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Summary - Enums

Combine data-carrying enums and Option to model an IP address type.

Enums summary

An enum states that a value is one of a fixed set of options. Variants may carry no data, or carry data in tuple or struct form. match forces every variant to be handled. Option<T> (Some/None) replaces null and is used with helpers like unwrap_or, map and is_some. Methods can be added to enums with impl.

Methods on an enum

In enums carrying data, a method does a separate calculation for each variant with match self:

enum Sekil {
    Daire(f64),
    Kare(f64),
}

impl Sekil {
    fn alan(&self) -> f64 {
        match self {
            Sekil::Daire(r) => std::f64::consts::PI * r * r,
            Sekil::Kare(a) => a * a,
        }
    }
}

When a new shape is added, the compiler tells you to update the alan method.

Practice Shape areas

Write the Sekil enum above and the alan method. Print the areas of Sekil::Daire(1.0) and Sekil::Kare(2.0) with the {:.2} format, separated by a space.

Starter code

main.rs
fn main() {
    // buraya yaz
}

Sample solution

main.rs
enum Sekil {
    Daire(f64),
    Kare(f64),
}

impl Sekil {
    fn alan(&self) -> f64 {
        match self {
            Sekil::Daire(r) => std::f64::consts::PI * r * r,
            Sekil::Kare(a) => a * a,
        }
    }
}

fn main() {
    println!("{:.2} {:.2}", Sekil::Daire(1.0).alan(), Sekil::Kare(2.0).alan());
}

Expected output

3.14 4.00

Questions and answers

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

1 What determines the memory size of an <code>enum Sekil { Daire(f64), Kare(f64) }</code> value?

  • The largest variant + a discriminant tag Correct answer
  • The sum of all variants
  • Always 1 byte
  • It changes at runtime

Explanation: Since an enum value is one variant at a time, room is made for the largest one, plus a tag saying which variant it is.

2 Option variant

Question 
let x: Option<i32> = ____(4);
Answer 
let x: Option<i32> = Some(4);

Explanation: If there is a value, it is wrapped in Some.

3 Group summary

Term Meaning
enum A { X, Y } Set of options
B::X(5) Variant with data
Option<T> Instead of null
match self Method by variant

All lessons in Beginner

1 Rust basics

  1. What is Rust?
  2. Project with Cargo
  3. Your first program
  4. println! and formatting
  5. Comments
  6. Summary - Rust basics

2 Variables

  1. Variables with let
  2. Mutability with mut
  3. Constants with const
  4. Static variables
  5. Shadowing
  6. Summary - Variables

3 Data types

  1. Integers
  2. Floating point
  3. Bool and char
  4. Tuple
  5. Array
  6. Summary - Data types

4 Strings

  1. &str vs String
  2. Creating a String
  3. Concatenating strings
  4. format! macro
  5. String length
  6. Summary - Strings

5 Project: Calculator

  1. Project skeleton
  2. Reading input
  3. Computing
  4. Error handling
  5. Loop and exit

6 Functions

  1. Defining with fn
  2. Parameters
  3. Return value
  4. Expressions and statements
  5. Early return
  6. Summary - Functions

7 Control flow

  1. if-else
  2. match basics
  3. loop
  4. while loop
  5. for loop
  6. Summary - Control flow

8 Ownership

  1. What is ownership?
  2. Move semantics
  3. Copy trait
  4. Functions and ownership
  5. Return and ownership
  6. Summary - Ownership

9 Borrowing

  1. References
  2. Mutable reference
  3. Borrowing rules
  4. Dangling reference
  5. References and functions
  6. Summary - Borrowing

10 Slices

  1. String slices
  2. Slice syntax
  3. Array slices
  4. Slices and functions
  5. Range types
  6. Summary - Slices

11 Project: Guessing game

  1. Game skeleton
  2. Reading input
  3. Comparing numbers
  4. Random number
  5. Loop and exit

12 Structs

  1. Struct definition
  2. Creating an instance
  3. Tuple struct
  4. Unit-like struct
  5. Field update
  6. Summary - Structs

13 Methods

  1. impl block
  2. self parameter
  3. &mut self
  4. Associated functions
  5. Multiple impl blocks
  6. Summary - Methods

14 Enums

  1. Enum definition
  2. Variants
  3. Variants with data
  4. Option<T>
  5. Option instead of null
  6. Summary - Enums

15 Pattern matching

  1. match patterns
  2. Value binding
  3. if let
  4. while let
  5. _ wildcard
  6. Summary - Pattern matching

16 Project: To-do CLI

  1. Task model
  2. Adding
  3. Listing
  4. Marking done
  5. Reading commands

17 Vectors

  1. Vec<T>
  2. Push and pop
  3. Index and get
  4. Looping a vector
  5. with_capacity
  6. Summary - Vectors

18 Advanced strings

  1. String and &str conversion
  2. chars and bytes
  3. Slicing strings
  4. contains and replace
  5. split and join
  6. Summary - Advanced strings

19 Error handling

  1. Result<T, E>
  2. panic!
  3. ? operator
  4. match and Result
  5. unwrap and expect
  6. Summary - Error handling

20 Beginner final

  1. Vowel counter
  2. Average calculator
  3. Bank account
  4. Temperature converter