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String and &str conversion

Learn conversion methods between String and &str.

Converting between String and &str

String is owned, growable text; &str is a borrowed view of text. Conversions:

let a: String = String::from("rust");
let b: String = "rust".to_string();
let c: String = "rust".to_owned();
let d: &str = &a;          // deref coercion
let e: &str = a.as_str();
let f: String = format!("{a}-{}", 2024);

Going from &str to String allocates new memory; the reverse is free.

Concatenation

The + operator takes ownership of the String on the left and needs a &str on the right:

let a = String::from("Merhaba");
let b = String::from("dünya");
let c = a + " " + &b;   // a was moved, b was lent
// println!("{a}");     // error: a was moved
println!("{c} {b}");

When joining several pieces, format! is more readable and moves no value. To append to an existing String, use push_str.

Practice Two calls

Write fn selamla(ad: &str) -> String returning format!("Merhaba {ad}"). Call it with a String (&s) and with a literal ("Can"); print the results separated by |.

Starter code

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

Sample solution

main.rs
fn selamla(ad: &str) -> String {
    format!("Merhaba {ad}")
}

fn main() {
    let s = String::from("Ada");
    println!("{}|{}", selamla(&s), selamla("Can"));
}

Expected output

Merhaba Ada|Merhaba Can

Questions and answers

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

1 If a function only reads text, what should the parameter type be?

  • &str Correct answer
  • String
  • &String
  • &mut String

Explanation: &str accepts both Strings (via &s) and literals; &String accepts only Strings.

2 Take a borrowed slice

Question 
let a = String::from("rust");
let e: &str = a.____();
Answer 
let a = String::from("rust");
let e: &str = a.as_str();

Explanation: as_str views the String's content as &str without copying.

3 Expression and result type

Term Meaning
"a".to_string() String
s.as_str() &str
format!("{x}") String (new)
a + &b String (a is moved)

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