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Summary - Borrowing

Write a function that increments a counter via & and &mut references.

Borrowing summary

&T reads without taking ownership; &mut T modifies (the variable must be mut). The rule: at any time either many &T or a single &mut T. A reference can't outlive the value it points to. Function signatures state the intent: T consumes, &T/&[T]/&str read, &mut T modifies.

Borrowing habits

Practical rules: ask for the weakest access possible in parameters (&str > &String > String); keep references short-lived; when you get a borrow-checker error, first look for a small value you could copy, then try reordering the operations. .clone() is the last resort, but it's fine for small values.

Practice Longest word

Write fn en_uzun(kelimeler: &[&str]) -> usize; it returns the character count of the longest word (chars().count() and max). Print the result for ["go", "rust", "çiçekçi"].

Starter code

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

Sample solution

main.rs
fn en_uzun(kelimeler: &[&str]) -> usize {
    kelimeler.iter().map(|k| k.chars().count()).max().unwrap_or(0)
}

fn main() {
    println!("{}", en_uzun(&["go", "rust", "çiçekçi"]));
}

Expected output

7

Questions and answers

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

1 Which of these compiles?

  • None
  • let mut s = String::new(); let r = &mut s; r.push('a'); println!("{s}"); Correct answer
  • let mut s = String::new(); let r = &mut s; println!("{s}"); r.push('a');
  • let s = String::new(); let r = &mut s;

Explanation: In the first, r is last used in push and then s is read; in the second, s is read while r is still going to be used (E0502); in the third, s isn't mut.

2 Mutable borrow

Question 
let mut liste = vec![1];
ekle(____ liste);
Answer 
let mut liste = vec![1];
ekle(&mut liste);

Explanation: Since the function will change the list, &mut liste is passed.

3 Group summary

Term Meaning
&T Shared read
&mut T Exclusive write
Borrow checker Checks the rules
Lifetime A reference can't outlive the value

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