Loops

Loops#

Course Announcements

Due this week:

VQ8 due Wed
CL4 due Fri
A3 due Sun
Take your exam!

Control Flow - Loops#

while
for
range, continue, break

SideNote: counters#

# Initialize a counter variable
counter = 0 
print(counter)

counter = counter + 1
print(counter)

counter = counter + 1
print(counter)

# shorthand - does same as above using +=
counter += 1
print(counter)

The idea here…is that as the code executes, the value of the counter increases. We’ll use these a lot in loops!

SideNote: `append`#

append is a list methods (we’ll talk about others soon) that adds an item to the end of an existing list in place

my_list = ['a', 'b', 'c']
my_list.append('d') ### NOTE THAT IT IS NOT ASSIGNED BACK TO my_list
my_list

['a', 'b', 'c', 'd']

# this is wrong - dont do this
my_list = ['a', 'b', 'c']
my_list = my_list.append('d') # DON'T DO THIS - avoid assigning back to a list
print(my_list)

None

Loops#

A loop is a procedure to repeat a piece of code.

`while` Loops#

A while loop is a procedure to repeat a piece of code while some condition is still met.

`while` Loop: live demo#

while is_sitting(studentA):
    # studentB moves one sheet of paper from podium to table

`for` Loops#

A for loop is a procedure a to repeat code for every element in a sequence.

`for` Loop: live demo#

students_in_line = ['Yawen', 'Rosalinda', 'Nathan']

brown_hair = []
other_hair = []

for student in students_in_line:
    if has_brown_hair(student):
        brown_hair.append(student)
    else:
        other_hair.append(student)         

Activity I: Comprehension Check#

Complete all questions in this Google Form (https://forms.gle/gV3w2GhNgD6Hh6WF8) and then click submit.

# TEST THINGS OUT
this_list = ['hi', 'bye', 'good', 'bad']
this_list.append('ok')
# this_list = this_list + ['ok']
this_list

['hi', 'bye', 'good', 'bad', 'ok']

Loops: Code#

`while` Loop: code#

shopping_budget = 20
bill = 0
index = 0
prices = [3, 4, 10, 3, 2, 15, 7]

while bill < shopping_budget:
    
    # add cost of item (prices) to bill
    bill = bill + prices[index]
    
    # increment index each time through the loop
    index = index + 1
    
    #print bill so we can see what's going on
    print(bill)

`for` loop: code#

# Define a list of items
shopping_list = ['bananas', 'cookies', 'ice cream']
in_stock = ['apples', 'bananas', 'cookies', 'chicken', 'cucumbers']
in_cart = []

# Loop across each element
for item in shopping_list:
    # print(item)
    if item in in_stock:
        in_cart.append(item)
    else: 
        print(item + ' not in stock')

print(in_cart)
print(item)

ice cream not in stock
['bananas', 'cookies']
ice cream

# Loop across items in a string
vowels = ['A', 'E', 'I', 'O', 'U', 'a', 'e', 'i', 'o', 'u']
my_string = 'python'

for char in my_string:
    if char not in vowels:
        print(char)

p
y
t
h
n

Activity II: `loops`#

Complete the coding question in this Google Form (https://forms.gle/isggkAXwhMy9BN9h6) and then click submit.

Write a function sum_odd() containing a loop that will add up all the odd numbers in an input list.

For example: sum_odd(['a', 3, 5, 6]) would return 8, b/c 3 + 5 is 8

# WRITE CODE
# define a function - input will be some list
# loop over each thing in the input list
# if thing is an integer and that integer is odd (% 2 -> 0....even)
def sum_odd(input_list):
    
    out_sum = 0
    
    for val in input_list:
        if type(val) == int and val % 2 !=0:
            out_sum = out_sum + val

    return out_sum

# can i do this using append?
# another possible solution
def sum_odd(input_list):
    
    val_list = []
    
    for val in input_list:
        if type(val) == int and val % 2 !=0:
            val_list.append(val)

    out_sum = sum(val_list) #add all numbers in val_list together

    return out_sum

sum([1,2,3])

# TEST FUNCTION
sum_odd(['a', 3, 5, 6])

Dictionaries: Indexing & Looping#

Reminder: Dictionaries are indexed by their key.

# recall: collections lecture
scores = {
    'Alondra': 85,
    'Holly': 58,
    'Brooke': 92,
    'Pauline': 47,
    'Minqi': 76
}

scores['Holly']

passing_score = 60
passed_students = []

for student in scores:
    if scores[student] >= passing_score:
        # print(student)
        passed_students.append(student)

print("Students who passed:", passed_students)

Students who passed: ['Alondra', 'Brooke', 'Minqi']

Q&A

Q: How does python know that item (for the grocery example) is the items on the list.
A: Because we are using index to specify the position (index) in the list and adding that to bill.

Q: do we have to define the variables every single time, or just in some of them
A: To use a variable, it must previously be defined. If in a function, must be defined inside the function definition or passed in as an input.

Q: I am confused why we used index in the while example. what would happen if we didn’t have index?
A: We would not have been able to specify which value in bills to add.

Q: Loops are more confusing than the concepts we have learned. How should I practice to familiarize with loops?
A: Deeply learn the concept first (know what a for and while loop are conceptually). Then, familiarize with the code. Then practice examples on your own. Explaining out loud what’s going on each iteration in the loop is great practice!

Q: May you explain the clearest difference between for and while loops?
A: for loops loop over a collection. while loops loop until a condition is no longer True.

Course Announcements

Due:

CL4 due Fri
A3 due Sun
Take your exam!

Notes:

E1 scores/exams will be available to view on PrairieLearn at 5PM Friday
- can sign up in PrairieTest for E1 retake on Friday 5/1 (tomorrow)
- can “report an issue” if you believe you were graded incorrectly on anything
Mid-course survey will be available on Saturday (linked from Canvas and course website); optional for EC
(to self) Discuss outcomes from Loops II activity

`range`, `continue` and `break`#

Activity III: Dictionaries#

Complete the coding question in this Google Form (https://forms.gle/RocHJ6rmMTxWYHyH8) and then click submit.

Define a function name_dictionary that will generate a dictionary that stores each unique letter in the input name as a different key, and the number of times each letter shows up in your name as the letter’s value.

For example, name_dictionary('Shannon')would return: {'S':1, 'h':1, 'a': 1, 'n': 3, 'o': 1}

An aside…there’s a shortcut! The += operator

out = out + 4

# OR 

out += 4

# THE CODE

# TEST IT OUT

`range`#

range is an operator to create a range of numbers, that is often used with loops.

for temp in [114, 115, 116, 117, 118]:
# for temp in range(114, 119): 
    print(temp)
    
    if(temp > 115):
        print('The tea is too hot!')

`continue`#

continue is a special operator to jump ahead to the next iteration of a loop.

for number in range(1, 11):
    if number % 2 == 0:
        continue  # Skip even numbers

    print(number)

`break`#

break is a special operator to break out of a loop.

`break` examples#

numbers = [4, 7, 10, 3, 8, 5]

for num in numbers:
    if num == 8:
        print("Found 8!")
        break  # Stop the loop as soon as we find 8

Altogether#

It is possible to use these all together…

find_first_even finds the first even number in a range
if that number is odd, continue
once you find the first even number break

numbers = [1, 3, 7, 9, 6, 4, 11, 6, 8]
range(len(numbers))

def find_first_even(numbers):
    for i in range(len(numbers)):
        print(i)
        if numbers[i] % 2 != 0:
            continue 
        out = numbers[i]
        break 
    return out

find_first_even(numbers = [1, 3, 7, 9, 6, 4, 11, 6, 8])

Activity IV: `range`, `break`, `continue`#

Please complete the Google Form here: https://forms.gle/SsRvuJ7XT1zqy7hZA

Define a function find_all_neg() that will extract all negative values from a list, returning the list of negative numbers.

You can assume all elements in the input list are numeric.

## FUNCTION

## TEST IT OUT

Code Style: Loops#

for/while statement with a colon at the end on first line
all code within the loop inside a code block (indented)

Good Code Style

number = 5
while number < 0:
    print(number)
    number = number + 1

Code Style to Avoid

number=5
while number<0:print(number);number=number+1 # avoid all on a single line

Activity V: Additional Loops Practice#

Please complete the Google Form here: https://forms.gle/yhCTF8qbhkyFHPeE9

Q1. Define a function remove_special_characters() that will remove any characters from a string if they are one of these characters: '!@#$%^&*();<>?/'

# FUNCTION
# input: 'hello!@'
# output: 'hello'

# TEST IT OUT

Loops

Contents

Loops#

Control Flow - Loops#

SideNote: counters#

SideNote: append#

Loops#

while Loops#

while Loop: live demo#

for Loops#

for Loop: live demo#

Activity I: Comprehension Check#

Loops: Code#

while Loop: code#

for loop: code#

Activity II: loops#

Dictionaries: Indexing & Looping#

range, continue and break#

Activity III: Dictionaries#

range#

continue#

break#

break examples#

Altogether#

Activity IV: range, break, continue#

Code Style: Loops#

Activity V: Additional Loops Practice#

SideNote: `append`#

`while` Loops#

`while` Loop: live demo#

`for` Loops#

`for` Loop: live demo#

`while` Loop: code#

`for` loop: code#

Activity II: `loops`#

`range`, `continue` and `break`#

`range`#

`continue`#

`break`#

`break` examples#

Activity IV: `range`, `break`, `continue`#