Function vander 

pub fn vander<T>(
    x: ArrayView<'_, T, Ix1>,
    n: Option<usize>,
    increasing: Option<bool>,
) -> Result<Array<T, Ix2>, &'static str>
where
    T: Clone + Zero + One + Mul<Output = T> + Div<Output = T>,

Create a matrix vander from a 1D array

Arguments

• x - Input 1D array
• n - Optional number of columns in the output (defaults to x.len())
• increasing - Optional boolean to determine order (defaults to false)

Returns

A Vandermonde matrix where each column is a power of the input array

Examples

use ndarray::array;
use scirs2_core::ndarray_ext::matrix::vander;

let x = array![1.0, 2.0, 3.0];

// Default behavior: decreasing powers from n-1 to 0
let v1 = vander(x.view(), None, None).unwrap();
assert_eq!(v1.shape(), &[3, 3]);
// Powers: x^2, x^1, x^0
assert_eq!(v1, array![
    [1.0, 1.0, 1.0],
    [4.0, 2.0, 1.0],
    [9.0, 3.0, 1.0]
]);

// Increasing powers: 0 to n-1
let v2 = vander(x.view(), None, Some(true)).unwrap();
assert_eq!(v2.shape(), &[3, 3]);
// Powers: x^0, x^1, x^2
assert_eq!(v2, array![
    [1.0, 1.0, 1.0],
    [1.0, 2.0, 4.0],
    [1.0, 3.0, 9.0]
]);

// Specify 4 columns
let v3 = vander(x.view(), Some(4), None).unwrap();
assert_eq!(v3.shape(), &[3, 4]);
// Powers: x^3, x^2, x^1, x^0
assert_eq!(v3, array![
    [1.0, 1.0, 1.0, 1.0],
    [8.0, 4.0, 2.0, 1.0],
    [27.0, 9.0, 3.0, 1.0]
]);