6. Lighthouse

Level 2 · Total a cart with a loop

From Jennifer Castillo, Support Team Lead

“Our order lookup shows quantities and prices in two separate columns. Can you add up this cart for me? Each quantity lines up with the price in the same position.”

Your goal

Set result to the cart total as a float: the sum of quantity times price for each position in quantities and prices, rounded to 2 decimals.

  • Store a number in result

Your code

Python

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    The lesson

    Repeating work with for

    A for loop runs the same block of code once for every item in a list:

    prices = [49.99, 129.00, 12.50]
for price in prices:
    print(price)

    On each pass, the loop variable (price) holds the next item. The indented lines are the loop body. When the list runs out, the loop ends and Python continues with the first unindented line.

    The accumulator pattern

    The most common loop in analysis code adds things up. You create a variable before the loop, then update it on every pass:

    total = 0
for price in prices:
    total += price      # same as total = total + price

    After the loop, total is 191.49. The same pattern counts things (count += 1), finds the largest value so far, or builds up a new list with append.

    Looping over two lists together

    Here quantities and prices sit in separate lists that line up by position. zip pairs them:

    quantities = [2, 1, 4]
prices = [49.99, 129.00, 12.50]

list(zip(quantities, prices))
# [(2, 49.99), (1, 129.0), (4, 12.5)]

    You can unpack each pair straight into two loop variables:

    total = 0
for qty, price in zip(quantities, prices):
    total += qty * price

    This reads almost like English: for each quantity and price, add the line total. If the lists have different lengths, zip stops at the shorter one without an error, so make sure your data lines up before trusting the result.

    When you need the position

    enumerate gives you the index alongside each item, which helps when you want to label lines:

    for i, price in enumerate(prices, start=1):
    print(f"Line {i}: ${price:.2f}")

    Avoid the older style for i in range(len(prices)): with prices[i] inside. It works, but zip and enumerate are clearer and harder to get wrong.

    Don’t forget the built-ins

    When you only need a plain sum of a list, sum(prices) is simpler than a loop. Loops earn their place when each step does some work first, such as multiplying by a quantity or skipping bad rows.

    Common mistakes

    • Putting total = 0 inside the loop. It resets on every pass, so you end up with only the last line.
    • Forgetting to indent the body. Python raises IndentationError or, worse, runs the line once after the loop ends.
    • Rounding inside the loop. Round the final total once.

    A look ahead

    In pandas you’ll rarely write this loop: (df["quantity"] * df["unit_price"]).sum() does the same thing for a whole column at once, and much faster. But knowing what that line does under the hood makes it far easier to debug.