Notes S-10/Python 7: Loop control, functions, and dictionaries

TODO: Additional exercise (Insurance) - PR shares it

Notes:

Functions: at least one problem set in the exam.
Dictionaries: less relevant for the exam (do not explicitly announce as not relevant at the beginning) - maybe as multiple-choice.

Cover loop termination, functions, and dictionaries, while retaining the planned exercise order and exam guidance.

Slide 115 — `break` and `continue`

break terminates the loop, whereas continue skips the rest of the current iteration.

for val in "string":
    if val == "i":
        break
    print(val)

print("The end")

for val in "string":
    if val == "i":
        continue
    print(val)

print("The end")

Slide 116 — Loop termination

Extend the code to end the loop when the user enters 0.

In-class exercise ../materials/session_10/StopLoop.py

items = []
while True:
    n = int(input("Enter a number: "))
    if n==0:
        break
    items.append(n)

Expand the dice game again.

Assign exercise 2 as homework.

Homework ../materials/session_10/dicegame3.py

import random

stake = int(input('How much money would you like to invest? '))
continue_ = True

while continue_:
    pasch = False
    try_ = 1
    while pasch==False and try_<4:
        dice1 = random.randint(1,6)
        dice2 = random.randint(1,6)
        print("The throw brought", dice1, "and", dice2)
        if dice1==dice2:
            pasch = True
            break
        try_ = try_ + 1

    if pasch==True:
        print("You have won!!! It's a double", dice1)
        payout = stake * dice1
        print("You get", payout, "Euro!")
        continue_ = bool(int(input('Would you like to continue? 1=yes, 0=no: ')))
        if continue_:
            stake = payout
    else:
        print("Unfortunately you have lost!!!")
        continue_ = False

Additional exercise TODO

An additional insurance exercise is shared by PR. TODO: Add its mapping when the material is available.

Slide 118 — Defining functions

Parameters receive call arguments, return sends a value to the caller, and a function call runs the reusable body.

def square(x):
    sq = x * x
    return sq


x = 2
y = square(x)
print(x, y)

def add(x, y):
    return x + y


x = 2
y = 3
z = add(x, y)
print(x, y, z)

def fibonacci(n):
    fib = [0, 1]

    for i in range(n):
        fib.append(fib[-2] + fib[-1])

    return fib


n = int(input("How many numbers: "))
print(fibonacci(n))

Slide 120 — Reusing functions

One function can call another, and defining reusable functions reduces duplication.

def square(x):
    sq = x * x
    return sq


def sumofSquares(numberlist):
    sum_ = 0

    for i in numberlist:
        squaredValue = square(i)
        sum_ += squaredValue

    return sum_


a = 2
b = 3

c2 = square(a) + square(b)
print(c2)

numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
total = sumofSquares(numbers)

print("Sum of the Square of List of Numbers:", total)

Slides 121–122 — Functions

Order, in-session: 4, 5, 1, 6 (if time permits). Assign 2,3 as homework.

Write a function count_digits(int)

In-class exercise ../materials/session_10/CountDigits.py

def count_digitsa(number):
    snum = str(number)
    count = len(snum)
    return count

def count_digitsb(number):
    count = 0
    while number>0:
        number = number//10
        count = count + 1
    return count

number = int(input("Please enter a number: "))
print("The number of Digits is", count_digitsa(number))
print("The number of Digits is", count_digitsb(number))

Write a Python function to multiply all the numbers in a list.

Homework ../materials/session_10/Listmult.py

def multi(x):
    n = len(x)
    prodX = 1.0
    for i in range(n):
        prodX = prodX * x[i]
    return prodX

li = [8, 2, 3, -1, 7, 5, 6 ]
print ("Product =", multi(li))

Write a function isHarshad(int)

Homework ../materials/session_10/Harshad.py

def isHarshad(num):
    num_string = str(num)
    num_sum = 0
    for i in range(len(num_string)):
        num_sum = num_sum + int(num_string[i])
    if (num%num_sum==0):
        return True
    else:
        return False

number = int(input("Please enter a number: "))
print(isHarshad(number))

Write a Python function to calculate the factorial of a number (n!).

In-class exercise ../materials/session_10/Factorial.py

def factorial(n):
    fac = 1
    for i in range(1, n+1):
        fac = fac * i
    return fac

n = int(input("Enter a number: "))
print ("Factorial =", factorial(n))

Extend the program by adding a function which calculates the number of possible combinations

In-class exercise ../materials/session_10/Combinations.py

def factorial(n):
    fac = 1
    for i in range(1, n+1):
        fac = fac * i
    return fac

def combinations(n,k):
    return (factorial(n)/(factorial(n-k)*factorial(k)))

n = int(input("Enter n: "))
k = int(input("Enter k: "))
print ("Combinations =", combinations(n,k))

Write a program that simulates a slotmachine

In-class exercise ../materials/session_10/SlotMachine.py

# Version 1: Simply 3 random numbers
import random

def run_Slotmachine():
    result = [0, 0, 0]
    for i in range(3):
        result[i] = random.randint(1,6)
    return result

result = run_Slotmachine()
if result[0]==result[1]==result[2]:
    print("You have won: ", result)
else:
    print("You have lost: ", result)


# Version 2: 3 random numbers generated by 100 Iterations
import random

def run_Slotmachine():
    result = [0, 0, 0]
    for i in range(3):
        for j in range(100):
            result[i] = random.randint(1,6)
    return result

result = run_Slotmachine()
if result[0]==result[1]==result[2]:
    print("You have won: ", result)
else:
    print("You have lost: ", result)


# Version 3: 3 random numbers generated by random Iterations
import random

def run_Slotmachine():
    result = [0, 0, 0]
    for i in range(3):
        for j in range(random.randint(1,100)):
            result[i] = random.randint(1,6)
    return result

result = run_Slotmachine()
if result[0]==result[1]==result[2]:
    print("You have won: ", result)
else:
    print("You have lost: ", result)


# Version 4: 3 random numbers generated by random Iterations
import random

def run_Slotmachine():
    result = [0, 0, 0]
    for i in range(3):
        for j in range(random.randint(1,100)):
            result[i] = random.randint(1,6)
    return result

result = run_Slotmachine()
if result[0]==result[1]==result[2]:
    print("You have won: ", result)
else:
    print("You have lost: ", result)















import random

def run_Slotmachine():
    result = [0, 0, 0]
    for i in range(3):
        for j in range(100):
            result[i] = random.randint(1,6)
    return result

result = run_Slotmachine()
if result[0]==result[1]==result[2]:
    print("You have won: ", result)
else:
    print("You have lost: ", result)

Exam relevance

At least one exam exercise will cover functions.

Slide 125 — Using an external module

The module name acts as a namespace prefix.

import math

print(math.sin(5))
print(math.sin(math.pi))

Slide 127 — Creating a dictionary

Dictionaries use curly braces to associate unique keys with values.

dict1 = {
    'Peter': 55,
    'Mary': 40,
    'Bob': 34,
}

dict1

Slide 128 — Accessing and modifying dictionaries

The second and third expressions below intentionally raise KeyError because those keys do not exist.

dict1['Mary']
dict1['Hans']
dict1[0]

a = 'Peter'
dict1[a]

dict1['Alice'] = 55
dict1

dict1['Mary'] = 41
dict1

stockprices = {
    'Date': [
        '1.1.2021',
        '2.1.2021',
        '1.1.2021',
        '2.1.2021',
    ],
    'BMW': [89, 91, 95, 92],
    'Telekom': [12, 10, 14, 17],
    'SAP': [114, 119, 125, 130],
}

These examples demonstrate lookup, insertion, replacement, and list values.

Slide 129 — Complex and nested dictionaries

Values can themselves be dictionaries or lists, and keys may be strings or numbers.

contacts = {
    'name': 'Alice',
    'age': 30,
    'address': {
        'street': '123 Main St',
        'city': 'Wonderland',
        'zip': '12345',
    },
    'phone_numbers': [
        {
            'type': 'home',
            'number': '123-456-7890',
        },
        {
            'type': 'work',
            'number': '987-654-3210',
        },
    ],
}

product_details = {
    'product_id': 101,
    'name': 'Laptop',
    'price': 799.99,
    'in_stock': True,
    'tags': ['electronics', 'computer'],
}

item_quantities = {
    1: 50,
    2: 30,
    3: 75,
}

students = {
    'student1': {
        'name': 'John',
        'age': 22,
        'courses': ['Math', 'Science'],
    },
    'student2': {
        'name': 'Jane',
        'age': 24,
        'courses': ['History', 'Literature'],
    },
}

Slide 130 — Accessing nested dictionary values

Use successive keys to access nested values; a list stored inside a dictionary can be modified with its list methods.

students['student1']
students['student1']['age']
students['student1']['courses']
students['student1']['courses'][0]

students['student1']['haircolor'] = 'blonde'
students['student1']['courses'].append('Informatics')

students['student1']

Slide 131 — Adding, changing, and deleting nested data

students['student3'] = {
    'name': 'Peter',
    'age': 25,
    'courses': ['Finance'],
}

students['student3']['weight'] = 23

del students['student3']['weight']
del students['student3']

students['student2']['courses'].append('Informatics')
students['student1']['courses'].remove('Math')
students['student1'] = {'age': 25}

The final assignment replaces the complete student1 dictionary rather than changing only its age.

Slide 132 — Save and load dictionaries with JSON

Write mode creates or replaces the file and json.dump() serializes the dictionary; read mode opens it and json.load() reconstructs the data.

import json

with open('students.db', 'w', encoding='utf-8') as file:
    json.dump(students, file)

with open('students.db', 'r', encoding='utf-8') as file:
    students = json.load(file)

☕ Break — 10 minutes

Slide 133 — Dictionaries and persistence

In-class exercise ../materials/session_10/Friends.py

friends = {'friend1' : {'name':'Peter', 'likes':['biking', 'techno'], 'food_intolerances':['pepper', 'cucumber']},
           'friend2' : {'name':'Maria', 'likes':['dancing', 'skiing'], 'food_intolerances':['nuts']}}

import json
file = open('friends.db', 'w')
json.dump(friends, file)

# Remove all variables from kernel namespace

import json
file = open('friends.db', 'r')
friends = json.load(file)

friends
friends['friend1']
friends['friend1']['name']
friends['friend2']['likes']

friends['friend3'] = {'name':'Jane', 'age':22, 'likes':['hiking', 'dancing'], 'food_intolerances':['gluten', 'milk']}

friends['friend1']['likes'].append('reading')

del friends['friend3']['age']

del friends['friend1']

file = open('friends.db', 'w')
json.dump(friends, file)

Dictionaries are less relevant to the exam; do not announce this explicitly at the beginning. They may appear in a multiple-choice question.

Generated file

friends.db is preserved in the exercise table, but it is not supplied as input material. Friends.py generates it at runtime.

Summary and announcements

TODO

Notes for improvement

Take notes on improvements and common questions during the session and add them to feedback.qmd afterwards.

Slide 115 — break and continue

Slide 116 — Loop termination

Slide 118 — Defining functions

Slide 120 — Reusing functions

Slides 121–122 — Functions

Slide 125 — Using an external module

Slide 127 — Creating a dictionary

Slide 128 — Accessing and modifying dictionaries

Slide 129 — Complex and nested dictionaries

Slide 130 — Accessing nested dictionary values

Slide 131 — Adding, changing, and deleting nested data

Slide 132 — Save and load dictionaries with JSON

Slide 133 — Dictionaries and persistence

Summary and announcements

Slide 115 — `break` and `continue`