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Slices and functions

Pass slices to functions to borrow data.

Slices and functions

Taking &str instead of &String, and &[T] instead of &Vec<T>, lets a function accept Strings, literals and sub-slices alike. The returned slice is a part of the input:

fn ilk_kelime(s: &str) -> &str {
    for (i, b) in s.bytes().enumerate() {
        if b == b' ' {
            return &s[..i];
        }
    }
    &s[..]
}

As long as the returned &str borrows s, the source can't be changed.

Built-in helpers

The standard library offers methods that produce slices without hand-written loops:

let s = "ad soyad yaş";
let ilk = s.split(' ').next().unwrap_or("");   // "ad"
let (sol, sag) = s.split_at(2);               // ("ad", " soyad yaş")
let temiz = "  rust  ".trim();                // "rust"

split_whitespace, lines and trim all return parts of the source as &str without copying.

Practice First word

Write fn ilk_kelime(s: &str) -> &str: find the first space with bytes().enumerate() and return &s[..i]; if there is no space, &s[..]. Print the results for "merhaba dünya" and "tek" separated by a space.

Starter code

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

Sample solution

main.rs
fn ilk_kelime(s: &str) -> &str {
    for (i, b) in s.bytes().enumerate() {
        if b == b' ' {
            return &s[..i];
        }
    }
    &s[..]
}

fn main() {
    println!("{} {}", ilk_kelime("merhaba dünya"), ilk_kelime("tek"));
}

Expected output

merhaba tek

Questions and answers

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

1 How is <code>ilk_kelime</code> called with a <code>String</code>?

  • ilk_kelime(metin)
  • ilk_kelime(*metin)
  • It can't be called
  • ilk_kelime(&metin) Correct answer

Explanation: &String is converted to &str automatically (deref coercion).

2 Slice parameter

Question 
fn en_buyuk(v: ____) -> i32 {
    *v.iter().max().unwrap()
}
Answer 
fn en_buyuk(v: &[i32]) -> i32 {
    *v.iter().max().unwrap()
}

Explanation: &[i32] accepts both vector and array references.

3 Method and result on " ad soyad "

Term Meaning
trim() "ad soyad"
split_whitespace().count() 2
len() 11
trim().split_at(2).0 "ad"

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