Iterating collections

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 V

Keys 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 = 34

HashMap::from([...]) builds a map directly from an array.

Exercise 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.

Show example 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);
}

Test yourself

Can a new key be added to the map while iterating it?
- Only in a BTreeMap
- No; the iteration borrows the map
- Yes, always
- Only in a HashMap
Show answer

No; the iteration borrows the map — Collect what to add in a separate vector first and add after the loop; or filter in place with retain.

Change the values

for v in fiyat.____() {
    *v *= 2;
}

Show answer

for v in fiyat.values_mut() {
    *v *= 2;
}

Method and what it gives

keys()

&K
values()

&V
values_mut()

&mut V
into_iter()

(K, V)

Next lesson: Summary - Collections

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