Summary - Const generics

Combine const fn and [T; N] to build a generic dot product.

Const generics summary

Const generics parameterize types by values: struct S<const N: usize>, fn f<const N: usize>. The size is part of the type; a mismatch is a compile error and no heap is needed. const fn computes at compile time. In stable Rust parameters are limited to integers/bool/char, and expressions like N + 1 can't be used in types. Use try_into for slice ↔ array, and chunks_exact for fixed pieces.

A sized matrix

A matrix carrying its dimensions in the type with two const parameters:

struct Matris<const R: usize, const C: usize>([[i32; C]; R]);

impl<const R: usize, const C: usize> Matris<R, C> {
    fn devrik(&self) -> Matris<C, R> {
        let mut t = [[0; R]; C];
        for i in 0..R {
            for j in 0..C {
                t[j][i] = self.0[i][j];
            }
        }
        Matris(t)
    }
}

Multiplication is checked in the signature too: fn carp<const K: usize>(&self, o: &Matris<C, K>) -> Matris<R, K>.

Exercise Matrix transpose

Write the Matris<const R: usize, const C: usize> type above and the devrik method. Print the inner array of the transpose of Matris([[1, 2, 3], [4, 5, 6]]) with {:?}.

Show example solution

struct Matris<const R: usize, const C: usize>([[i32; C]; R]);

impl<const R: usize, const C: usize> Matris<R, C> {
    fn devrik(&self) -> Matris<C, R> {
        let mut t = [[0; R]; C];
        for i in 0..R {
            for j in 0..C {
                t[j][i] = self.0[i][j];
            }
        }
        Matris(t)
    }
}

fn main() {
    let m = Matris([[1, 2, 3], [4, 5, 6]]);
    println!("{:?}", m.devrik().0);
}

Test yourself

What is the type of the transpose of a <code>Matris<2, 3></code>?
- Vec<Vec<i32>>
- Matris<3, 2>
- Matris<2, 3>
- Matris<6>
Show answer

Matris<3, 2> — Rows and columns swap; the type system expresses this in the signature: fn devrik(&self) -> Matris<C, R>.

Transpose type

fn devrik(&self) -> Matris____ {
    ..
}

Show answer

fn devrik(&self) -> Matris<C, R> {
    ..
}

Group summary

<const N: usize>

Value parameter
const fn

Compile-time computation
try_into

Slice → array
Matris<R, C>

Sized type

Next lesson: Struct patterns

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