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LESSON 21 OF 414 · CALCULATIONS WITH NUMBERS

Price After Discount in Scala: Computing with Percentages

Lesson Read the full explanation

We created two variables to store the original price of a product in dollars and the discount percentage applied to that price.

Use these two variables and arithmetic operations to compute and print the price after the discount.


Example:

For example, if price was 150 and discount was 10, the answer would be 135.

Why? Because the discount is 10%. 10% of 150 dollars is 15 dollars. And 150 - 15 dollars is 135 dollars.

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    SCALA · CALCULATIONS WITH NUMBERS

    Price After Discount in Scala, explained

    The task

    The starter code holds two values: the original price of a product in dollars and the discount as a percentage. The exercise is to print the price you pay after the discount. For a price of 150 and a discount of 10, the discount is 15 dollars and the final price is 135.

    Two ways to see it

    There are two equivalent routes to the same number.

    • Subtract the discount. Compute the discount amount, price * discount / 100, and take it off the price: price - price * discount / 100.
    • Keep the remaining share. A 15% discount means you still pay 100 - 15 = 85 percent of the price. So the final price is price * (100 - discount) / 100.

    The second route needs one multiplication and one division, and price appears only once in it. The lesson uses it.

    Step by step

    1. Work out the percentage you keep: 100 - discount. With a discount of 15.0 this is 85.0. Wrap it in parentheses so the subtraction happens before the multiplication.
    2. Multiply the price by that percentage: 70.0 * 85.0 is 5950.0.
    3. Divide by 100 to turn the percentage into a part of the price: 5950.0 / 100 is 59.5.
    4. Print the result with println.

    The solution in Scala

    val price = 70.0
val discount = 15.0
println(price * (100 - discount) / 100)

    The output is:

    59.5

    The subtracting version prints the same value for these numbers: price - price * discount / 100 also gives 59.5.

    Details that matter in Scala

    The values are written 70.0 and 15.0, with a decimal point, so Scala infers the type Double for both. Writing val price = 70 would infer Int instead. A Double calculation keeps decimals, and its output always includes a decimal part: with a price of 150.0 and a discount of 10.0, the same expression prints 135.0, not 135.

    The parentheses are not optional. Compare:

    println(price * (100 - discount) / 100)
println(price * 100 - discount / 100)

    The second line subtracts only 0.15 from 7000.0 and prints 6999.85, which is not a discounted price. Multiplication and division run before subtraction, so a subtraction that must happen first needs parentheses.

    If the values were Int, integer division would matter too. With a price of 70 and a discount of 15, price * (100 - discount) / 100 is 5950 / 100, which truncates to 59. The .5 is lost, so a price with cents needs Double.

    Edge cases and common mistakes

    • Printing the discount instead of the price. The amount taken off is 10.5 here. The price you pay is 59.5.
    • Using the discount as a fraction. price * discount multiplies by 15, not by 0.15. The percentage must be divided by 100.
    • A discount of 0 or 100. A discount of 0 returns the full price, and a discount of 100 returns 0.0. The formula covers both.
    • Hard-coded numbers. Use price and discount, so the answer follows the values.

    Key takeaways

    • The price after a discount is price * (100 - discount) / 100.
    • Parentheses make the subtraction happen before the multiplication.
    • Use Double for prices, since Int division drops the fraction.
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