# Day 1 Morning Exercises

## Arrays and `for` Loops

``````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.