CS110_L3_Notes.py

Python3 Editor

#------------------------------------------------------------
#        Collections of Data
#------------------------------------------------------------

# If you have a whole bunch of data that all represent different data points for the same thing,
# it would be best to store that data in a collection of some sort. 
# Data collections are abstract data types rather than primitive. Lists, Tuples, Sets, and Dictionaries
# are some of the collection types in Python. Each can store different types of data in one instance.

print('''------------------------------------------------------------
        Lists
------------------------------------------------------------''')

# Lists are exactly what they sound like. They store data with an order, like a shopping list or a to-do list.
# Create them by using the square brackets.
list1 = [5, "Bob", "Hello", 2.7, 10]
print("list1 =", list1)

# In Python, you can store all different types of data within the same list, as shown above.
# However, beware of logic errors that may arise when using a list containing different types.
# Generally, if using a list, best practice would be to have all values be the same type.

# For all collections of data, you can see if a value is in the collection by using the keyword 'in'.
print( "Bob" in list1 )    # True
print( 20 in list1 )       # False

# Lists have indices too, just like strings. Remember how I said strings are lists of characters? They follow the same rules.
# First item in the list is at index 0. The length of a list is the number of items in the list. 
# The last item in the list is at index len(list)-1, or -1.
print("There are", len(list1), "items in the list.")

print("First item =", list1[0])
print("Fourth item =", list1[3])
print("Last item =", list1[-1])

# Can change a value in the list using the index as well.
list1[3] = 3.6
print('list1 =', list1)

# Can create sublists by slicing the list, just like a string.
# list[start:end], the value at the end index is not included in the sublist
sublist1 = list1[1:3]
print("sublist1 =", sublist1)

sublist2 = list1[:3]    # starts at 0 by default
print("sublist2 =", sublist2)

sublist3 = list1[2:]    # goes to last value by default
print("sublist3 =", sublist3)

# You can also reverse a list using slicing, just like a string.
reverse = list1[::-1]
print("reverse =", reverse)
print()

# You can also have lists inside of lists, or multi-dimensional lists.
mdlist = [
    [90, 80, 70, 5],
    [10, 20, 30, 15],
    [50, 60, 40, 55]
]
# Each list within the list is a ROW. Each item in the row is further indexed, we refer to the second index as the COLUMN index.
# To access a value within a multi-dimensional list, you start from the outside and work your way in.
# For a 2D List, you would use list[row][column]
print("mdlist[0][0] =", mdlist[0][0])    # 90
print("mdlist[1][3] =", mdlist[1][3])    # 15

# Each inner list has a length as well. Since there are 3 rows, len(mdlist) = 3. 
print("There are", len(mdlist), "rows in mdlist.")
# In the first row, there are 4 values.
print("There are", len(mdlist[0]), "columns in the first row of mdlist.")

# Need to add a new value to the end of the list? Use the append function.
list1.append("Goodbye")
print("list1 =", list1)

# If you use append to add a list of items to another list, they will be added as one value (a list)
list1.append([1, 9])
print("list1 =", list1)

# If you want each individual value to be added to the list as separate items, use the extend function instead.
list1.extend([3, 4])
print("list1 =", list1)

# You can insert a value into the list at a specific index using the insert function.
# It will insert it into the list such that the new value is at the specified index. 
# Every value at that index or greater is moved right by one index.
list1.insert(2, 'A')
print("list1 =",list1)
print()

# There are 3 ways to remove from a list: del, remove, and pop

# del deletes the item at that index. All values at indices higher than this will move left by 1.
del list1[7]
print("list1 =", list1)

# remove will remove a specific value from the list. However if the value is NOT in the list, this will cause an error
list1.remove('Goodbye')
print("list1 =", list1)

# pop can be used in two different ways
# If you call it with nothing in the parentheses it will remove the last item and return it.
x = list1.pop()
print("x =", x)
print("list1 =", list1)

# You can also call it by passing the index of the item to remove. 
# It will remove the item at that index from the list and return the value to you (so you can store and use it)
y = list1.pop(2)
print("y =", y)
print("list1 =", list1)
print()

# There are 2 methods you can use to sort a list.
# However, both of them need all of the items in the list to be the same type.
list2 = [3, 5, 9, 7, 10, 8]

# The built-in method sorted will sort the list and return it sorted, without changing the original list.
sortedlist2 = sorted(list2)
print("list2 =", list2)
print("sortedlist2 =", list2)

# Lists also have a sort function defined to sort them, but this will sort them "in place", meaning it changes the original list.
list2.sort()
print("list2.sort() =", list2)

# You can find out how many times a specific value appears in a list using the count function.
list3 = [2, 6, 5, 4, 6, 2, 6, 7]
print("list3 =", list3)
print("6 appears", list3.count(6), "times")

# You can find the index where a value first occurs in a list using the index function.
print("5 is located at index", list3.index(5))
print()

print('''-------------------------------------------------------------
        Tuples
-------------------------------------------------------------''')

# Tuples are similar to lists, except you CANNOT change their individual values.
# Create a tuple using parentheses.

tuple1 = (5, 2, "Bob")
print("tuple1 =", tuple1)

# tuples are also indexed, so you can access each piece with the index
print("First item in tuple1 =", tuple1[0])

# tuples also have a length
print("There are", len(tuple1), "items in tuple1.")
print()

# However, you cannot change individual values of a tuple. 
# Uncomment the next line and you will see that it causes a TypeError: 'tuple' object does not support item assignment
# tuple1[0] = 10

# If you need to change anything about the values in a tuple, you will have to create a new tuple,
# copy over the values that remain the same, and change the ones needed to be changed in the new tuple.

print('''--------------------------------------------------------------
         Sets
--------------------------------------------------------------''')

# Sets are different than the other data collections we are discussing because they are NOT indexed, meaning there is no order.
# Sets also do not allow duplicate values to be stored within them. Every value in a set is unique.
# Use the curly brackets and list items separated by commas to create a set.
set1 = {'birch', 'elm', 'fir'}
print("set1 =", set1)

# Every time you press the run button, the set above may print out in a different order than we defined it as.

# To access individual values, you would need to use a loop and access all of the values at once. We'll look at this in a later lesson.

# Use 'in' and 'not in' to check if a value exists already in a set.
print('birch' in set1)        # True since 'birch' is in the set
print('birch' not in set1)    # False since 'apple' is not in the set

# Use the add function to add values to a set. 
# If the value is already in the set, then it will not be added and no error will occur.
set1.add('apple')
set1.add('birch')
print("set1 =", set1)

# If you need to add multiple items to the set, use the update function.
set1.update(['cedar', 'cypress', 'ash'])
print("set1 =", set1)

# You can also view the number of items in a set using the len function.
print("There are", len(set1), "items in set1.")

# There are 2 ways to remove from the set: remove and discard
# remove will remove the item from the set, but if it does not exist in the set, it will cause an error
set1.remove('birch')
print("set1 =", set1)
# Uncomment the line below to see the error.
#set1.remove('orange')

# To avoid this error, use the discard function instead. If the item is in the set, it will remove it.
# Otherwise, it will do nothing.
set1.discard('apple')
print("set1 =", set1) # properly removes 'apple'
set1.discard('orange') # 'orange' is not in the set, but no error occurs
print("set1 =", set1)

# Want a random item in the set removed and returned? Use the pop function. However, if the set is empty, it will
removed = set1.pop()
print("removed =", removed)
print("set1 =", set1)

# You can clear all items from a set and change it to an empty set using the clear function
set1.clear()
print("set1 =", set1)
print()

# If you use the del keyword on the set, it will delete the variable altogether and no more operations can be done on the set
# Uncomment the 2 lines below to see that it causes a NameError, since the variable set1 no longer exists.
#del set1
#print("set1 =", set1)

# Sets have many functions defined just for them to combine sets and create subsets. You can read more about those in the documentation.

print('''--------------------------------------------------------------
         Dictionaries
--------------------------------------------------------------''')

# Dictionaries store data in key-value pairs. Keys can be string literals, integer literals, float literals, or tuples. 
# Values can be any type, including collections of data like other dictionaries, sets, and lists.
# You as the programmer determine the keys and set their values. 
# Dictionaries are great when you have a bunch of different TYPES of data, each representing a different part of one object.
# It allows you to essentially define multiple variables for each property without creating individual variables 
# and still know which values represent that object.

# Create a dictionary using the curly brackets. Inside, write the key, followed by a colon, then the value and a comma.
# So long as there is another value to be added to the dictionary, the value before it should have a comma at the end, like a list.
square = {
    'x': 200,
    'y': 150,
    'color': 'red',
    'side': 50
}
print("square =", square)

# The keys of a dictionary allow it to be indexed, like a list. 
# However, in a dictionary, the index is the key, which is not necessarily an integer.
print( square['x'] )        # 200
print( square['color'] )    # red
# If you try to access a key that does not exist, you will get an error.
# To avoid this error, you can use the get() function instead:
print( square.get('x') )
# 'name' is not in the dictionary, so trying to access it with [] would cause KeyError
# using the get() function, it will return None instead
print( square.get('name') )

# Just like lists, you can change the value stored to a key the same way.
square['x'] = 100
print("square =", square)

# Just like any other set, you can check if a KEY is in the dictionary using the 'in' keyword.
print( 'color' in square )        # True
print( 'name' in square )         # False
print( 'name' not in square )     # True

# If you want to know all the keys in a dictionary, use the keys() function
print( square.keys() )
# This will return a dict_keys object. To use it as a list, you would need to change its type to a list using the list() function
print("Keys:", list(square.keys()) )

# Want a list of values? Use the values() function. It returns a dict_values object, so change its type to a list
print("Values:", list(square.values()) )

# Want a list containing all key-value pairs? Use the items() function
# This function will return a dict_items object, where each item is represented as a tuple (key, value)
print("Items:", list(square.items()) )

# Want to add a new key-value pair to the dictionary? Just set its value as if it was already in the dictionary.
square['name'] = 'Bob'
print("square =", square)

# Want to add or update multiple key-value pairs in the dictionary? Use the update function and pass in another dictionary
square.update({
    'x': 50,
    'color': 'orange',
    'outline': 'blue'
})
print("square =", square)

# Remove a key-value pair from the dictionary using the pop function.
square.pop('name')
print("square =", square)

# Clear all key-value pairs from a dictionary using the clear function
square.clear()
print("square =", square)
print("\n")

# Just like with lists, you can have multiple layers in a dictionary. Let's take a look at an example and how to access them
person = {
    'name': 'Bob',
    'age': 23,
    'job': 'Student',
    'pets': [
        {
            'name': 'Spot',
            'age': 4,
            'type': 'dog'
        },
        {
            'name': 'Salem',
            'age': 12,
            'type': 'cat'
        }
    ]
}

# to print the person's name, we would write
print( person['name'] )

# But how about if we wanted to know something about the pets?
# What if we wanted the name of his second pet?
print( person['pets'] )    # this will give the whole list of pets
print()
print( person['pets'][1] )    # this will give the item at index 1 in the pets list, which is the second pet in the list
print()
# since person['pets'][1] is a dictionary, to get the pet's name, we would then need to access the 'name' key for that pet
print( person['pets'][1]['name'] )    # this will give us the second pet's name

( around 261 lines Python3 code )

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Created on 2020-10-21T16:45:21Z

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