第 1 天：上午練習

陣列和 for 迴圈

(返回練習)

// Copyright 2022 Google LLC
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

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

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

// ANCHOR: tests
#[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],
        ]
    );
}
// ANCHOR_END: tests

// ANCHOR: main
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);
}

加分題

這需要用到更進階的概念。表面上看起來我們可以使用多維度切片 (&[&[i32]]) 做為轉置的輸入型別，進而讓函式處理任何大小的矩陣。但實際上這很快就會失敗，也就是說傳回型別不能是 &[&[i32]]，因為它需要擁有您傳回的資料。

您可以嘗試使用 Vec<Vec<i32>> 這類代碼，但這也不能立即見效，因為從 Vec<Vec<i32>> 轉換成 &[&[i32]] 並不容易，因此您現在也無法輕易使用 pretty_print。

一旦瞭解特徵和泛型，我們就能使用 std::convert::AsRef 特徵，對任何可當做切片參照的資料進行抽象化。

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]]);
}

此外，型別本身不會強制規定子項切片的長度必須相同，因此這類變數可能含有無效的矩陣。