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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.

Practice 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.

Starter code

Main.kt
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))
    // Calculate and print the total
}

Sample solution

Main.kt
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)
}

Expected output

8.0

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.

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Complete the sumOf call

Question 
val total = cart.____ { it.price }
Answer 
val total = cart.sumOf { it.price }

Explanation: cart.sumOf { it.price } sums all prices.

2 What does cart.sumOf { it.price } do?

  • Sums the prices Correct answer
  • Gives the item count
  • Takes the first item
  • Empties the list

Explanation: sumOf { it.price } takes each item's price and adds them.

3 Match the total concepts

Term Meaning
sumOf { it.price } Price sum
it List item
cart.size Item count
total Subtotal

All lessons in Beginner

1 Kotlin basics

  1. What is Kotlin?
  2. Your first program
  3. The main function
  4. Comments
  5. String templates
  6. Summary - Kotlin basics

2 Variables

  1. val and var
  2. Type inference
  3. const
  4. lateinit
  5. by lazy
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. Boolean and Char
  4. String
  5. Any and Unit
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Elvis operator
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String templates
  3. String methods
  4. substring
  5. split and replace
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. if expression
  4. when
  5. when expression
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. Range
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Arrays

  1. Creating arrays
  2. Array access
  3. Array size
  4. Array loop
  5. Array methods
  6. Summary - Arrays

11 Lists

  1. Creating lists
  2. List operations
  3. List access
  4. List iteration
  5. List methods
  6. Summary - Lists

12 Maps and Sets

  1. Creating sets
  2. Set operations
  3. Creating maps
  4. Map operations
  5. Map iteration
  6. Summary - Maps and Sets

13 Functions

  1. Defining functions
  2. Return values
  3. Expression body
  4. Default parameters
  5. Named arguments
  6. Summary - Functions

14 Null safety

  1. Nullable types
  2. ?, !! and ?:
  3. let
  4. Null check
  5. lateinit
  6. Summary - Null safety

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. Regex
  2. matches and find
  3. replace
  4. StringBuilder
  5. trim and pad
  6. Summary - Advanced strings

17 Enums

  1. enum class
  2. Enum values
  3. Enum properties
  4. when with enum
  5. Enum and range
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. try expression
  5. Custom exceptions
  6. Summary - Exceptions

19 Class basics

  1. class
  2. Constructor
  3. Properties
  4. Methods
  5. this
  6. Summary - Class basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together