multidimensional-array - 使用ndarray实现逐行操作

Question

我想执行一个循环操作，该操作将执行以下操作(操作必须逐个元素地进行)。

let mut spec = array![] // matrix mxn
let exponencial = array![] // 1xn

for k in 0..N {
    spectrum [k, ..] = spectrum [k, ..] * exponential;
}

如何正确有效地执行此操作？我的意图是将其实现为一个循环，以与其他语言的实现进行比较。我必须使用一些中间临时数组吗？
尝试1

use ndarray::prelude::*; // 0.13.1

fn main() {
    // All rows in same time
    let mut spec = array![[1, 2, 3, 4], [5, 6, 7, 8]]; // mxn
    let exponencial = array![[1, 1, 1, 1]]; // matrix 1xn

    let _result = spec.assign(&(&spec * &exponencial));
    // or spec = &spec*&exponencial;

    //  In the form of a loop. one row each iteration.
    // This is the implementation that I want to make to be able
    // to implement it to be able to compare with other languages
    // 1)
    let mut spec = array![[1, 2, 3, 4], [5, 6, 7, 8]]; // mxn
    let exponencial = array![[1, 1, 1, 1]]; // matrix 1xn
    spec.slice_mut(s![0, ..])
        .assign(&(&(spec.row(0)) * &exponencial.row(0)));
}

playground

error[E0502]: cannot borrow `spec` as immutable because it is also borrowed as mutable
  --> src/main.rs:18:21
   |
17 |     spec.slice_mut(s![0, ..])
   |     ---- mutable borrow occurs here
18 |         .assign(&(&(spec.row(0)) * &exponencial.row(0)));
   |          ------     ^^^^ immutable borrow occurs here
   |          |
   |          mutable borrow later used by call

尝试2

use ndarray::prelude::*; // 0.13.1

fn main() {
    // All rows in same time
    let mut spec = array![[1, 2, 3, 4], [5, 6, 7, 8]]; // mxn
    let exponencial = array![[1, 1, 1, 1]]; // matrix 1xn

    let result = spec.assign(&(&spec * &exponencial));
    // or spec = &spec*&exponencial;

    //  In the form of a loop. one row each iteration.
    // This is the implementation that I want to make to be able
    // to implement it to be able to compare with other languages
    // 2)
    let mut spec = array![[1, 2, 3, 4], [5, 6, 7, 8]]; // mxn
    let exponencial = array![[1, 1, 1, 1]]; // matrix 1xn
    spec.row_mut(0) = &spec.row(0) * &exponencial.row(0);
}

playground

error[E0308]: mismatched types
  --> src/main.rs:17:23
   |
17 |     spec.row_mut(0) = &spec.row(0) * &exponencial.row(0);
   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `ndarray::ViewRepr`, found struct `ndarray::OwnedRepr`
   |
   = note: expected struct `ndarray::ArrayBase<ndarray::ViewRepr<&mut {integer}>, _>`
              found struct `ndarray::ArrayBase<ndarray::OwnedRepr<{integer}>, _>`

error[E0070]: invalid left-hand side of assignment
  --> src/main.rs:17:21
   |
17 |     spec.row_mut(0) = &spec.row(0) * &exponencial.row(0);
   |     --------------- ^
   |     |
   |     cannot assign to this expression

Answer 1

除了bnaecker's answer之外，如果要在循环中实现它，还可以使用 axis_iter_mut() 沿轴进行迭代:

use ndarray::prelude::*;

fn main() {
    let mut spec = array![
        [1, 2, 3, 4],
        [5, 6, 7, 8]
    ];
    
    let expo = array![2, 2, 2, 2];
    
    for mut row in spec.axis_iter_mut(Axis(0)) {
        row *= &expo;
    }
    
    println!("{}", spec);
}

[[2, 4, 6, 8],
 [10, 12, 14, 16]]