CodeBlanks Logo Icon CodeBlanks
Login Register

_ wildcard

Catch all remaining cases in one arm with _.

The _ wildcard

match must be exhaustive: every possible value must fit an arm. For types with many values such as integers, _ collects the rest. _ doesn't bind the value; if you want to bind it, use a name (diger => ..):

fn gun_turu(g: u8) -> &'static str {
    match g {
        1..=5 => "hafta içi",
        6 | 7 => "hafta sonu",
        _ => "geçersiz",
    }
}

Ignoring parts

_ is used inside patterns too: (_, y) ignores the first element, Some(_) ignores the content. .. skips several parts:

let sayilar = (2, 4, 8, 16, 32);
let (ilk, .., son) = sayilar;
println!("{ilk} {son}");   // 2 32

let _kullanilmayan = 5;   // a name starting with _ gives no warning

_x binds the value but silences the "unused variable" warning; a lone _ doesn't bind at all.

Practice Day type

Write the function fn gun_turu(g: u8) -> &'static str above. Print the results for 3, 7 and 9 on one line separated by |.

Starter code

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

Sample solution

main.rs
fn gun_turu(g: u8) -> &'static str {
    match g {
        1..=5 => "hafta içi",
        6 | 7 => "hafta sonu",
        _ => "geçersiz",
    }
}

fn main() {
    println!("{}|{}|{}", gun_turu(3), gun_turu(7), gun_turu(9));
}

Expected output

hafta içi|hafta sonu|geçersiz

Questions and answers

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

1 Why doesn't <code>match g { 1..=5 =&gt; .., 6 | 7 =&gt; .. }</code> (g: u8) compile?

  • It compiles
  • The values 0 and 8..=255 aren't covered Correct answer
  • | can't be used
  • The range is wrong

Explanation: The compiler lists the missing values: "patterns `0_u8` and `8_u8..=u8::MAX` not covered".

2 Catch the rest

Question 
match g {
    1..=5 => "hafta içi",
    6 | 7 => "hafta sonu",
    ____ => "geçersiz",
}
Answer 
match g {
    1..=5 => "hafta içi",
    6 | 7 => "hafta sonu",
    _ => "geçersiz",
}

Explanation: _ catches every value that didn't fit the earlier arms.

3 Pattern and effect

Term Meaning
_ Fits anything, doesn't bind
_x Binds, no warning
(ilk, .., son) Skips the middle
Some(_) Ignores the content

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