Problem Solving with Python

Breaking Down Problems

The most important skill in programming is not memorising syntax -- it is problem decomposition: breaking a large, complex problem into smaller, manageable parts. Each small part can then be solved independently and combined into a complete solution.

Follow this four-step process:

From Pseudocode to Python

Pseudocode is an informal, plain-English description of what your program should do. It lets you focus on the logic without worrying about syntax. Once your pseudocode is clear, translating it to Python is straightforward.

SET total to 0
SET count to 0
FOR each number in the list:
    ADD number to total
    ADD 1 to count
SET average to total / count
PRINT average

total = 0
count = 0
for number in numbers:
    total += number
    count += 1
average = total / count
print(average)

Notice how each line of pseudocode maps almost directly to a line of Python. Good pseudocode is language-independent -- anyone should be able to understand it, even if they do not know Python.

A Complete Example: Password Validator

Problem: Write a program that checks if a password meets these rules: at least 8 characters long, contains at least one digit, and contains at least one uppercase letter.

PSEUDOCODE:
GET password from user
SET is_long to length of password >= 8
SET has_digit to False
SET has_upper to False
FOR each character in password:
    IF character is a digit: SET has_digit to True
    IF character is uppercase: SET has_upper to True
IF is_long AND has_digit AND has_upper:
    PRINT "Password is strong"
ELSE:
    PRINT which rules are not met

def validate_password(password):
    is_long = len(password) >= 8
    has_digit = False
    has_upper = False

    for char in password:
        if char.isdigit():
            has_digit = True
        if char.isupper():
            has_upper = True

    if is_long and has_digit and has_upper:
        return "Password is strong!"
    else:
        issues = []
        if not is_long:
            issues.append("at least 8 characters")
        if not has_digit:
            issues.append("at least one digit")
        if not has_upper:
            issues.append("at least one uppercase letter")
        return "Password needs: " + ", ".join(issues)

# Testing
print(validate_password("Hello123"))   # Strong!
print(validate_password("abc"))        # Needs: 8 chars, digit, uppercase

Key Vocabulary

Worked Examples

# Pseudocode: SET largest to first item, loop through rest, compare
def find_largest(numbers):
    largest = numbers[0]
    for num in numbers[1:]:
        if num > largest:
            largest = num
    return largest

print(find_largest([34, 67, 12, 89, 45]))  # Output: 89

def count_vowels(text):
    vowels = "aeiouAEIOU"
    count = 0
    for char in text:
        if char in vowels:
            count += 1
    return count

print(count_vowels("BrightPath AI Tutoring"))  # Output: 7

def reverse_list(items):
    reversed_items = []
    for i in range(len(items) - 1, -1, -1):
        reversed_items.append(items[i])
    return reversed_items

print(reverse_list([1, 2, 3, 4, 5]))  # Output: [5, 4, 3, 2, 1]

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.