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Index and get

Access vector elements safely with index and get.

Index and get

There are two ways to reach an element. v[i] gives the value directly but panics out of bounds. v.get(i) returns an Option<&T>:

let v = vec![10, 20, 30];
let a = v[1];          // 20
let b = v.get(2);      // Some(&30)
let c = v.get(5);      // None
// let d = v[5];       // panic: index out of bounds

If you are sure the index is valid, use []; in uncertain cases, like an index coming from the user, use get.

First, last and searching

Frequently used read methods also return Option:

let v = vec![4, 8, 15];
println!("{:?} {:?}", v.first(), v.last());   // Some(4) Some(15)
println!("{}", v.contains(&8));               // true
println!("{:?}", v.iter().position(|&x| x == 15));   // Some(2)

get_mut(i) gives a mutable reference: if let Some(x) = v.get_mut(0) { *x += 1; }.

Practice Ways of access

For let v = vec![10, 20, 30];, print the results of v[1], v.get(2) and v.get(5) on one line ({:?} for the Options).

Starter code

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

Sample solution

main.rs
fn main() {
    let v = vec![10, 20, 30];
    println!("{} {:?} {:?}", v[1], v.get(2), v.get(5));
}

Expected output

20 Some(30) None

Questions and answers

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

1 Why doesn't <code>let v = vec![String::from("a")]; let s = v[0];</code> compile?

  • v[0] tries to move the String out of the vector; &v[0] or clone is needed Correct answer
  • Index 0 is invalid
  • String can't be indexed
  • It compiles

Explanation: Moving a non-Copy element out by index is forbidden (E0507). Take a reference or copy it.

2 Safe access

Question 
let v = vec![10, 20, 30];
let c = v.____(5);   // None
Answer 
let v = vec![10, 20, 30];
let c = v.get(5);   // None

Explanation: get returns None instead of panicking out of bounds.

3 Expression and result (v = [10, 20, 30])

Term Meaning
v[1] 20
v.get(2) Some(30)
v.get(5) None
v[5] Panic

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