Session 09/Python 6: For and while loops

Explain loop mechanics with sums and products, then distinguish the planned in-class and homework exercises.

flowchart

  subgraph AFTER_BREAK["<b>After break: while loops</b>"]
    direction TB
    B1["<b>Condition-controlled while loops</b> (25 min)"]
    B2["<b>💻 Exercise: Number guessing 1</b> (15 min)"]
    B3["<b>💻 Exercise: Number guessing 2</b> (18 min)"]
    B4["<b>💻 Exercise: Repeated dice game</b> (20 min)"]
    B5["<b>Homework: shopping list and loan</b> (9 min)"]
    B6["<b>Loop review and questions</b> (3 min)"]
    B1 --> B2 --> B3 --> B4 --> B5 --> B6
  end

  subgraph BEFORE_BREAK["<b>Before break: for loops</b>"]
    direction TB
    A1["<b>Ranges and for loops</b> (25 min)"]
    A2["<b>Demo: loop sums and products</b> (10 min)"]
    A3["<b>💻 Exercise: Message</b> (13 min)"]
    A4["<b>💻 Exercise: Average</b> (17 min)"]
    A5["<b>💻 Exercise: Fibonacci</b> (20 min)"]
    A6["<b>for-loop homework</b> (5 min)"]
    A1 --> A2 --> A3 --> A4 --> A5 --> A6
  end

Slide 109 — Ranges and `for` loops

range() accepts a start, an exclusive stop, and an optional step.

list(range(10))
list(range(6, 10))
list(range(1, 10, 2))

for i in range(3):
    print(i)

for i in [2, 9, 7]:
    x = i * 2
    print(x)

for i in ['apple', 'orange']:
    print(i)

for can iterate over ranges and directly over list elements.

Slides 110–111 — `for` loops

Explain loops using \(\sum_i^n\) and \(\prod_i^n\).

In-class demo for_demo.py

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


n = 5
for i in range(n):
    print(i, end="")


n = 5
for i in range(n):
    for j in range(n):
        print(i, j)


n = 5
for i in range(n):
    for j in range(n):
        print(i, j, end=" | ")
    print()


n = 5
for i in range(n):
    for j in range(i+1):
        print(i, j, end=" | ")
    print()


s = "Hallo"
n = 5
for i in range(n):
    print(s[i])


s = "Hallo"
for i in range(len(s)):
    print(s[i])


s = "Hallo"
for i in range(len(s)):
    print(s[i], end="")


s = "Hallo"
for i in range(len(s)):
    for j in range(i+1):
        print(s[i], end="")
    print()


s = "Hallo"
for i in s:
    print(i)

Note: 3–4 and 6–7 are homework.

Write a program that will ask the user for a message and the number of times they want that message displayed.

In-class exercise Message.py

message = input("Enter the message: ")
n = int(input("How many times should the message be displayed: "))
for i in range(n):
    print(message)

Write a program that will calculate the average (mean) of a set of numbers.

In-class exercise Average.py

n = int(input("How many numbers do you want to enter: "))
numbers = []
for i in range(n):
    numbers.append(int(input("Enter number: ")))
sum = 0
for i in range(n):
    sum = sum + numbers[i]
average = sum/n
print("The average is: ", average)

Write a program that will output the numbers from 1 to 100.

Homework Tens.py

for i in range(1,101):
    if i%10==0:
        print(i, " divisible by 10!")
    else:
        print(i)

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9.

Homework MultipleSum.py

msum = 0
n = int(input("Enter the upper bound: "))
for i in range(1,n):
    if(i%3==0 or i%5==0):
        msum = msum + i
print(msum)

Write a program that generates a list containing the Fibonacci sequence.

In-class exercise Fibonacci.py

n = int(input("How many numbers should be generated: "))
fib = [0,1]
for i in range(n):
    fib.append(fib[-2]+fib[-1])
print(fib)

Write a program to generate n random numbers between 1 and 10.

Homework Frequencies.py

import random

n = int(input("How many random numbers do you want to generate? "))
freqlist = [0,0,0,0,0,0,0,0,0,0]
for i in range(n):
    x = random.randint(1,10)
    freqlist[x-1] = freqlist[x-1] + 1
print(freqlist)

Write a program that displays a word entered by the user as a triangle of letters.

Homework Triangle.py

word = input("Enter a word: ")

for i in range(len(word)):
    for j in range(i+1):
        print(word[i], end="")
    print()

Break — 10 minutes

Slide 112 — `while` loops

A while loop repeats while its condition remains true. Its body must change the relevant state to avoid an infinite loop.

count = 0

while count < 4:
    print("The count is:", count)
    count = count + 1

names = ['Ann', 'Tom', 'Peter', 'Jens', 'Sue']

while names[0] != 'Jens':
    print(names[0])
    names.remove(names[0])

Slides 113–114 — `while` loops

Assign exercise 1, 5 as homework.

Create a program that will keep track of items for a shopping list.

Homework ShoppingList.py

sl = []
item=input("Insert list item: ")
while item != "":
    sl.append(item)
    item=input("Insert list item: ")
for i in sl:
    print(i)


sl = []
item="x"
while item != "":
    item=input("Insert list item: ")
    if item!="":
        sl.append(item)
for i in sl:
    print(i)


sl = []
item="x"
while item != "":
    item=input("Insert list item: ")
    if item=="":
        break
    sl.append(item)
for i in sl:
    print(i)

Write a Python program to guess a number between 1 to 10.

In-class exercise Guess1.py

import random
target_num = random.randint(1, 10)
guess_num = 0
while target_num != guess_num:
    guess_num = int(input('Guess a number between 1 and 10 until you get it right: '))
print('Well guessed!')

Rewrite the program to have the user guess a number between 1 to 100

In-class exercise Guess2.py

import random
target_num = random.randint(1, 100)
guess_num = 0
while target_num != guess_num:
    guess_num = int(input("Guess a number: "))
    if(target_num<guess_num):
        print("lower")
    elif(target_num>guess_num):
        print("higher")
print('Well guessed!')

Expand the previously created dice game.

In-class exercise dicegame2.py

import random

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

while continue_:
    dice1 = random.randint(1,6)
    dice2 = random.randint(1,6)

    if dice1 == dice2:
        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 they lost!!! The throw brought", dice1, "and", dice2)
        continue_ = False

Write a program to create the history of a loan.

Note: Remind students that exercise 5 is the same task they completed in Excel.

Homework Loan.py

amount = float(input("Amount of the loan: "))
repayment = float(input("Repayment: "))
irate = float(input("Interest rate (in % and annual): "))

period = 0
while amount>0.0:
    period += 1
    interest = amount * irate/100.0/12.0
    if (amount>repayment):
        amount = amount - repayment
    else:
        repayment = amount
        amount = amount - repayment
    print ("Month:", period, " Repayment:", repayment, " interest:", round(interest,2), " Remaining:", amount)

Summary and announcements

Notes for improvement

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

Slide 109 — Ranges and for loops

Slides 110–111 — for loops

Slide 112 — while loops

Slides 113–114 — while loops

Summary and announcements

Slide 109 — Ranges and `for` loops

Slides 110–111 — `for` loops

Slide 112 — `while` loops

Slides 113–114 — `while` loops