Chapter 5 - Vecs

Exercise 1

fn array_and_vec() -> ([i32; 4], Vec<i32>) {
    let a = [10, 20, 30, 40]; // a plain array
    let v = // TODO: declare your vector here with the macro for vectors

    (a, v)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_array_and_vec_similarity() {
        let (a, v) = array_and_vec();
        assert_eq!(a, v[..]);
    }
}

We have 2 ways to declare a vector:

Declare with the macro vec!:

fn array_and_vec() -> ([i32; 4], Vec<i32>) {
    let a = [10, 20, 30, 40]; // a plain array
    let v = vec![10, 20, 30, 40]

    (a, v)
}

Declare with the Vec::new function - remember to make the v mutable:

fn array_and_vec() -> ([i32; 4], Vec<i32>) {
    let a = [10, 20, 30, 40]; // a plain array
    let mut v: Vec<i32> = Vec::new();
    v.push(10);
    v.push(20);
    v.push(30);
    v.push(40);

    (a, v)
}

Exercise 2

fn vec_loop(mut v: Vec<i32>) -> Vec<i32> {
    for element in v.iter_mut() {
        // TODO: Fill this up so that each element in the Vec `v` is
        // multiplied by 2.
        ???
    }

    // At this point, `v` should be equal to [4, 8, 12, 16, 20].
    v
}

fn vec_map(v: &Vec<i32>) -> Vec<i32> {
    v.iter().map(|element| {
        // TODO: Do the same thing as above - but instead of mutating the
        // Vec, you can just return the new number!
        ???
    }).collect()
}

Looking at the vec_loop function, we're using the for loop to iterate through all element of vector v, so to multiple each element by 2, we have to assign the new value to the element.
element = element * 2 or element *= 2
But you will see this error cannot multiply '&mut i32' by {integer}.
The problem is the element here is the reference to the element of the vector, so we have to dereference it first.

fn vec_loop(mut v: Vec<i32>) -> Vec<i32> {
    for element in v.iter_mut() {
        // TODO: Fill this up so that each element in the Vec `v` is
        // multiplied by 2.
        *element *= 2; // or *element = *element * 2
    }

    // At this point, `v` should be equal to [4, 8, 12, 16, 20].
    v
}

For the vec_map function, we're using the map function to iterate through all element of vector v, so to multiple each element by 2, we have to return the new value.

fn vec_map(v: &Vec<i32>) -> Vec<i32> {
    v.iter().map(|element| {
        // TODO: Do the same thing as above - but instead of mutating the
        // Vec, you can just return the new number!
        element * 2
    }).collect()
}

Conclusion

The fifth chapter of Rustlings - If ends here.
TIL:

How to declare a vector with vec! macro and Vec::new function
Vector's loop and map

Thanks for reading and please add comments below if you have any questions