Lecture 10 – More Iteration (for loops)

Data 6, Summer 2021

Review

for n in [2, 4, 6, 8]:
    print(n * 5)

10
20
30
40

Quick Check 1

# Don't run this cell until after you've guessed what it does!
actions = ["ate", "slept", "drank"]
feelings = ["happy", "energized", "confused"]

for k in range(len(actions)):
    print("I " + actions[k] + " and I am " + feelings[-k-1] + ".")

I ate and I am confused.
I slept and I am energized.
I drank and I am happy.

Example: missing number

Quick Check 2

def missing_number(nums):
    for n in range(1, len(nums) + 2):
        if n not in nums:
            return n

# Should be 3
missing_number([1, 2, 6, 4, 5])

3

# Should be 6
missing_number([7, 2, 3, 5, 9, 8, 4, 1])

6

Example: Luhn's algorithm

# Ignore this code
def int_to_list(n):
    return [int(i) for i in str(n)]

int_to_list(5457623898234113)

[5, 4, 5, 7, 6, 2, 3, 8, 9, 8, 2, 3, 4, 1, 1, 3]

def luhns_algorithm(cc):
    # Step 1
    check_digit = cc[-1]
    
    even_sum = 0
    for i in range(0, len(cc), 2):
        # Step 2
        even_element = cc[i] * 2
        if even_element > 9:
            even_element = even_element - 9
            
        # Step 3
        even_sum += even_element
    
    # Step 4
    odd_sum = 0
    for i in range(1, len(cc) - 2, 2):
        odd_sum += cc[i]
        
    # Step 5
    total_sum = even_sum + odd_sum
    
    # Step 6
    return (total_sum + check_digit) % 10 == 0

luhns_algorithm(int_to_list(5457623898234113))

True

What if I accidentally swap two digits?

luhns_algorithm(int_to_list(5475623898234113))

False

Now Luhn's algorithm can tell me the credit card number is invalid, which likely means I made a typo.