Vectors

• A vector is a similar collection type provided by the standard library that is allowed to grow or shrink in size.

• Vectors are implemented using generics.

• More often, you’ll create a Vec<T> with initial values and Rust will infer the type of value you want to store, so you rarely need to do type annotation.

• References:

  • Storing Lists of Values with Vectors

  • Control Flow

  • Iterator

  • Closure

vecs1.rs

fn array_and_vec() -> ([i32; 4], Vec<i32>) {
    let a = [10, 20, 30, 40]; // Array

    // Create a vector called `v` which contains the exact same elements as in the array `a`.
    // Use the vector macro.
    // let v = ???;
    let v = vec![10, 20, 30, 40];
    (a, v)
}

fn main() {
    // You can optionally experiment here.
}

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

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

• Rust conveniently provides the vec! macro, which will create a new vector that holds the values you give it.

• You can specify the vector types the type within angle brackets like Vec<i32>.

• More often, you’ll create a vector with initial values and Rust will infer the type of value you want to store, so you rarely need to do this type annotation.

• In this exercise we just need to init new vector using vec! macro.

  let v = vec![10, 20, 30, 40];

vecs2.rs

fn vec_loop(input: &[i32]) -> Vec<i32> {
    let mut output = Vec::new();

    for element in input {
        // Multiply each element in the `input` slice by 2 and push it to
        // the `output` vector.
        output.push(element * 2);
    }

    output
}

fn vec_map_example(input: &[i32]) -> Vec<i32> {
    // An example of collecting a vector after mapping.
    // We map each element of the `input` slice to its value plus 1.
    // If the input is `[1, 2, 3]`, the output is `[2, 3, 4]`.
    input.iter().map(|element| element + 1).collect()
}

fn vec_map(input: &[i32]) -> Vec<i32> {
    // Here, we also want to multiply each element in the `input` slice
    // by 2, but with iterator mapping instead of manually pushing into an empty
    // vector.
    // See the example in the function `vec_map_example` above.
    input.iter().map(|element| element * 2).collect()
}

fn main() {
    // You can optionally experiment here.
}

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

    #[test]
    fn test_vec_loop() {
        let input = [2, 4, 6, 8, 10];
        let ans = vec_loop(&input);
        assert_eq!(ans, [4, 8, 12, 16, 20]);
    }

    #[test]
    fn test_vec_map_example() {
        let input = [1, 2, 3];
        let ans = vec_map_example(&input);
        assert_eq!(ans, [2, 3, 4]);
    }

    #[test]
    fn test_vec_map() {
        let input = [2, 4, 6, 8, 10];
        let ans = vec_map(&input);
        assert_eq!(ans, [4, 8, 12, 16, 20]);
    }
}

• In this exercise we have 2 task:

• First is to iterate through a collection manually using for and multiply each element with 2.

  for element in input {
    output.push(element * 2);
  }

  • Multiple element by 2.

  • Then push it into output variable.

  • You can read more about it in Looping Through a Collection with for.
• Second is to iterate through a collection using iterators and multiply each element with 2.

  input.iter().map(|element| element * 2).collect()

  • Calling iter() to a collection will return Iter type.

  • Then calling map() to produces a new iterator which calls given closure on each element of the original iterator. Ref: trait.iterator method.map.
  • collect() can take anything iterable, and turn it into a relevant collection.

  • You can read more about iterators in here: Iteration and the trait here" Iterator Trait.