Looping a vector

Iterate vector elements with for.

Loops and vectors

The three ways of looping over a vector mean three different kinds of ownership:

let mut v = vec![1, 2, 3];
for x in &v { println!("{x}"); }        // &i32: reads
for x in &mut v { *x *= 2; }             // &mut i32: modifies
for x in v { println!("{x}"); }         // i32: consumes, v is gone

If you also need the position, use enumerate: for (i, x) in v.iter().enumerate().

Iterator methods

Instead of loops, iterator methods are often clearer:

let notlar = vec![70, 85, 90, 45];
let toplam: i32 = notlar.iter().sum();
let gecen: Vec<&i32> = notlar.iter().filter(|&&n| n >= 50).collect();
let artirilmis: Vec<i32> = notlar.iter().map(|n| n + 5).collect();
println!("{toplam} {:?} {:?}", gecen, artirilmis);
// 290 [70, 85, 90] [75, 90, 95, 50]

Exercise Raise and list

For let mut fiyat = vec![100, 250, 40];, add a tenth of each price to itself with for f in &mut fiyat. Then print each line as 1: 110 with enumerate (positions start at 1).

Show example solution

fn main() {
    let mut fiyat = vec![100, 250, 40];
    for f in &mut fiyat {
        *f += *f / 10;
    }
    for (i, f) in fiyat.iter().enumerate() {
        println!("{}: {}", i + 1, f);
    }
}

Test yourself

Why does <code>for x in &v</code> fail when calling <code>v.push(..)</code> inside the loop?
- push is forbidden in loops
- x has the wrong type
- It doesn't fail
- v can't be changed while the loop borrows it
Show answer

v can't be changed while the loop borrows it — A growing vector could move to new memory and invalidate the loop's references; the borrow checker prevents this.

Loop with position

for (i, x) in v.iter().____() {
    println!("{i}: {x}");
}

Show answer

for (i, x) in v.iter().enumerate() {
    println!("{i}: {x}");
}

Loop and type of x

for x in &v

&i32
for x in &mut v

&mut i32
for x in v

i32 (consumes)
for (i, x) in v.iter().enumerate()

(usize, &i32)

Next lesson: with_capacity

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