Quick Logistics Review

Using Python

When running a Python file, you can use flags on the command line to inspect your code further. Here are a few that will come in handy. If you want to learn more about other Python flags, take a look at the documentation.

  • Using no flags will run the code in the file you provide and return you to the command line.

    python3 lab01.py
  • -i: The -i option runs your Python script, then opens an interactive session. To exit, type exit() into the interpreter prompt. You can also use the keyboard shortcut Ctrl-D on Linux/Mac machines or Ctrl-Z Enter on Windows.

    If you edit the Python file while running it interactively, you will need to exit and restart the interpreter in order for those changes to take effect.

    python3 -i lab01.py
  • -m doctest: Runs doctests in a particular file. Doctests are marked by triple quotes (""") and are usually located within functions.

    python3 -m doctest lab01.py

Using OK

In CS 88, we use a program called Ok for autograding labs, homeworks, and projects. You should have Ok in the starter files downloaded at the start of this lab. For more information on using Ok commands, learn more here. To use Ok to run doctests for a specified function, run the following command:

python3 ok -q <specified function>

By default, only tests that did not pass will show up. You can use the -v option to show all tests, including tests you have passed:

python3 ok -v

You can also use the debug printing feature in OK by writing

print("DEBUG:", x)

Finally, when you have finished all the questions in lab01.py, you must submit the assignment using the --submit option:

python3 ok --submit

For more Ok commands, visit here.

Topics

Consult this section if you need a refresher on the material for this lab. It's okay to skip directly to the questions and refer back here should you get stuck.

Division

Let's compare the different division-related operators in Python:

True Division
(decimal division)
The / Operator
Floor Division
(integer division)
The // Operator
Modulo
(similar to a remainder)
The % Operator
>>> 1 / 5
0.2

>>> 25 / 4
6.25

>>> 5 / 0
ZeroDivisionError
>>> 1 // 5
0

>>> 25 // 4
6

>>> 5 // 0
ZeroDivisionError
>>> 1 % 5
1

>>> 25 % 4
1

>>> 5 % 0
ZeroDivisionError

Note that floor division and modulo both return an integer while true division always returns a floating point number.

One useful technique involving the % operator is to check whether a number x is divisible by another number y:

x % y == 0

For example, in order to check if x is an even number:

x % 2 == 0

Functions

If we want to execute a series of statements over and over, we can abstract them away into a function to avoid repeating code.

For example, let's say we want to know the results of multiplying the numbers 1-3 by 3 and then adding 2 to it. Here's one way to do it:

>>> 1 * 3 + 2
5
>>> 2 * 3 + 2
8
>>> 3 * 3 + 2
11

If we wanted to do this with a larger set of numbers, that'd be a lot of repeated code! Let's write a function to capture this operation given any input number.

def foo(x):
    return x * 3 + 2

This function, called foo, takes in a single argument and will return the result of multiplying that argument by 3 and adding 2.

Now we can call this function whenever we want this operation to be done:

>>> foo(1)
5
>>> foo(2)
8
>>> foo(1000)
3002

Applying a function to some arguments is done with a call expression.

Call expressions

A call expression applies a function, which may or may not accept arguments. The call expression evaluates to the function's return value. It has a familiar notation borrowed from Math.

The syntax of a function call:

  add   (    2   ,    3   )
   |         |        |
operator  operand  operand

Every call expression requires a set of parentheses delimiting its comma-separated operands.

To evaluate a function call:

  1. Evaluate the operator
  2. Evaluate the operands (from left to right).
  3. Apply the operator to the operands (the values of the operands).

If an operand is a nested call expression, then these two steps are applied to that operand in order to evaluate it.

return and print

Most functions that you define will contain a return statement. The return statement will give the result of some computation back to the caller of the function and exit the function. For example, the function square below takes in a number x and returns its square.

def square(x):
    """
    >>> square(4)
    16
    """
    return x * x

When Python executes a return statement, the function terminates immediately. If Python reaches the end of the function body without executing a return statement, it will automatically return None.

In contrast, the print function is used to display values in the Terminal. This can lead to some confusion between print and return because calling a function in the Python interpreter will print out the function's return value.

However, unlike a return statement, when Python evaluates a print expression, the function does not terminate immediately.

def what_prints():
    print('Hello World!')
    return 'Exiting this function.'
    print('CS 88 is awesome!')

>>> what_prints()
Hello World!
'Exiting this function.'

Notice also that print will display text without the quotes, but return will preserve the quotes.


Control

Boolean Operators

You briefly encountered Boolean operators last week, but here you will see there is a close connection between them and conditionals.

Python supports three boolean operators: and, or, and not:

>>> a = 4
>>> a < 2 and a > 0
False
>>> a < 2 or a > 0
True
>>> not (a > 0)
False
  • and evaluates to True only if both operands evaluate to True. If at least one operand is False, then and evaluates to False.
  • or evaluates to True if at least one operand evaluates to True. If all operands are False, then or evaluates to False.

What do you think the following expression evaluates to? Try it out in the Python interpreter.

>>> True and not False or not True and False

It is difficult to read complex expressions, like the one above, and understand how a program will behave. Using parentheses can make your code easier to understand. Just so you know, Python interprets that expression in the following way:

>>> (True and (not False)) or ((not True) and False)

This is because boolean operators, like arithmetic operators, have an order of operation:

  • not has the highest priority
  • and
  • or has the lowest priority

To make your code more readable, and and or work on more than just booleans (True, False). Other Python values can be considered "false-y," including 0, None, and '' (the empty string). All other values are considered "truth-y."

Short Circuiting

What do you think will happen if we type the following into Python?

1 / 0

Try it out in Python! You should see a ZeroDivisionError. But what about this expression?

True or 1 / 0

It evaluates to True because Python's and and or operators short-circuit. That is, they don't necessarily evaluate every operand.

Operator Evaluates from left to right until: Example
AND The first "false-y" value False and 1 / 0 evaluates to False
OR The first "truth-y" value True or 1 / 0 evaluates to True

If and and or do not short-circuit, they just return the last value. This means that and and or don't always return booleans when using truth-y and false-y values.

Here's a little example:

def divides(x, y):
    """
    return True if x divides y
    """
    return x and y % x == 0

If Statements

You can review the syntax of if statements in Section 1.5.4 of Composing Programs.

Tip: We sometimes see code that looks like this:

if x > 3:
    return True
else:
    return False

This can be written more concisely as return x > 3. If your code looks like the code above, see if you can rewrite it more clearly!

While Loops

You can review the syntax of while loops in Section 1.5.5 of Composing Programs.


Error Messages

By now, you've probably seen a couple of error messages. They might look intimidating, but error messages are very helpful for debugging code. The following are some common types of errors:

Error Types Descriptions
SyntaxError Contained improper syntax (e.g. missing a colon after an if statement or forgetting to close parentheses/quotes)
IndentationError Contained improper indentation (e.g. inconsistent indentation of a function body)
TypeError Attempted operation on incompatible types (e.g. trying to add a function and a number) or called function with the wrong number of arguments
ZeroDivisionError Attempted division by zero

Using these descriptions of error messages, you should be able to get a better idea of what went wrong with your code. If you run into error messages, try to identify the problem before asking for help. You can often Google unfamiliar error messages to see if others have made similar mistakes to help you debug.

For example:

>>> square(3, 3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: square() takes 1 positional argument but 2 were given

Note:

  • The last line of an error message tells us the type of the error. In the example above, we have a TypeError.
  • The error message tells us what we did wrong — we gave square 2 arguments when it can only take in 1 argument. In general, the last line is the most helpful.
  • The second to last line of the error message tells us on which line the error occurred. This helps us track down he error. In the example above, TypeError occurred at line 1.

Required Questions

What Would Python Display (Part 1)?

Question 1: WWPD: Control

Use Ok to test your knowledge with the following "What Would Python Display?" questions:

python3 ok -q control -u
>>> def xk(c, d):
...     if c == 4:
...         return 6
...     elif d >= 4:
...         return 6 + 7 + c
...     else:
...         return 25
>>> xk(10, 10)
______
23
>>> xk(10, 6)
______
23
>>> xk(4, 6)
______
6
>>> xk(0, 0)
______
25
>>> def how_big(x):
...     if x > 10:
...         print('huge')
...     elif x > 5:
...         return 'big'
...     elif x > 0:
...         print('small')
...     else:
...         print("nothin'")
>>> how_big(7)
______
'big'
>>> how_big(12)
______
huge
>>> how_big(1)
______
small
>>> how_big(-1)
______
nothin'
>>> n = 3
>>> while n >= 0:
...     n -= 1
...     print(n)
______
2 1 0 -1

Hint: Make sure your while loop conditions eventually evaluate to a false value, or they'll never stop! Typing Ctrl-C will stop infinite loops in the interpreter.

>>> positive = 28
>>> while positive:
...    print("positive?")
...    positive -= 3
______
Infinite Loop
>>> positive = -9
>>> negative = -12
>>> while negative:
...    if positive:
...        print(negative)
...    positive += 3
...    negative += 3
______
-12 -9 -6

Question 2: WWPD: Veritasiness

Use Ok to test your knowledge with the following "What Would Python Display?" questions:

python3 ok -q short_circuiting -u
>>> True and 13
______
13
>>> False or 0
______
0
>>> not 10
______
False
>>> not None
______
True
>>> True and 1 / 0 and False
______
Error (ZeroDivisionError)
>>> True or 1 / 0 or False
______
True
>>> True and 0
______
0
>>> False or 1
______
1
>>> 1 and 3 and 6 and 10 and 15
______
15
>>> 0 or False or 2 or 1 / 0
______
2
>>> not 0
______
True
>>> (1 + 1) and 1
______
1
>>> 1/0 or True
______
Error
>>> (True or False) and False
______
False

Coding Practice

Question 3: Fix the Bug

The following snippet of code doesn't work! Figure out what is wrong and fix the bugs.

def both_positive(x, y):
    """Returns True if both x and y are positive.

    >>> both_positive(-1, 1)
    False
    >>> both_positive(1, 1)
    True
    """
return x and y > 0 # You can replace this line!
return x > 0 and y > 0

The original line (return x and y > 0) will check that two things are true:

  1. x
  2. y > 0

When will x be considered True? In Python, any number that is not 0 is considered True. Thus, the first doctest will fail: x = -1 and -1 != 0, and y = 1 > 0, so both clauses are True.

Use OK to test your code:

python3 ok -q both_positive

Question 4: Sum Digits

Write a function that takes in a nonnegative integer and sums its digits. (Using floor division and modulo might be helpful here!)

def sum_digits(n):
    """Sum all the digits of n.

    >>> sum_digits(10) # 1 + 0 = 1
    1
    >>> sum_digits(4224) # 4 + 2 + 2 + 4 = 12
    12
    >>> sum_digits(1234567890)
    45
    >>> x = sum_digits(123) # make sure that you are using return rather than print
    >>> x
    6
    """
"*** YOUR CODE HERE ***"
total = 0 while n > 0: total, n = total + n % 10, n // 10 return total

Use OK to test your code:

python3 ok -q sum_digits

Optional Questions

What Would Python Display (Part 2)?

Question 5: WWPD: What If?

Use Ok to test your knowledge with the following "What Would Python Display?" questions:

python3 ok -q what_if -u

Hint: print (unlike return) does not cause the function to exit!

>>> def ab(c, d):
...     if c > 5:
...         print(c)
...     elif c > 7:
...         print(d)
...     print('foo')
>>> ab(10, 20)
______
10 foo
>>> def bake(cake, make):
...     if cake == 0:
...         cake = cake + 1
...         print(cake)
...     if cake == 1:
...         print(make)
...     else:
...         return cake
...     return make
>>> bake(0, 29)
______
1 29 29
>>> bake(1, "mashed potatoes")
______
mashed potatoes 'mashed potatoes'

More Coding Practice

Question 6: Falling Factorial

Let's write a function falling, which is a "falling" factorial that takes two arguments, n and k, and returns the product of k consecutive numbers, starting from n and working downwards.

If k is larger than n, only multiply up to n consecutive numbers!

def falling(n, k):
    """Compute the falling factorial of n to depth k.

    >>> falling(6, 3)  # 6 * 5 * 4
    120
    >>> falling(4, 3)  # 4 * 3 * 2
    24
    >>> falling(4, 1)  # 4
    4
    >>> falling(4, 0)
    1
    """
"*** YOUR CODE HERE ***"
total, stop = 1, n-k while n > stop: total, n = total*n, n-1 return total

Use OK to test your code:

python3 ok -q falling

Question 7: Double Eights

Write a function that takes in a number and determines if the digits contain two adjacent 8s.

def double_eights(n):
    """Return true if n has two eights in a row.
    >>> double_eights(8)
    False
    >>> double_eights(88)
    True
    >>> double_eights(2882)
    True
    >>> double_eights(880088)
    True
    >>> double_eights(12345)
    False
    >>> double_eights(80808080)
    False
    """
"*** YOUR CODE HERE ***"
prev_eight = False while n > 0: last_digit = n % 10 if last_digit == 8 and prev_eight: return True elif last_digit == 8: prev_eight = True else: prev_eight = False n = n // 10 return False

Use OK to test your code:

python3 ok -q double_eights