Lab 2: Python and Value/Reference Semantics

A PDF version of this document is located here.

Learning objectives:

Most of the code you write in this course will be in Python or Scheme, both of which have reference semantics. This lab will help you adjust to that. This lab also gives you practice in writing recursive code as well as reading a language specification, both of which will be necessary for projects 2 through 5.

Use the following commands to download and unpack the distribution code:

$ wget
$ tar xzf starter-files.tar.gz
  1. Value and reference semantics. Consider two fragments of code in C++ and Python:

    C++ Python
    class Foo { ... };
    int main() {
      Foo x = Foo();
      Foo y = x;
      Foo &z = y;
      x = Foo();
    class Foo:
    x = Foo()
    y = x
    z = y
    x = Foo()

    For each program, draw a memory diagram that represents the state of the program just before it terminates.

  2. Reference semantics and swap. Alyssa P. Hacker is trying to write a Python function to swap the contents of two lists. Here is what she has tried so far:

    def swap_contents(list1, list2):
        """Modify list1 to contain the contents of list2 and vice versa.
        >>> list1 = [1, 2, 3]
        >>> list2 = ['hello', 'world']
        >>> swap_contents(list1, list2)
        >>> list1
        ['hello', 'world']
        >>> list2
        [1, 2, 3]
        tmp = list1
        list1 = list2
        list2 = tmp
    1. Explain why this code does not swap the contents of the two lists.

    2. Modify the code so that it does swap the contents. Starter code for this problem can be found in To run the doctests on your implementation:

      $ python3 -m doctest
  3. Recursion. Implement the flatten() function in Python. Given a nested list structure, it produces a new flattened list that contains all elements from the original structure. Use isinstance(item, list) to determine if item is a list.

    def flatten(item):
        """Produce a flattened version of the given structure.
        If item is a list, returns a new, flat list containing all
        the items contained within list or any of its elements. If
        item is not a list, returns a list containing item.
        >>> flatten(3)
        >>> items = [1, 2, 3]
        >>> flattened = flatten(items)
        >>> flattened
        [1, 2, 3]
        >>> flattened is items  # verify flattened is a new list
        >>> flatten([[], [1, 2], [[3, [4, 5]], 6], 7])
        [1, 2, 3, 4, 5, 6, 7]
        # your code here

    Starter code for this problem can be found in To run the doctests on your implementation:

    $ python3 -m doctest
  4. Literals and operators. Implement a BitVector type in C++17, representing a growable sequence of booleans. Your BitVector must support the following operations:

    • String literals that end with the _bv suffix construct a BitVector from the string with bits in order from left to right. A 0 character specifies false and a 1 character specifies true. Example:

      "1010"_bv  -->  [ true, false, true, false ]

      In other words, the literal (external representation) "1010"_bv gets converted to the data structure (internal representation) of a BitVector object that contains the elements true, false, true, false.

    • The size() member function returns the size of the BitVector. The return type should be size_t.

    • The push_back() member function takes in a bool and appends it to the end of the BitVector.

    • The [] operator returns the boolean at the given position. You may return by value or reference (i.e. you do not have to support modifying a BitVector using the [] operator).

    • The bitwise operators &, |, and ^, when applied to two BitVectors, should result in a BitVector that consists of the AND, OR, and XOR of the two BitVectors, respectively. If they differ in size, then the operators should treat the smaller as if it were padded on the right with zeros.

    • The << stream insertion operator when applied to a BitVector should insert each boolean as a 0 or 1. Example:

      cout << "1010"_bv;  -->  prints 1010

    Write your code in BitVector.hpp. We have provided a test case in BitVector_test.cpp and expected output in BitVector_test.correct.

    You may find this reference helpful.

    To compile and run tests, use the included Makefile:

    $ make bv