Calculating totals

Learn to compute the subtotal and the tax-inclusive total.

Calculating totals (sumOf)

To sum the prices of all items, use sumOf. it represents each item in the list.

val total = cart.sumOf { it.price }
println(total)  // 8.0

Here it.price takes each item's price and adds them. With two items (5.0 + 3.0) the result is 8.0.

sumOf is a list function; it takes an expression in curly braces. it is Kotlin's automatic name; no loop variable needed. You'll see the details in the arrays topic.

Exercise Calculate and print the total

Add val total = cart.sumOf { it.price } to the previous cart (two items). Print the total with println(total). The total should be 8.0.

Show example solution

data class Product(val name: String, val price: Double)

fun main() {
    val cart = mutableListOf<Product>()
    cart.add(Product("Apple", 5.0))
    cart.add(Product("Bread", 3.0))
    val total = cart.sumOf { it.price }
    println(total)
}

Tax calculation

For a tax-inclusive total, the tax rate is applied to the subtotal. For example 18% tax: val tax = total * 0.18.

val subtotal = cart.sumOf { it.price }
val tax = subtotal * 0.18
val grandTotal = subtotal + tax
println(grandTotal)  // 9.44

Here 8.0 + 1.44 = 9.44. The tax rate 0.18 (18%) is multiplied by the subtotal.

In this lesson we only calculate the subtotal; tax can be added later.

Test yourself

Complete the sumOf call

val total = cart.____ { it.price }

Show answer

val total = cart.sumOf { it.price }

What does cart.sumOf { it.price } do?
- Sums the prices
- Gives the item count
- Takes the first item
- Empties the list
Show answer

Sums the prices — sumOf { it.price } takes each item's price and adds them.
Match the total concepts

sumOf { it.price }

Price sum
it

List item
cart.size

Item count
total

Subtotal

Next lesson: Listing the cart

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