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String slices

Reference part of a string with &str slices.

String slices

A slice is a reference to a contiguous part of a collection; it doesn't take ownership. A string slice has the type &str and is taken with a byte range:

let s = String::from("merhaba dünya");
let ilk = &s[0..7];     // "merhaba"
let son = &s[8..];      // "dünya"
println!("{ilk} / {son}");

The range is in byte positions, not characters. "ü" takes two bytes, so "dünya" is 6 bytes.

Why are slices safe?

Because a slice borrows its source, the source can't be changed while the slice is alive:

let mut s = String::from("rust dili");
let kelime = &s[0..4];
// s.clear();          // error E0502: kelime is still going to be used
println!("{kelime}");

So the bug of an index pointing into a cleared source is caught at compile time. String literals ("merhaba") are in fact &str slices embedded in the program.

Practice Two pieces

For let s = String::from("rust dili");, take the slices &s[0..4] and &s[5..] and print them with | between.

Starter code

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

Sample solution

main.rs
fn main() {
    let s = String::from("rust dili");
    let a = &s[0..4];
    let b = &s[5..];
    println!("{}|{}", a, b);
}

Expected output

rust|dili

Questions and answers

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

1 What happens with <code>let s = String::from("çay"); let x = &amp;s[0..1];</code>?

  • Compile error
  • x = ""
  • Runtime panic: 1 is not a char boundary Correct answer
  • x = "ç"

Explanation: "ç" is two bytes; byte 1 falls in the middle of a character and slicing panics.

2 First four bytes

Question 
let s = String::from("rust dili");
let ilk = &s[____];
Answer 
let s = String::from("rust dili");
let ilk = &s[0..4];

Explanation: The range 0..4 takes bytes 0, 1, 2, 3: "rust".

3 Expression and result

Term Meaning
&s[0..7] "merhaba"
&s[8..] "dünya"
"çay"[0..1] Panic
"dünya".len() 6

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