References and functions
- Rust course
- Beginner
- Lesson 52/115
- 4 min
Pass references into functions and return them.
References and functions
A function signature states clearly what will be done with the value:
fn f(v: Vec<i32>)— takes ownership (consumes it).fn f(v: &Vec<i32>)or&[i32]— only reads.fn f(v: &mut Vec<i32>)— changes it in place.
fn toplam(v: &[i32]) -> i32 { v.iter().sum() }
fn ikiye_katla(v: &mut Vec<i32>) {
for x in v.iter_mut() { *x *= 2; }
}iter and iter_mut
iter() gives elements as &T, iter_mut() as &mut T. The second is used to change elements in place, and each element is dereferenced with *:
let mut fiyatlar = vec![10.0, 20.0];
for f in fiyatlar.iter_mut() {
*f *= 1.2;
}
println!("{:?}", fiyatlar); // [12.0, 24.0]Writing for x in &mut v is the same as v.iter_mut().
Practice Read and modify
Write the functions fn toplam(v: &[i32]) -> i32 and fn ikiye_katla(v: &mut Vec<i32>). For let mut v = vec![1, 2, 3];, call ikiye_katla(&mut v) first, then toplam(&v), and print v and the sum.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn toplam(v: &[i32]) -> i32 {
v.iter().sum()
}
fn ikiye_katla(v: &mut Vec<i32>) {
for x in v.iter_mut() {
*x *= 2;
}
}
fn main() {
let mut v = vec![1, 2, 3];
ikiye_katla(&mut v);
let t = toplam(&v);
println!("{:?} {}", v, t);
}Expected output
[2, 4, 6] 12
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 For a function that only reads a vector, which parameter is most flexible?
- &mut Vec<i32>
- &Vec<i32>
- &[i32] Correct answer
- Vec<i32>
Explanation: A slice reference accepts &Vec<i32>, arrays and sub-slices; &Vec accepts only a vector.
2 Change in place
for x in v.____() {
*x += 1;
}for x in v.iter_mut() {
*x += 1;
}Explanation: iter_mut gives mutable references to the elements.
3 Signature and intent
| Term | Meaning |
|---|---|
v: Vec<T> |
Consumes |
v: &[T] |
Reads |
v: &mut Vec<T> |
Modifies |
v.iter_mut() |
&mut T elements |