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CSC110 Week 1: Representing Data

Exercise 1: Data types in Python

  1. In the table below, write down the Python type for each Python literal. The first row is done for you.

    Data Type Python Literal
    int 12345
    12345.0
    True
    'You are doing great :)'
    {'David', 'Tom'}
    [1, 2, 3, 4, 5]
    {'cool': ['David', 'Tom', 'You']}

  2. What are two differences between the set and list data type in Python?

  3. Without using the Python console, complete the table below.

    Python Expression Value Type of Value
    9 // 3
    9 / 3
    5 + 5.0
    2 == 2.0
    3 in {1, 2}
    'x' + 'y'
    [1, 2] + [2, 3]
    ['hi', 'bye'][0]
    {'hi': 'bye'}['hi']

    When you are finished, check your work using the Python console. If you have extra time, play around with more expressions in the console to see if they return what you would expect. Some fun ones to try: 0 == False and 1 == True. :)

  4. Imagine that you are the owner of a neighbourhood grocery store that sells many kinds of food. Think about different kinds of data you would have to keep track of. For each data type below, write a description of a piece of data that would have that type. We have completed the first row for you.

    Data Type Example data of that type
    Boolean “Is my store wheelchair accessible?”
    Real Number
    String
    Set
    List
    Mapping

Exercise 2: Variables

  1. Suppose we execute this code in the Python console:

    >>> x = 4
>>> y = x + 2
>>> z = {y: x}

    Complete the value-based memory model table below to show what x, y, and z refer to.

    Variable Value
    x
    y
    z

  2. Repeat the above exercise, now with the following code:

    >>> a = 'cat'
>>> b = [a[0], a[1], a[2]]
>>> c = ('c' in a)
>>> d = ('cat' in b)
    Variable Value
    a
    b
    c
    d

  3. When things go wrong. As part of reviewing this lecture, your friend Bob opens the Python console and tries to assign a variable x to the integer value 5. This is what he types and then sees:

    >>> 5 = x
Traceback (most recent call last):
... [some output omitted] ...
File "<input>", line 1
    5 = x
    ^
SyntaxError: cannot assign to literal here. Maybe you meant '==' instead of '='?

    Read what your friend typed and the error message, and then write down: (1) a brief explanation of the problem, and (2) how to correct the problem.

Exercise 3: Practice with comprehension expressions

Here is a summary of the three types of comprehensions, for your reference:

Comprehension type Syntax
set comprehension { <expression> for <variable> in <collection> }
list comprehension [ <expression> for <variable> in <collection> ]
dict comprehension { <key_expr>: <value_expr> for <variable> in <collection> }

  1. Suppose we assign the variable numbers = [1, 2, 3].

    1. Fill in the table below.

      Expression Value
      numbers[0]
      numbers[1]
      numbers[2]
      numbers[0] ** 3
      numbers[1] ** 3
      numbers[2] ** 3

    2. Write a comprehension that evaluates to the list of every integer in numbers cubed (i.e., raised to the power of 3).

      
>>>


[1, 8, 27]   # Evaluating your expression should produce this

    3. Write a comprehension that evaluates to a dictionary mapping every integer in numbers to three times that integer.

      
>>>


{1: 3, 2: 6, 3: 9}   # Evaluating your expression should produce this

      Hint: the identity dictionary comprehension has the following form:

      >>> {x : x for x in numbers}

    4. Write a comprehension that evaluates to the given output dictionary shown.

      
>>>



{3: 1, 6: 2, 9: 3}   # Evaluating your expression should produce this

    5. Write a comprehension that is a translation of the set builder expression \(\{\frac{x}{x + 1} \mid x \in \texttt{numbers} \}\).

Additional exercises

  1. Order of operations. When combining multiple operations in the same expression (e.g., 3 + 5 * 2), we recommend using parentheses to make it clear what order the operations are evaluated in. However, you might see code where the programmer didn’t include parentheses, and you’ll need to figure out what the code is doing yourself.

    Python follows the same order of arithmetic operations that you’re used to from mathematics (sometimes referred to as “BEDMAS” or “PEMDAS”), with the modulo % operator at the same precedence as multiplication/division.

    In the table below, add parentheses to indicate the order that operations are evaluated, and then write the value of the expression. Make sure to check your work using the Python console!

    Python Expression Expression with parentheses Value
    3 + 5 * 2
    3 / 5 * 2
    3 * 5 / 2
    3 + 5 % 2
    4 + 8 / 2 ** 2 / -2

  2. Terminology recap. For each piece of code, circle all terms that can be used to describe it.

    1. 4 + 5

      1. literal
      2. expression
      3. statement

    2. 'Hi ' + 'there'

      1. literal
      2. expression
      3. statement

    3. 'Hi'

      1. literal
      2. expression
      3. statement

    4. 'fries' in {'fries': 5.99, 'steak': 25.99, 'soup': 8.99}

      1. literal
      2. expression
      3. statement

    5. distance1 = ((1 - 2) ** 2 + (3 - 5) ** 2) ** 0.5

      1. literal
      2. expression
      3. statement

    6. {1, 2, 3} == {3, 1, 2}

      1. literal
      2. expression
      3. statement

  3. Error message follow-up (1). Recall Exercise 2, Question 3 from above. Your friend Bob entered an invalid piece of code into the Python console, and saw the following:

    >>> 5 = x
Traceback (most recent call last):
  ... [some output omitted] ...
  File "<input>", line 1
    5 = x
    ^
SyntaxError: cannot assign to literal here. Maybe you meant '==' instead of '='?

    Rather than listen to your great explanation, he then read the error message and tried typing in something different, but then saw a different error message. 😔

    >>> 5 == x
Traceback (most recent call last):
  ... [some output omitted] ...
  File "<input>", line 1, in <module>
NameError: name 'x' is not defined

    Explain this new error.

  4. Error message follow-up (2). Below, we have written several assignment statements. For each one, determine whether it is valid Python code. If it is invalid, explain what the problem is and how to fix it (if it makes sense to do so).

    Try doing this without the Python console first, and then check your work in the Python console.

    1. 5 = x

    2. x = 5.0

    3. x = hello!

    4. x = true

    5. x = {}

    6. x = [1, 'hi', 3]

    7. x = {1: 'two', 'three'}