Question

我是Rust的新手。我的代码如下：

use std::*;

fn DFS(A: i32, grid: &mut [[i32; 500]; 500], visited: &mut [i32; 500]) -> (usize, usize) {
    let mut s = Vec::new();
    s.push(A);
    visited[A as usize] = 1;
    let mut flag;
    let mut max_height = 0;
    let mut ans_vertex: usize = A as usize;
    let mut x;

    'outer: while let Some(top) = s.pop() {
        s.push(top);
        x = top as usize;

        flag = 0;
        'inner: for i in 1..500 {
            if visited[grid[x][i] as usize] == 0 && grid[x][i] != 0 {
                flag = 1;
                s.push(grid[x][i]);
                visited[grid[x][i] as usize] = 1;
                break 'outer;
            }
        }
        if s.len() > max_height {
            max_height = s.len();
            ans_vertex = s.pop() as usize;
        }
        if flag != 0 {
            s.pop();
        }
    }
    println!("{}, {}", ans_vertex, max_height);
    return (ans_vertex, max_height);
}

fn fc(grid: &mut [[i32; 500]; 500]) {
    for i in 1..500 {
        for j in 1..500 {
            grid[i][j] = 0;
        }
    }

    grid[1][2] = 1;
    grid[2][1] = 1;
    grid[2][3] = 1;
    grid[3][2] = 1;
    grid[3][4] = 1;
    grid[4][3] = 1;
}

fn main() {
    let mut visited: [i32; 500] = [0; 500];
    let mut grid: [[i32; 500]; 500] = [[0; 500]; 500];

    fc(&mut grid);
    let B = DFS(1, &mut grid, &mut visited);
    println!("{}", B.0);
}

我已尝试将usize更改为u32和其他类型，但我没有得到任何结果。当我运行rustc newdia.rs时，它会显示：

  newdia.rs:26:17: 26:33 error: non-scalar cast: `core::option::Option<i32>` as `usize`
  newdia.rs:26          ans_vertex = s.pop() as usize;
                                 ^~~~~~~~~~~~~~~~
  error: aborting due to previous error

Answer 1

Vec::pop()会返回Option<T>，因为如果Vec为空，则弹出窗口没有值。如果您确定在致电Vec::pop()之前您的Vec包含至少1个值，则可以使用Option::unwrap()，这会将您的Option<i32>转换为i32（如果{{{}则会发生恐慌1}}返回Vec::pop，因为Vec是空的。）

None

您还可以选择使用ans_vertex = s.pop().unwrap() as usize;或match

来区别对待案例

if let

错误：非标量强制转换：`core :: option :: Option <i32>`as`usize`

1 个答案: