Building programs with Python

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)

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

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.

Slicing strings

A section of an array is called a slice. We can take slices of character strings as well:

element = 'oxygen'
print('first three characters:', element[0:3])
print('last three characters:', element[3:6])

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.

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 = []

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, :]?

Visualising data using libraries

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.

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 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. (Note that if the body of a conditional is a single statement, we can write it on the same line as the if.)

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'