Challenges

Python basics: Variables, Objects, Arrays, Lists etc

What’s inside the box?

Draw diagrams showing what variables refer to what values after each statement in the following program:

weight = 70.5
age = 35
# Take a trip to the planet Neptune
weight = weight * 1.14
age = age + 20

Sorting out references

What does the following program print out?

first, second = 'Grace', 'Hopper'
third, fourth = second, first
print(third, fourth)

Arrays, Lists, etc.

Slicing strings

What is the value of element[:4]? What about element[4:]? Or element[:]?

What is element[-1]? What is element[-2]? Given those answers, explain what element[1:-1] does.

Slicing From the End

Use slicing to access only the last four characters of a string or entries of a list.

string_for_slicing = "Observation date: 02-Feb-2013"
list_for_slicing = [["fluorine", "F"], ["chlorine", "Cl"], ["bromine", "Br"], ["iodine", "I"], ["astatine", "At"]]

"2013"
[["chlorine", "Cl"], ["bromine", "Br"], ["iodine", "I"], ["astatine", "At"]]

Would your solution work regardless of whether you knew beforehand the length of the string or list (e.g. if you wanted to apply the solution to a set of lists of different lengths)? If not, try to change your approach to make it more robust.

Overloading

+ usually means addition, but when used on strings or lists, it means “concatenate”. Given that, what do you think the multiplication operator * does on lists? In particular, what will be the output of the following code?

counts = [2, 4, 6, 8, 10]
repeats = counts * 2
print(repeats)
  1. [2, 4, 6, 8, 10, 2, 4, 6, 8, 10]
  2. [4, 8, 12, 16, 20]
  3. [[2, 4, 6, 8, 10],[2, 4, 6, 8, 10]]
  4. [2, 4, 6, 8, 10, 4, 8, 12, 16, 20]

The technical term for this is operator overloading: a single operator, like + or *, can do different things depending on what it’s applied to.

Repeating actions using loops

From 1 to N

Python has a built-in function called range that creates a list of numbers: range(3) produces [0, 1, 2], range(2, 5) produces [2, 3, 4]. Using range, write a loop to print the first 3 natural numbers:

1
2
3

Turn a String Into a List

Use a for-loop to convert the string “hello” into a list of letters:

["h", "e", "l", "l", "o"]

Hint: You can create an empty list like this:

my_list = []

Computing powers with loops

Exponentiation is built into Python:

print(5 ** 3)
125

Write a loop that calculates the same result as 5 ** 3 using multiplication (and without exponentiation).

Reverse a string

Write a loop that takes a string, and produces a new string with the characters in reverse order, so Newton becomes notweN.

Making choices

How many paths?

Which of the following would be printed if you were to run this code? Why did you pick this answer?

  • A
  • B
  • C
  • B and C
if 4 > 5:
    print('A')
elif 4 <= 5:
    print('B')
elif 4 < 5:
    print('C')

What Is Truth?

True and False are special words in Python called booleans which represent true and false statements. However, they aren’t the only values in Python that are true and false. In fact, any value can be used in an if or elif. After reading and running the code below, explain what the rule is for which values are considered true and which are considered false.

if '':
    print('empty string is true')
if 'word':
    print('word is true')
if []:
    print('empty list is true')
if [1, 2, 3]:
    print('non-empty list is true')
if 0:
    print('zero is true')
if 1:
    print('one is true')

Modularising your code using functions

Combining Strings

“Adding” two strings produces their concatenation: 'a' + 'b' is 'ab'. Write a short function called fence that takes two parameters called original and wrapper and returns a new string that has the wrapper character at the beginning and end of the original. A call to your function should look like this:

print(fence('name', '*'))

*name*

How do function parameters work?

We actually used the same variable name fahr in our main code and and the function. But it’s important to note that even though they share the same name, they don’t refer to the same thing. This is because of variable scoping.

Within a function, any variables that are created (such as parameters or other variables), only exist within the scope of the function.

For example, what would be the output from the following:

f = 0
k = 0

def multiply_by_10(f):
  k = f * 10
  return k

multiply_by_10(2)
multiply_by_10(8)

print(k)
  1. 20
  2. 80
  3. 0

Does the sum of a list equal a given value?

Write a function to take a list of numbers and another value, and return whether or not the sum of the list of numbers is equal to that value.

Following the function definition, a call to your function should look like this:

is_sum_equal([1,2,3], 6))
True
is_sum_equal([2,4,6], 100)
False

This is really useful, since it means we don’t have to worry about conflicts with variable names that are defined outside of our function that may cause it to behave incorrectly.

Readable Code

Revise a function you wrote for one of the previous exercises to try to make the code more readable. Then, collaborate with one of your neighbors to critique each other’s functions and discuss how your function implementations could be further improved to make them more readable.

How to deal with problems in your code

Identifying Variable Name Errors

  1. Read the code below (or open the file error_name_ch.py in code folder), and (without running it) try to identify what the errors are.
  2. Run the code, and read the error message. What type of NameError do you think this is? In other words, is it a string with no quotes, a misspelled variable, or a variable that should have been defined but was not?
  3. Fix the error.
  4. Repeat steps 2 and 3, until you have fixed all the errors.
for number in range(10):
    # use a if the number is a multiple of 3, otherwise use b
    if (Number % 3) == 0:
        message = message + a
    else:
        message = message + "b"
print(message)

Identifying Syntax Errors

  1. Read the code below (or open the file error_syntax_ch.py in code folder), and (without running it) try to identify what the errors are.
  2. Run the code, and read the error message. Is it a SyntaxError or an IndentationError?
  3. Fix the error.
  4. Repeat steps 2 and 3, until you have fixed all the errors.
def another_function
  print("Syntax errors are annoying.")
   print("But at least python tells us about them!")
  print("So they are usually not too hard to fix.")

Identifying Index Errors

  1. Read the code below, and (without running it) try to identify what the errors are.
  2. Run the code, and read the error message. What type of error is it?
  3. Fix the error.
seasons = ['Spring', 'Summer', 'Fall', 'Winter']
print('My favorite season is ', seasons[4])

Reading Error Messages

Read the python code (or open the file error_index_ch.py in code folder) and the resulting traceback below, and answer the following questions:

  1. How many levels does the traceback have?
  2. What is the function name where the error occurred?
  3. On which line number in this function did the error occurr?
  4. What is the type of error?
  5. What is the error message?
# This code has an intentional error. Do not type it directly;
# use it for reference to understand the error message below.
def print_message(day):
    messages = {
        "monday": "Hello, world!",
        "tuesday": "Today is tuesday!",
        "wednesday": "It is the middle of the week.",
        "thursday": "Today is Donnerstag in German!",
        "friday": "Last day of the week!",
        "saturday": "Hooray for the weekend!",
        "sunday": "Aw, the weekend is almost over."
    }
    print(messages[day])

def print_friday_message():
    print_message("Friday")

print_friday_message()

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in print_friday_message
  File "<stdin>", line 11, in print_message
KeyError: 'Friday'

Debug with a Neighbour

Take a function that you have written today, and introduce a tricky bug. Your function should still run, but will give the wrong output. Switch seats with your neighbor and attempt to identify - or debug - the problem that they introduced into their function.

Not Supposed to be the Same

You are assisting a researcher with Python code that computes the Body Mass Index (BMI) of patients (open the file error_silent_ch.py in code folder). The researcher is concerned because all patients seemingly have identical BMIs, despite having different physiques. BMI is calculated as weight in kilograms divided by the the square of height in metres.

patients = [[70, 1.8], [80, 1.9], [150, 1.7]]

def calculate_bmi(weight, height):
    return weight / (height ** 2)

for patient in patients:
    height, weight = patients[0]
    bmi = calculate_bmi(height, weight)
    print("Patient's BMI is: %f" % bmi)

Patient's BMI is: 21.604938
Patient's BMI is: 21.604938
Patient's BMI is: 21.604938

Reading and analysing Patient data using libraries

Thin slices

From our previous topic challenges, the expression element[3:3] produces an empty string, i.e., a string that contains no characters. If data holds our array of patient data, what does data[3:3, 4:4] produce? What about data[3:3, :]?

Data Visualisation

Make your own plot

Create a plot showing the standard deviation of the inflammation data for each day across all patients. Hint: data.std(axis=0) gives you standard deviation.

Moving plots around

Modify the program to display the three plots on top of one another instead of side by side.

Making choices

How many paths?

Which of the following would be printed if you were to run this code? Why did you pick this answer?

  • A
  • B
  • C
  • B and C
if 4 > 5:
    print('A')
elif 4 <= 5:
    print('B')
elif 4 < 5:
    print('C')