Python Practice 21/22

Fix the bug: the till is a penny short

The shop started charging 10c for a carrier bag last month. Since then the end-of-day reconciliation has drifted - not every day, and never by much. A penny here, a penny there. Nobody can find the broken line, because no line looks broken.

total_cents takes a basket of prices in dollars and returns the total in whole cents, which is the only thing the card processor accepts. Today's basket is a $2.50 notebook, a $1.25 pen, a $0.75 eraser and the 10c bag. Add it up on paper: $4.60, so 460 cents. Run the code. It prints 459.

Nothing throws. Nothing warns. The number even looks reasonable - without the receipt in your hand you would never think to question it. The till just charges a penny less than it should, and the books stop balancing.

The part worth noticing: this code was correct for a year. $2.50, $1.25 and $0.75 are halves and quarters, and a float stores those exactly. The bag is the value that broke it.

Your task: fix total_cents so it returns the exact number of cents in the basket. Today's basket is 460.

You'll practice:

Checking a plausible-looking number against the answer a human would write down
Converting money to whole cents before the arithmetic instead of after

# The card processor only takes whole cents. This basket - a $2.50 notebook, a
# $1.25 pen, a $0.75 eraser and a 10c bag - is $4.60, so this should print 460.
# It runs without a single error. It prints 459. Fix total_cents.
def total_cents(prices):
    total = 0.0
    for price in prices:
        total += price
    return int(total * 100)

print(total_cents([2.50, 1.25, 0.75, 0.10]))