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Programming 103 - Advanced programming concepts with Python

< Course overview
< Previous course module

Table of Contents

Introduction

This course uses the programming language Python to introduce more programming concepts.

This module ends with a series of exercises.

Have fun!

Data: Dictionaries

The first course module introduced the datatypes string, integer (int), floating point (float) and boolean. The previous course module introduced the datatype list.

This chapter will introduce you to the datatype dictionary, usualy refered to as dict.

Dictionaries are used to map keys to values. While that might sound scary, it really isn't.

Remember lists? my_list = ['a', 'b', 'c']. When we want to retreive data from my_list we can use my_list[0] to retreive the value 'a'. This is because in our list example, the value 'a' is what is stored at the index 0.

Dictionaries work in a similar way, but instead of making use of an index, a key is used to retreive data.

A simple dict example

Let's look at a simple dictionary example:

my_dict = {'key1': 'value1', 'key2':'value2'}

In the example above, we can find the following keys and values:

Key Value
key1 value1
key2 value2

Retreiving data from the example dictionary works like this:

my_dict = {'key1': 'value1', 'key2':'value2'}
my_dict['key1']

The resulting output of my_dict['key1'] would be 'value1'.

Dicts hold other data

Lists could hold any other type of data as their value. It is, for example, possible to have a list of lists.

Dicts work very much the same way, dicts can store strings, ints, lists or possibly even dicts.

Have a look at the following example (note: you can scroll the example horizontally).

my_dict = {'my_string': 'this is a string', 'my_int': 8, 'my_float': 8.001, 'my_list': ['apple', 'banana'], 'my_dict': {'my_key': 'my_awesome_value'}}

Let's make the above example more readable by slightly changing the formatting.

my_dict = {
    'my_string': 'this is a string',
    'my_int': 8,
    'my_float': 8.001,
    'my_list': [
        'apple', 
        'banana'
        ],
    'my_dict': {
        'my_key': 'my_awesome_value'
        }
    }

How would one retreive the value my_awesome_value out of the above example dict my_dict?

Adding and removing dict elements

Being able to make and read a dict is nice, but sometimes dicts require changes.

Adding or changing dict elements

Adding something to a dict is simple. Let's look at how it's done:

# We have some dict, called 'my_dict'
my_dict = {'a': 'value a', 'b': 'value b'}
# Now we want to add {'c': 'value c'} to "my_dict"
my_dict.update({'c': 'value'})

So to add a dict, we use .update(). Notice the () at the end of update. Remember functions? Very much like we can send arguments to a function, .update() requires information in the form of the extra key value pair that we want to add.

Notice how the new key value pair is basically a dict too.

Note: using .update() on a dict will overwrite existing keys and their value.

Removing dict elements

To remove something from a dict we can use .pop(), .pop() needs a valid key (one of the keys from the dict) to remove something from the dict.

Let's look what using pop() looks like in practise:

# We have some dict, called 'my_dict'
my_dict = {'a': 'value a', 'b': 'value b'}
# Now we want to remove the key and value for 'a'
# Pop will return the value corresponding with the key we just removed
my_dict.pop('a')
'value a'
# Let's see what 'my_dict' looks like now
my_dict
{'b': 'value b'}

There is more!

There are many more things that can be done with and to dicts. However, for now, let's have a look at what exactly happened when we used .update() and .pop(). This brings us to the next chapter.

Objects

You might have heard someone say that Python is an 'object oriented language' or even 'everything in Python is an object'. If you haven't, give it some time, you'll see it.

But what does all of that mean in practise?

In practise this means that, in Python, all the elements we work with (such as, for example, strings or dicts) have their own build in "functions" called methods.

This isn't all there is to say about objects (not at all), but being able to use methods and to understand the difference between methods and functions is rather important.

Methods

Let's have a look at a simple example object, in the form of a string.

my_string = 'hello world'

Finding methods

Strings come with an array of usefull methods, you can use Google to get an overview or, in the case of Python, use Python's help() function to get more information. To use Python's build in help, start Python by simply running python. Next, use:

help(str)

Note: Python's help is a "vi-like", you can quit by pressing q, search with /, or use commands such as gg and G.

Using methods

Functions are ran by calling them, such as in this example:

print('Hello world')

When we talked about dicts, we used .update() and .pop(), these are examples of dict methods.

Methods are called by using the . (dot) and then the method name. Much like with functions, methods also require () (parenthesis). In the case of the dict method .pop() we saw that we had to give .pop() the key that we wanted to remove from the dict.

Let's look at some methods that can be found on all Python strings.

# Our base string, stored in the variable "string"
string = 'Hello World'

# Let's use an obvious method: upper()
string.upper()
'HELLO WORLD'

# We can also do the opposite
string.lower()
'hello world'

# With split(), we can split a string on every occurence of the string that we send in.
# In this example, we split the string in the variable "string" at the string " ".
# Note how the result comes in a list
string.split(' ')
['Hello', 'World']

# A more complex split example would be:
string = 'Jon|[email protected]|Banana Road 20|Some place|Planet Earth'
string.split()
['Jon', '[email protected]', 'Banana Road 20', 'Some place', 'Planet Earth']

upper(), lower() and split() are just a few examples of all available string methods (in Python!).

Methods != functions

While we could say that methods are basically functions for objects, it's important that you are able to spot the difference.

The . (dot) notation makes it very easy to spot the difference.

Classes

While discussing objects, we only looked at build in objects. You might have guessed that it is also possible to build you own objects, in Python, these are refered to as a class.

Classes are often called 'objects' by other programmers. If you plan to continue using Python, make sure to use the right word from now on when talking about classes, because remember, "everything is an object", so calling something an object doesn't provide others with a lot of useful information.

Python classes will be covered in the Python course. For now it's simply important that you realise that it's possible to define your own objects in object oriented languages.

Test yourself

Exercise 0: Start a band

Turn the table below into a Python dict stored in the variable band. Oh and print it, by now, that should be easy.

key value
drums Lars
vocals James
guitar Dave

Exercise 1: Kick someone out

Use the band dict from the previous exercise. Remove 'Dave' from the dict. Store the new version of the dict in the band variable again.

Exercise 2: Add a member

Use the band dict from the previous exercise. Update the dict in band to include {'guitar': 'Kirk', 'bass': 'Cliff'} Print your new band variable.

Exercise 3: Update the band

Use the band dict from the previous exercise. Update the dict in band so {'bass': 'Cliff'} is updated to {'bass': Jason}. Print your new band variable.

Exercise 4: Split 'em up

Use the string method .split() to turn this string:

my_band = 'Lars James Kirk Cliff'

Into this list:

['Lars', 'James', 'Kirk', 'Cliff']

Print your list.

Exercise 5: Split them some more

Use the string method .split() to turn this string:

my_band = 'Lars, James, Kirk, Cliff'

Into this list:

['Lars', 'James', 'Kirk', 'Cliff']

Print your list.

Exercise 5.1: Show 'em all

Use the string method .split() to turn this string:

my_band = 'Lars:James:Kirk:Cliff'

Into this list:

['Lars', 'James', 'Kirk', 'Cliff']

Use a for loop to print every name on a new line.

Exercise 6: Master of methods

So far we used string and dict methods. Let's do something with list methods.

Use this list of ints:

unsorted_years = [2003, 1986, 2008, 1983, 1997, 1996, 1988, 1991, 2016, 1984]

Using a list method, use a single line of code to sort this list of years.

Exercise 6.1: Methods for all

Use this list of ints:

unsorted_years = [2003, 1986, 2008, 1983, 1997, 1996, 1988, 1991, 2016, 1984]

Store a sorted list of years in the variable sorted_years. Print the original unsorted years list. Print the sorted_years list.

What did you just learn about variables? (If you hadn't noticed yet, compare your unsorted_years with the unsorted_years in this exercise!)

Exercise 7: Join in

Look at this example using the .join() method.

some_words = ['Hello', 'I', 'am', 'learning', 'to', 'program']
print(' '.join(some_words))

The result of the above code is:

Hello I am learning to program

Use the join() method to turn the list below into a string. Elements should be seperated by :.

Use:

user_data = ['Cliff', 'Burton', '1962', '1986', 'Bass']

Store the result in a variable named member, print member.

The result of the print should look like this:

Cliff:Burton:1962:1986:Bass

Exercise 8: Discriminate

Have a look at this list of words:

word_list = [
    'MASTER',
    'Justice',
    'RIDE',
    'load',
    'Garage',
]

Use the list as provided above (you can copy paste it). Use a for loop to loop over the list. Use string methods to only print words that are printed in CAPITALS only or small letters only.

Exercise 9: The rest isn't that important

Have a look at this list of words:

word_list = [
    'MASTER',
    'Justice',
    'RIDE',
    'load',
    'Garage',
]

Use the list as provided above (you can copy paste it). Use a for loop to loop over the list. Store all entries that use mixed casing (both upper and lower) in a list. Store the list in the variable ELSE. Store all entries that use UPPER casing in a list. Store the list in the variable MATTERS. Store all entries that use lower casing in a list. Store the list in the variable NOTHING.

Print 'Nothing:', loop over the list in NOTHING and print every result to a new line. Print 'Else:', loop over the list in ELSE and print every result to a new line. Print 'Matters:', loop over the list in MATTERS and print every result to a new line.

Exercise 10: Let's write a phonebook

Calculators are fun, but this time, we'll write something else. A phonebook (if you don't know what a phonebook is, a phonebook is very much like the contacts app on your phone, but in print, so it updates slowly).

The phonebook requires a few things.

  • A data variable. This should be a list.
  • A user_add() function. Which you will write yourself.
  • A user_remove() function. Which you will write youself.
  • A list_users() function. Which you will write yourself.

Phonebook entries require some kind of setup. Structure your entries in dicts like this:

{'name_first': '', 'name_last': '', 'email': '', 'phone': ''}

Note how all values in the dict are strings, this will make working with the data much easier.

Data variable

The data variable should simply be a list. In this list we will be storing user data. Every user will get their own dict.

user_add

The user_add function should update the list in data to include a new user_dict (as described above).

Your function should accept four arguments: name_first, name_last, email and phone.

Your function should return the new user dictionary.

user_remove

The user_remove function should remove a user from the list in data. Use a for loop or look up an alternative method to find users in the list based on their first name.

list_users

The list_users function should return a list of all users. Use a for loop or look up an alternative method to list all users. Be sure to print all userdata (first_name, last_name, email and phone).

Aim for an output that is similair to:

First name: Bob
Last name: Ross
email: [email protected]
phone: 555666777888

Exercise 11: Divide

Reuse the code from the previous exercise.

Do the exact same thing, but make your output look like this:

--------------------
First name: Jon
Last name: Ross
email: [email protected]
phone: 555666777888
--------------------
First name: Son
Last name: Goku
email: [email protected]
phone: 555444333222

Exercise 12: Make it look nice

Use the code from the previous exercise. When printing results, use the len() function to make sure that all the dividers (---) are the same length. Use the lenght of the longest line you are printing to decide the maximum ammount of - to use.

The example above would then look like this:

---------------------------
First name: Jon
Last name: Ross
email: [email protected]
phone: 555666777888
---------------------------
First name: Son
Last name: Goku
email: [email protected]
phone: 555444333222

Bonus: Objectively better

Using dicts and lists is fun. But doing the same thing with classes is much more efficient. If you are about to continue learning Pyhon, build the phonebook from exercises 10-12, this time, using a python class.

Start off by finding out how to write your own classes.

Turn the phonebook functions into methods.

Next course module >