session-3.md

Session 3 Problems

In session 3 we learned convenient ways to store your data using:

1. Lists
2. Dictionaries

A little theory

Try to answer the questions then use the python console to check.

For the following questions assume numbers = [0,1,2,3,4]

• What is []?
• What does numbers[-1] evaluate to?
• What does numbers.append(9) make the list numbers look like?
• What does numbers.insert(2,9) make the list numbers look like?

For the following questions assume names = ["Alex", "Mia", "John", "Laura", "Joyce"]

• What does names.pop() return and what does the list names become?
• What does names.remove("mia") return and what does the list names become?

For the following questions assume person = {"name":"Alex", "age":27, "height":158}

• How would you change the value associated with the key "name" from "Alex" to "John"?
• How would you add the key-value pair {"graduate":True} to the dictionary person?
• What would list(person.keys()) look like?

Data analysis

For the following list:

# Copy and paste into your script
temperatures = [12,35,32,13,6,-23,-5,35,-42,-24,45,23,-64,8,2,-4]

• Calculate the length of the list temperatures
• Calculate the sum of all temperatures
• Find the maximum temperature
• Find the minimum temperature
• Calculate the mean temperature (mean = sum of temperatures divided by the number of temperature measurements)

HINT: The for loop is very useful when dealing with lists. Below is an example of using a for loop to determine the longest name in a list of names.

names = ["Alex", "Mia", "John", "Laura", "Joyce"]
longest_name = "" # initialise with an empty string

for name in names:
	if len(name) > len(longest_name):
		longest_name = name

print("The longest name is: " + longest_name)
# The longest name is Laura

Square map

Write a function called square_map that takes in a list of numbers (integers or floats) and returns a list of the same length only each element has been squared.

Sample input/output:

>>> numbers = [1,2,3,5]
>>> square_map(numbers)
[1, 4, 9, 25]

Append Check

Write a function called my_append that adds an element to a list provided that element is not already in the list. Remember that lists are passed by reference! The example below shows that the my_append function does not actually return anything, it "mutates" (edits) the animals list.

Sample input/output:

>>> animals = ["cat", "dog", "elephant", "pig"]
>>> my_append(animals, "dog")
>>> print(animals)
["cat", "dog", "elephant", "pig"]
>>> my_append(animals, "zebra")
>>> print(animals)
["cat", "dog", "elephant", "pig", "zebra"]

NOTE: Try not to mutate lists with functions in the future. We are just trying to show you how lists work with functions.

Pretty Print

The output of the inbuilt python print function doesn't look very good when printing a dictionary. Let's right a function called pprint to make the output look nicer.

def pprint(aDict):
	# write your code inside this function

Sample input/output:

>>> person = {"name":"Alex", "age":27, "height":158}
>>> pprint(person)
{
	name:	Alex
	age:	27
	height:	158
}

HINT: Use the person.items() method and a for loop.

Into the wild

Instead of going to work tomorrow you decide to go on an adventure. You have a number of items that you would like to take. However, you can only carry 7kg. Your job is to pack you bag with as many items as you can while ensuring that the weight of the backpack does not exceed 7kg. It is best to repesent the backpack as a dictionary:

>>> print(backpack)
{
	items:	["toothbrush", "sandwhich", "bon soy","book on python" ]
	weight:	2.750
}

Note this is not an optimisation problem. Put what ever you want in the backpack. Just make sure it is weighs less than 7kg.

Below is a list of your items that you could possibly take where each item is represented as a dictionary:

# copy and paste into your code if you like.
items = [
	{"name": "tooth brush", "weight": 0.050},
	{"name": "sandwhich", "weight": 0.100},
	{"name": "iPod", "weight": 0.250},
	{"name": "tomahawk", "weight": 2.000},
	{"name": "bon soy", "weight": 1.000},
	{"name": "sling shot", "weight": 0.800},
	{"name": "book on python", "weight": 1.600},
	{"name": "water", "weight": 1.800},
	{"name": "tissues",  "weight": 0.150},
	{"name": "first aid kit", "weight": 0.750},
	{"name": "flint", "weight": 1.075},
	{"name": "magnifying glass", "weight": 0.450}
]

BONUS: What is the best selection of items to maximise the number of items while keeping the weight below 7kg (i.e. make it an optimisation problem)?