5. Lighthouse

Level 2 · How many boxes does the warehouse need?

From Debra Marshall, Warehouse Manager

“We have an order for 23 soy candles and each shipping box holds 6. How many boxes do I need to pull? A half-full box still counts as a box.”

Your goal

Set result to the number of boxes needed as an int: every box holds per_box candles and a partly filled box still counts as one box.

  • Store a number in result

Your code

Python

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    ResultPress Run to see yours

    Mote checks your result

    Press Run. Each row that matches the goal becomes a firefly in your lantern.

    The lesson

    Three kinds of division

    Python has three operators that deal with division, and analysts use all of them.

    23 / 6     # 3.8333333333333335  true division, always a float
23 // 6    # 3                   floor division, whole part only
23 % 6     # 5                   modulo, the remainder

    / gives the exact answer as a float. // throws away the fractional part and rounds down to the whole number below. % (read “modulo” or “mod”) gives what is left over after the floor division. The two always fit together:

    (23 // 6) * 6 + (23 % 6)   # 18 + 5 = 23

    Where analysts meet this

    • Packing and batching: how many full pallets, how many boxes, how many pages of 50 results.
    • Time conversions: 135 // 60 is 2 hours and 135 % 60 is 15 minutes.
    • Every Nth row: order_id % 10 == 0 picks one order in ten for a quick sample.
    • Even and odd: n % 2 == 0 is true for even numbers.

    Rounding up with math.ceil

    Floor division always rounds down, which is wrong when a partial box still has to ship. You have two clean options.

    The first uses the math module, which ships with Python. import makes it available, and math.ceil (short for ceiling) rounds up to the next whole number:

    import math

math.ceil(23 / 6)   # 4
math.ceil(24 / 6)   # 4  exactly 4 boxes, no extra
math.floor(23 / 6)  # 3

    The second uses // and % together: take the full boxes, then add one more box if anything is left over. You’ll write that kind of logic yourself once you learn if in a later level.

    math.ceil returns an int, which is exactly what you want for a count of boxes. Compare this with the built-in round(), which rounds to the nearest number: round(23 / 6) is 4 here, but round(25 / 6) is also 4 when you need 5 boxes. Rounding to nearest is the wrong tool for “how many containers”.

    Converting between int and float

    Sometimes you need to change a number’s type on purpose:

    int(3.83)      # 3   cuts off the decimals (towards zero)
float(4)       # 4.0
int("23")      # 23  text that looks like a number becomes a number

    int() truncates rather than rounds, so int(3.99) is 3. Don’t use it as a rounding function.

    Common mistakes

    • Using // when the business rule says partial units count. 23 candles in boxes of 6 is 4 boxes, not 3.
    • Using round() for container counts. It rounds to the nearest number, which can leave candles on the floor.
    • Forgetting import math. Without it, math.ceil fails with NameError: name 'math' is not defined.

    Interview note

    “What does % do?” comes up surprisingly often in screening tests, usually as “find even order IDs” or “convert minutes into hours and minutes”. Knowing that // and % are a matched pair covers both.