2. Iterations, lists, dictionaries, comprehensions

Iterations

Iterations are code strucures which allow us to repeat sections of code while a condition is true, or a fixed number of time.
They allow us to do more with less code!

Definite loops (For Loop)

A Definite loop iterates over a a fixed set of values.
Real life iterations: Knocking on every door in your dorm. Calling each person in the contact list on your phone.
In Python, the for statement is used for definite loops
The for loop uses an iterator to select each item from the list or range and take action in the loop body.
The range() function is useful for getting an iterator of numbers.
The for loop can iterate over any value that is iterable.

# Example: Using the range() function to iterarate over a sequence

for i in range(5):
    print(i)

In the example above:

the print() statement repeats as part of the loop body.
the variable i is the iterator. It stores the current value of the range(5) for each iteration of the loop
range(5) is an example of the iterable - the thing we are looping over.
range(5) by definition creates an iterable of sequential numbers from 0 to 4: 0,1,2,3,4
Therefore the code repeats 5 times…

# Example: strings are iterable by their individual characters
for ch in "testing":
    if ch == "t":
        print(ch)

t
t

Questions about the example above:

What is the iterator variable?
What is the iterable?
How many times does this loop “iterate”?
Why does it only print two t’s?

The break command

break keyword exits the loop immediately.
Commonly used when there is no longer a reason to loop (you achieved your goal).
You can add an else to the for loop to execute when break does not happen.

# Example: find a letter in a text string

text = input("Enter Some Text:")
find = input("Enter character to find:")
for ch in text:
    if ch == find:
        print(f"Found {find} in {text}!")
        break
else:
    print(f"Unable to find {find} in {text}!")

Enter Some Text: testing
Enter character to find: n
Found n in testing!

Enter Some Text: testing
Enter character to find: x
Unable to find x in testing!

Code Challenge 2.1

Write a program to accept a password as input. If the password input is “secret” display “access granted”. Otherwise say “invalid password”.

Repeat the above up to 5 times. When the correct password is entered, stop looping. When 5 loops have exhausted, print “you are locked out”.

Solution

valid_password = "secret"
for i in range(5):
    pw = input("Enter Password:")
    if pw == valid_password:
        print("Access Granted!")
        break
    else:
        print("Invalid Password.")
else:
    print("You are locked out")

Indefinite loops

Indefinite loops are based on external input and are non-deterministic.
Unlike definite loops we do not know when the loop will stop.
Examples: get tutoring when you fail an exam (you may never fail an exam). When the temperature drops below 0 turn on the de-icer.
In Python, we use while statement for indefinite loops.
The classic indefinite loop is a sentinel-controlled loop that repeats until a specific event occurs.

# Example: Will I ever say the magic word?
while True:
    word = input("Say the magic word!")
    if word == 'please':
        break
    print("You didn't say the magic word!")

In the above example the word “please” is the sentinel value

# Example: Sentinel loop
count = 0
while True:
    raw = input("Enter a number or type 'stop':")
    if raw == 'stop':
        break
    count = count + 1

print(f"You entered {count} items.")

Code Challenge 2.2

Write a program to accept numbers until the user enters: 0

The program should count the number of positive and negative numbers entered, and print those values after the 0 is entered.

Solution

pos = 0
neg = 0

while True:
    num = int(input("Enter an integer:"))
    if num > 0:
        pos = pos + 1
    elif num < 0:
        neg = neg + 1
    else:
        break

print(f"Number of + numbers entered: {pos}")
print(f"Number of - numbers entered: {neg}")

Lists

lists are iterable, sequences of values
The values in the list are mutable - you can change them.
items in the list can be accessed by a zero-based index.

# Example: Items in the list
items = [ 'milk', 'bread' ,'cheese', 'apples' ]

print("The first item is:", items[0])
print("The second item is:", items[1])
print("The last item is:", items[-1])

The first item is: milk
The second item is: bread
The last item is: apples

# Lists are iterable
items = [ 'milk', 'bread' ,'cheese', 'apples' ]
for item in items:
    print(item)

milk
bread
cheese
apples

Questions about the example above:

What is the iterator variable?
What is the iterable?
How many times does this loop “iterate”?

The in operator

The in operator checks for existence of an item in a list.

# Example: whats in the list?
numbers = [10, 15, 20]
print(f"5 in {numbers}?", 5 in numbers)
print(f"20 in {numbers}?", 20 in numbers)

5 in [10, 15, 20]? False
20 in [10, 15, 20]? True

List Methods

There are numerous list methods which allow you to add, remove, and find values in the list, etc…

https://docs.python.org/3/library/stdtypes.html?highlight=list#mutable-sequence-types

# Example: manipulating a list

# An empty list
colors = []

# Add "blue" to the end
colors.append("blue")

# add "red" to the beginning
colors.insert(0, "red")

# add "white" in the 2nd position
colors.insert(1, "white")

# print ['red', 'white', 'blue']
print(colors)

# remove the last color
blue = colors.pop(-1)

# remove "white"
white = colors.remove("white")

# print ['red']
print(colors)

['red', 'white', 'blue']
['red']

Code Challenge 2.3

Write a sentinel controlled loop to input a color until “quit” is entered. Add each color to a list only when the color is not already in the list. Print the list each time in the loop.

Solution

colors = []

while True:
    color = input("Enter a color:")
    if color == 'quit':
        break
    if color not in colors:
        colors.append(color)
        op = "added to"
    else:
        op = "already in"

    print(f"{color} {op} {colors}")

list comprehensions

List comprehensions allow us to create lists from operations on existing lists.

Consider the following:

numbers = [1, 2, 4, 5, 7]
evens = []
for num in numbers:
    if num % 2 == 0: #even
        evens.append(num)
print(evens)

[2, 4]

# Same thing as a list comprehension
numbers = [1, 2, 4, 5, 7]
evens = [ num for num in numbers if num % 2 == 0]
print(evens)

[2, 4]

# This comprehension makes a list out of the first letter in each work
words = ["welcome", "other", "rent", "math" ]
firsts = [ word[0] for word in words ]
print(firsts)

['w', 'o', 'r', 'm']

Dictionaries

The dict type is designed to store key-value pairs. In Python this is known as a mapping type.
font = {'name': 'Arial','size': 8}
Python dictionaries are mutable which means you can change the values of the keys after they have been set.
Dictionary values are accessed and set by key not by index.
font['name'] = 'Courier'
the keys are unique in the dictionary

Dictionary Methods

Like str and list, the dict type has its own set of built-in functions. https://docs.python.org/3/library/stdtypes.html#mapping-types-dict

font = {'name': 'Arial','size': 8}

print(font.keys()) # this is an iterable

print(font.values()) # this is an iterable

print(font['name'])

font['name'] = 'Courier'

print(font['name'])

print('size' in font)

print(font.get('style', 'normal')) # get with default value

dict_keys(['name', 'size'])
dict_values(['Arial', 8])
Arial
Courier
True
normal

Complex Data Stuctures

We can combine lists and dictionaries to create complex data structures in python.

These allow us to represent real-world data in code

students = [
    { 'name' : 'abby', 'grades' : [100,80,90] },
    { 'name' : 'bob', 'grades' : [100,90,90] },
    { 'name' : 'chris', 'grades' : [90,100,100] }
]

# just print each student name
for student in students:
    print(student['name'])

abby
bob
chris

# print each student name and average grade
for student in students:
    avg_grade = sum(student['grades'])/len(student['grades'])
    print(f"{student['name']}  {avg_grade:.2f}")

abby  90.00
bob  93.33
chris  96.67

Code Challenge 2.4

Write a program to create a shopping list:

loop until “quit” is entered
input a grocery item
input a quantity
save the item as the key in the dictionary and quantity as the value
if the item is in the dictionary already, add the quantity to the existing value

Solution

items = {}

while True:
    item = input("Enter shopping item, or 'quit': ")
    if item == 'quit':
        break
    qty = int(input("Enter quantity: "))
    if item in items.keys():
        items[item] = items[item] + qty
    else:
        items[item] = qty

    print("ITEMS:", items)