[英]Assigning values in a 2D array
我在Rust中編寫了一些代碼(主要是作為POC)。 代碼采用2D數組,將其傳遞給第二個函數進行一些矩陣數學運算(我知道有一個標准庫可以做到這一點,但我想習慣事情的運作方式)。
問題是對2D陣列的分配導致了問題。
我的代碼看起來像這樣
fn main()
{
// first create a couple of arrays - these will be used
// for the vectors
let line1: [i32; 4] = [4, 2, 3, 3];
let line2: [i32; 4] = [3, 4, 5, 7];
let line3: [i32; 4] = [2, 9, 6, 2];
let line4: [i32; 4] = [5, 7, 2, 4];
// create two holding arrays and assign
let array_one = [line1, line3, line4, line2];
let array_two = [line2, line1, line3, line4];
// let's do the multiply
let result = matrix_multiply(&array_one, &array_two);
println!("{:?}", result);
}
fn matrix_multiply(vec1:&[&[i32;4];4], vec2:&[&[i32;4];4]) -> [[i32; 4];4]
{
// we need to deference the parameters passed in
let vec_one:[[i32;4];4] = vec1;
let vec_two:[[i32;4];4] = vec2;
// we need to store the sum
let mut sum = 0;
// we need to create the arrays to put the results into
let mut result = [[0i32; 4]; 4];
// loop through the two vectors
for vone in 0..4
{
for vtwo in 0..4
{
for k in 0..4
{
sum = sum + vec1[[vone].k] * vec2[[k].vtwo];
}
result[[vec_one].vec_two] = sum;
sum = 0;
}
}
return result;
}
我也試過result[vec_one][vec_two] = sum
但是當我來編譯時,看起來分配給數組有問題。
我在這做錯了什么?
這是你的錯誤,我相信(至少其中一個):
<anon>:15:34: 15:44 error: mismatched types:
expected `&[&[i32; 4]; 4]`,
found `&[[i32; 4]; 4]`
(expected &-ptr,
found array of 4 elements) [E0308]
<anon>:15 let result = matrix_multiply(&array_one, &array_two);
^~~~~~~~~~
問題是,數組的引用或取消引用不能在它們的嵌套的多個層次上進行。 這是因為[&[i32; 4]; 4]
[&[i32; 4]; 4]
[&[i32; 4]; 4]
和[[i32; 4]; 4]
[[i32; 4]; 4]
[[i32; 4]; 4]
在內容和大小方面完全不同 - 前一個數組包含四個指向其他數組的指針(4 * 4 = 16/8 * 4 =總共32個字節,具體取決於您的機器的體系結構),而后者包括順序排列的四個數組(4 * 4 * 4 =總共64個字節)。 根本沒有辦法從[[i32; 4]; 4]
[[i32; 4]; 4]
[[i32; 4]; 4]
到&[&[i32; 4]; 4]
&[&[i32; 4]; 4]
&[&[i32; 4]; 4]
沒有重建外部數組,Rust不會為你做,因為它太多魔法了。
你真的不需要使用內部引用; 事實上,你可能甚至不需要引用在所有通過這些陣列:陣列的Copy
類型Copy
為好,這樣你就可以按值傳遞它們。 它們足夠小,不會對性能產生任何影響,編譯器可能會自動優化它:
fn main() {
// first create a couple of arrays - these will be used
// for the vectors
let line1: [i32; 4] = [4, 2, 3, 3];
let line2: [i32; 4] = [3, 4, 5, 7];
let line3: [i32; 4] = [2, 9, 6, 2];
let line4: [i32; 4] = [5, 7, 2, 4];
// create two holding arrays and assign
let array_one = [line1, line3, line4, line2];
let array_two = [line2, line1, line3, line4];
// let's do the multiply
let result = matrix_multiply(array_one, array_two);
println!("{:?}", result);
}
fn matrix_multiply(vec1: [[i32; 4]; 4], vec2: [[i32; 4]; 4]) -> [[i32; 4]; 4] {
// we need to create the arrays to put the results into
let mut result = [[0i32; 4]; 4];
// loop through the two vectors
for vone in 0..4 {
for vtwo in 0..4 {
let mut sum = 0;
for k in 0..4 {
sum += vec1[vone][k] * vec2[k][vtwo];
}
result[vone][vtwo] = sum;
}
}
result
}
( 在這里試試)
根據當前的社區實踐(大括號定位,間距等),我還使你的代碼更加慣用,並且我已經修復了訪問數組的奇怪語法。
如果你寫
let line1: [i32; 4] = [4, 2, 3, 3];
let line2: [i32; 4] = [3, 4, 5, 7];
let line3: [i32; 4] = [2, 9, 6, 2];
let line4: [i32; 4] = [5, 7, 2, 4];
// create two holding arrays and assign
let array_one = [line1, line3, line4, line2];
array_one
的類型將是[[i32;4];4]
因為行數組被復制到array_one
。 通過&array_one
借用這個給你一些類型&[[i32;4];4]
,這與&[&[T; 4]; 4]
&[&[T; 4]; 4]
&[&[T; 4]; 4]
(你試圖調用的功能是什么)。
選項1 - 創建[&[T; 4]; 4]
[&[T; 4]; 4]
[&[T; 4]; 4]
:
let array_one = [&line1, &line3, &line4, &line2];
some_function(&array_one);
...
fn some_function(matrix: &[&[i32;4];4]) {...}
選項2 - 更改功能坐標:
let array_one = [line1, line3, line4, line2];
some_function(&array_one);
...
fn some_function(matrix: &[[i32;4];4]) {...}
如果您對處理任意大小的多維數組感興趣,也許您會發現我的實驗性多陣列箱有用。 它基本上嘗試為兩個或多個維度提供類似於Box<[T]>
, &[T]
和&mut[T]
的類型。 這是一個未經測試的例子,只是為了了解我想要做的事情:
extern crate multiarray;
use multiarray::*;
fn main() {
// the backing memory will be a dynamically allocated
// linear array with 4 elements in row-major (C-style) order.
let mut matrix = Array2D::new([2, 2], 0i32);
matrix[[0,0]] = 1; matrix[[0,1]] = 2;
matrix[[1,0]] = 3; matrix[[1,1]] = 4;
let mut square = Array2D::new([2, 2], 0i32);
// the borrow methods create reference-like proxies
mat_product(matrix.borrow(), matrix.borrow(),
square.borrow_mut());
}
fn mat_product(a: Array2DRef<i32>, b: Array2DRef<i32>,
mut c: Array2DRefMut<i32>) {
let rows = a.extents()[0]; // extent of 1st dimension
let intr = a.extents()[1]; // extent of 2nd dimension
let cols = b.extents()[1]; // extent of 2nd dimension
assert!(intr == b.extents()[0]);
assert!(rows == c.extents()[0]);
assert!(cols == c.extents()[1]);
for i in 0..rows {
// the i-th row of a and c...
let a_row_i = a.eliminated_dim(0, i);
let mut c_row_i = c.reborrow_mut().eliminated_dim(0, i);
for j in 0..cols {
c_row_i[j] = dot_product(a_row_i, b.eliminated_dim(1, j));
// ^^^j-th column of b^^^
}
}
}
fn dot_product(a: Array1DRef<i32>, b: Array1DRef<i32>) -> i32 {
a.zip(b).fold(0, |acc,(&x,&y)| acc + x * y)
}
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