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.