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Summary - Variables

Combine let, mut, const and shadowing to build a counter program.

Variables summary

let creates an immutable binding; let mut a mutable one; const a compile-time constant (type required, UPPER_CASE); static a global value with a single address; shadowing a new variable with the same name (its type can change). Rust choosing immutability by default lets you see immediately which values change just by reading the code.

Multiple bindings and tuples

A tuple can be destructured with a single let to define several variables:

let (ad, yas) = ("Ada", 36);
let (mut x, y) = (1, 2);
x += y;
println!("{ad} {yas} {x}");  // Ada 36 3

Parts you won't use are skipped with _: let (_, yas) = kisi;.

Practice All together

Define const HEDEF: i32 = 10;. In main, destructure with let (baslangic, adim) = (1, 3);, create let mut deger = baslangic; and do deger += adim; while deger < HEDEF. Print the result.

Starter code

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

Sample solution

main.rs
const HEDEF: i32 = 10;

fn main() {
    let (baslangic, adim) = (1, 3);
    let mut deger = baslangic;
    while deger < HEDEF {
        deger += adim;
    }
    println!("{}", deger);
}

Expected output

10

Questions and answers

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

1 Which compiles?

  • let x = 1; let x = "bir"; Correct answer
  • let x = 1; x = 2;
  • let mut x = 1; x = "bir";
  • const X = 1;

Explanation: Shadowing can change the type; the others fail because of immutability, a type mismatch and a missing type, respectively.

2 Destructure

Question 
let ____ = (10, 20);
println!("{}", a + b);
Answer 
let (a, b) = (10, 20);
println!("{}", a + b);

Explanation: The tuple pattern (a, b) binds the two values to separate variables.

3 Group summary

Term Meaning
let Immutable
let mut Mutable
const Compile-time constant
Shadowing New variable, same name

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