Iterating collections
- Rust course
- Intermediate
- Lesson 58/115
- 4 min
Learn iteration patterns over HashMap and Vec.
Iterating collections
Maps are iterated in three ways, each a different kind of borrow:
for (k, v) in &harita { } // (&K, &V)
for (k, v) in &mut harita { } // (&K, &mut V): values can be changed
for (k, v) in harita { } // (K, V): the map is consumed
harita.keys() // &K
harita.values() // &V
harita.values_mut() // &mut VKeys are never given mutably; changing a key would break its hash.
Updating and filtering values
To apply the same operation to all values, use values_mut; to remove entries failing a condition, retain:
let mut fiyat = HashMap::from([("kalem", 5), ("defter", 12), ("silgi", 2)]);
for v in fiyat.values_mut() {
*v *= 2;
}
fiyat.retain(|_, v| *v > 5);
let toplam: u32 = fiyat.values().sum(); // 10 + 24 = 34HashMap::from([...]) builds a map directly from an array.
Practice Update prices
Start with HashMap::from([("kalem", 5), ("defter", 12)]). Double all prices with values_mut. Print the sorted vector of keys and the sum of the values (u32) separated by a space.
Starter code
use std::collections::HashMap;
fn main() {
// buraya yaz
}Sample solution
use std::collections::HashMap;
fn main() {
let mut fiyat: HashMap<&str, u32> = HashMap::from([("kalem", 5), ("defter", 12)]);
for v in fiyat.values_mut() {
*v *= 2;
}
let mut anahtarlar: Vec<&str> = fiyat.keys().copied().collect();
anahtarlar.sort();
let toplam: u32 = fiyat.values().sum();
println!("{:?} {}", anahtarlar, toplam);
}Expected output
["defter", "kalem"] 34
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Can a new key be added to the map while iterating it?
- Only in a BTreeMap
- No; the iteration borrows the map Correct answer
- Yes, always
- Only in a HashMap
Explanation: Collect what to add in a separate vector first and add after the loop; or filter in place with retain.
2 Change the values
for v in fiyat.____() {
*v *= 2;
}for v in fiyat.values_mut() {
*v *= 2;
}Explanation: values_mut gives &mut references to the values.
3 Method and what it gives
| Term | Meaning |
|---|---|
keys() |
&K |
values() |
&V |
values_mut() |
&mut V |
into_iter() |
(K, V) |