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Slicing strings

Learn the correct way to slice a substring.

Slicing strings

Slicing text with a byte range panics if the range falls in the middle of a character. That's why Rust doesn't allow single-index access like s[0] at all:

let s = "Çiçekçi";
// let c = s[0];        // error: str can't be indexed
// let x = &s[0..1];    // panic: Ç is two bytes
println!("{}", s.is_char_boundary(2));   // true
println!("{}", &s[0..2]);                // Ç

char_indices

If you want to slice at a character boundary without copying, char_indices gives the byte position of each character:

let s = "Çiçekçi";
for (i, c) in s.char_indices().take(4) {
    print!("{i}:{c} ");     // 0:Ç 2:i 3:ç 5:e
}
let (son, _) = s.char_indices().nth(3).unwrap();
println!("{}", &s[..son]);   // Çiç

The byte where the 4th character starts (5) is where the first three characters end.

Practice Safe first three

For let s = "Çiçekçi";, find the byte position of the 4th character with char_indices().nth(3). Print the slice &s[..konum] and the position separated by a space.

Starter code

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

Sample solution

main.rs
fn main() {
    let s = "Çiçekçi";
    let (konum, _) = s.char_indices().nth(3).unwrap();
    println!("{} {}", &s[..konum], konum);
}

Expected output

Çiç 5

Questions and answers

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

1 What is the simplest way to safely take the first 3 characters of a text?

  • s[3]
  • s.bytes().take(3)
  • s.chars().take(3).collect::<String>() Correct answer
  • &s[0..3]

Explanation: chars advances by character boundaries; &s[0..3] may panic with Turkish letters.

2 Byte positions

Question 
for (i, c) in s.____() {
    println!("{i}: {c}");
}
Answer 
for (i, c) in s.char_indices() {
    println!("{i}: {c}");
}

Explanation: char_indices yields (byte position, character) pairs.

3 Expression and result on "Çiçekçi"

Term Meaning
&s[0..2] "Ç"
&s[0..1] Panic
s.is_char_boundary(1) false
s.char_indices().nth(3) Some((5, 'e'))

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