Unit 4 · Lesson 3

Iterating Over Lists

Combine loops with lists to search, filter, transform, and analyze data.

Use for item in list to iterate

Access indices with enumerate()

Build filtered lists

Find max/min values and positions

Looping Through Lists

The most natural way to process every item in a list:

colors = ["red", "green", "blue"] for color in colors: print(color) # red, green, blue

When You Need the Index Too

# Option 1: enumerate() — preferred! for i, color in enumerate(colors): print(f"{i}: {color}") # Option 2: range(len()) — when you need index math for i in range(len(colors)): print(f"{i}: {colors[i]}")

Common list patterns: Sum all values (accumulator), find max/min, filter into a new list, transform each element, search for a value. You learned the patterns in Unit 3 — now apply them to real lists!

Practice Time

Challenges

📊

Challenge 1: Grade Calculator

Guided

Calculate the average of a list of grades.

Instructions: Given grades, calculate the total and average.

Hint: for g in grades: total += g

🔍

Challenge 2: Search & Count

Guided

Count how many times a target appears in a list.

Instructions: Count how many times target appears in numbers. Store in count.

Hint: for n in numbers: if n == target: count += 1

🏆

Challenge 3: High Scores

Solo

Find the highest and lowest scores and their positions.

Instructions: Find highest, lowest, high_pos (index of highest), and low_pos (index of lowest). Don't use built-in max()/min().

Hint: Use for i, s in enumerate(scores): then compare s to highest/lowest, updating both value and position.

🔽

Challenge 4: List Filter

Stretch

Build a new list containing only the even numbers.

Instructions: From numbers, create evens containing only the even values (in order).

Hint: for n in numbers: if n % 2 == 0: evens.append(n)