Skill 9: Iteration with Lists (Python)

1. Understanding Local Variables in Loops

What to Teach

Local variables store and accumulate values during iteration. Python uses regular variable assignment (no special var keyword needed) and standard assignment operators.

Basic Variable Usage

# initialize accumulator variables
total = 0
count = 0

# variables can be updated
total = total + 10
count = count + 1

total += 10
count += 1

When to Use Local Variables

In python, mutation is used pervasively. In our class, we try to only use it for accumulating values during iteration, though as students encounter libraries that rely on mutation, they will inevitably have to understand other uses, e.g.:

• Counting occurrences
• Building results incrementally
• Tracking state that changes over time

Teaching Examples

Good Example:

def calculate_total(prices: list[float]) -> float:
    """calculate total of all prices"""
    total = 0.0
    
    for price in prices:
        total += price
    
    return total

def test_calculate_total():
    assert calculate_total([1.0, 2.5, 3.0]) == 6.5
    assert calculate_total([]) == 0.0
    assert calculate_total([10.0]) == 10.0
    assert calculate_total([1.25, 2.75, 0.5]) == 4.5

What to Point Out:

• Variables are initialized before the loop
• Use meaningful names that describe what's being accumulated
• Variables maintain their values across loop iterations

Common Student Mistakes to Watch For

1. Forgetting to initialize accumulator variables

   # BAD - total not initialized
   def bad_sum(numbers: list[int]) -> int:
       for num in numbers:
           total += num # total is not defined
       return total

2. Declaring variables inside the loop

   # BAD - reinitializing inside loop
   def bad_accumulate(numbers: list[int]) -> int:
       for num in numbers:
           total = 0 # gets reset every iteration
           total += num
       return total

---

2. Basic for ... in Loop Structure

What to Teach

Python's for ... in loops iterate through every element in a list. The loop variable takes on the value of each element in sequence.

Basic Syntax

for element in list:
    # do something with element

Teaching Examples

Examples:

def sum_numbers(numbers: list[int]) -> int:
    """calculate sum of all numbers in list"""
    total = 0
    
    for number in numbers:
        total += number
    
    return total

def test_sum_numbers():
    assert sum_numbers([1, 2, 3, 4, 5]) == 15
    assert sum_numbers([]) == 0
    assert sum_numbers([-1, 1, -2, 2]) == 0
    assert sum_numbers([10]) == 10

def count_long_words(words: list[str]) -> int:
    """count words longer than 5 characters"""
    count = 0
    
    for word in words:
        if len(word) > 5:
            count += 1
    
    return count

def test_count_long_words():
    assert count_long_words(["hello", "world", "python", "programming"]) == 2
    assert count_long_words(["cat", "dog", "bird"]) == 0
    assert count_long_words([]) == 0
    assert count_long_words(["elephant", "mouse", "rhinoceros"]) == 2

What to Point Out:

• Loop variable (number, word) represents current element
• Loop body executes once for each list element
• Loop variable is automatically updated by Python
• Use descriptive names for loop variables

Common Student Mistakes to Watch For

1. Using index-based loops unnecessarily

   # BAD - using indices when not needed
   def bad_sum(numbers: list[int]) -> int:
       total = 0
       for i in range(len(numbers)):
           total += numbers[i] # should use `for num in numbers`
       return total

2. Trying to modify the loop variable

   # BAD - trying to change loop variable
   def bad_process(numbers: list[int]) -> list[int]:
       result = []
       for num in numbers:
           num = num * 2 # this doesn't affect the original list
           result.append(num)
       return result

3. Confusing element vs index

   # BAD - treating element as index
   def bad_access(items: list[str]) -> str:
       result = ""
       for item in items:
           result += items[item] # item is the value, not index
       return result