Day 1 Morning Exercises

Arrays and for Loops

(back to exercise)

fn transpose(matrix: [[i32; 3]; 3]) -> [[i32; 3]; 3] {
    let mut result = [[0; 3]; 3];
    for i in 0..3 {
        for j in 0..3 {
            result[j][i] = matrix[i][j];
        }
    }
    return result;
}

fn pretty_print(matrix: &[[i32; 3]; 3]) {
    for row in matrix {
        println!("{row:?}");
    }
}

#[test]
fn test_transpose() {
    let matrix = [
        [101, 102, 103], //
        [201, 202, 203],
        [301, 302, 303],
    ];
    let transposed = transpose(matrix);
    assert_eq!(
        transposed,
        [
            [101, 201, 301], //
            [102, 202, 302],
            [103, 203, 303],
        ]
    );
}

fn main() {
    let matrix = [
        [101, 102, 103], // <-- the comment makes rustfmt add a newline
        [201, 202, 203],
        [301, 302, 303],
    ];

    println!("matrix:");
    pretty_print(&matrix);

    let transposed = transpose(matrix);
    println!("transposed:");
    pretty_print(&transposed);
}

Bonus question

It requires more advanced concepts. It might seem that we could use a slice-of-slices (&[&[i32]]) as the input type to transpose and thus make our function handle any size of matrix. However, this quickly breaks down: the return type cannot be &[&[i32]] since it needs to own the data you return.

You can attempt to use something like Vec<Vec<i32>>, but this doesn’t work out-of-the-box either: it’s hard to convert from Vec<Vec<i32>> to &[&[i32]] so now you cannot easily use pretty_print either.

Once we get to traits and generics, we’ll be able to use the std::convert::AsRef trait to abstract over anything that can be referenced as a slice.

use std::convert::AsRef;
use std::fmt::Debug;

fn pretty_print<T, Line, Matrix>(matrix: Matrix)
where
    T: Debug,
    // A line references a slice of items
    Line: AsRef<[T]>,
    // A matrix references a slice of lines
    Matrix: AsRef<[Line]>
{
    for row in matrix.as_ref() {
        println!("{:?}", row.as_ref());
    }
}

fn main() {
    // &[&[i32]]
    pretty_print(&[&[1, 2, 3], &[4, 5, 6], &[7, 8, 9]]);
    // [[&str; 2]; 2]
    pretty_print([["a", "b"], ["c", "d"]]);
    // Vec<Vec<i32>>
    pretty_print(vec![vec![1, 2], vec![3, 4]]);
}

In addition, the type itself would not enforce that the child slices are of the same length, so such variable could contain an invalid matrix.