license info is on GitHub.

Challenge Notebook

Problem: Implement n stacks using a single array.

Constraints

  • Are the stacks and array a fixed size?
    • Yes
  • Are the stacks equally sized?
    • Yes
  • Does pushing to a full stack result in an exception?
    • Yes
  • Does popping from an empty stack result in an exception?
    • Yes
  • Can we assume the user passed in stack index is valid?
    • Yes
  • Can we assume this fits memory?
    • Yes

Test Cases

  • Test the following on the three stacks:
    • Push to full stack -> Exception
    • Push to non-full stack
    • Pop on empty stack -> Exception
    • Pop on non-empty stack

Algorithm

Refer to the Solution Notebook. If you are stuck and need a hint, the solution notebook's algorithm discussion might be a good place to start.

Code

In [ ]:
class Stacks(object):

    def __init__(self, num_stacks, stack_size):
        # TODO: Implement me
        pass

    def abs_index(self, stack_index):
        # TODO: Implement me
        pass

    def push(self, stack_index, data):
        # TODO: Implement me
        pass

    def pop(self, stack_index):
        # TODO: Implement me
        pass

Unit Test

The following unit test is expected to fail until you solve the challenge.

In [ ]:
# %load test_n_stacks.py
import unittest


class TestStacks(unittest.TestCase):

    def test_pop_on_empty(self, num_stacks, stack_size):
        print('Test: Pop on empty stack')
        stacks = Stacks(num_stacks, stack_size)
        stacks.pop(0)

    def test_push_on_full(self, num_stacks, stack_size):
        print('Test: Push to full stack')
        stacks = Stacks(num_stacks, stack_size)
        for i in range(0, stack_size):
            stacks.push(2, i)
        stacks.push(2, stack_size)

    def test_stacks(self, num_stacks, stack_size):
        print('Test: Push to non-full stack')
        stacks = Stacks(num_stacks, stack_size)
        stacks.push(0, 1)
        stacks.push(0, 2)
        stacks.push(1, 3)
        stacks.push(2, 4)

        print('Test: Pop on non-empty stack')
        self.assertEqual(stacks.pop(0), 2)
        self.assertEqual(stacks.pop(0), 1)
        self.assertEqual(stacks.pop(1), 3)
        self.assertEqual(stacks.pop(2), 4)

        print('Success: test_stacks\n')


def main():
    num_stacks = 3
    stack_size = 100
    test = TestStacks()
    test.assertRaises(Exception, test.test_pop_on_empty, num_stacks,
                      stack_size)
    test.assertRaises(Exception, test.test_push_on_full, num_stacks,
                      stack_size)
    test.test_stacks(num_stacks, stack_size)


if __name__ == '__main__':
    main()

Solution Notebook

Review the Solution Notebook for a discussion on algorithms and code solutions.