如何在 python 中执行三次样条插值？

Question

我有两个列表来描述函数 y(x)：

 x = [0,1,2,3,4,5]
y = [12,14,22,39,58,77]

我想执行三次样条插值，以便在 x 的域中给定一些值 u，例如

u = 1.25

我能找到 y(u)。

我 在 SciPy 中找到了这个，但我不确定如何使用它。

原文由 user112829 发布，翻译遵循 CC BY-SA 4.0 许可协议

Answer 1

简短回答：

 from scipy import interpolate

def f(x):
    x_points = [ 0, 1, 2, 3, 4, 5]
    y_points = [12,14,22,39,58,77]

    tck = interpolate.splrep(x_points, y_points)
    return interpolate.splev(x, tck)

print(f(1.25))

长答案：

scipy 将样条插值中涉及的步骤分成两个操作，很可能是为了提高计算效率。

1. 使用 splrep() 计算描述样条曲线的系数。 splrep 返回包含系数的元组数组。

2. 这些系数被传递到 splev() 以实际评估所需点的样条 x （在本例中为 1.25）。 x 也可以是数组。 Calling f([1.0, 1.25, 1.5]) returns the interpolated points at 1 , 1.25 , and 1,5 , respectively.

诚然，这种方法对于单次评估不方便，但由于最常见的用例是从少数几个函数评估点开始，然后重复使用样条曲线找到插值，因此在实践中通常非常有用。

原文由 youngmit 发布，翻译遵循 CC BY-SA 4.0 许可协议

Answer 2

如果未安装 scipy：

 import numpy as np
from math import sqrt

def cubic_interp1d(x0, x, y):
    """
    Interpolate a 1-D function using cubic splines.
      x0 : a float or an 1d-array
      x : (N,) array_like
          A 1-D array of real/complex values.
      y : (N,) array_like
          A 1-D array of real values. The length of y along the
          interpolation axis must be equal to the length of x.

    Implement a trick to generate at first step the cholesky matrice L of
    the tridiagonal matrice A (thus L is a bidiagonal matrice that
    can be solved in two distinct loops).

    additional ref: www.math.uh.edu/~jingqiu/math4364/spline.pdf
    """
    x = np.asfarray(x)
    y = np.asfarray(y)

    # remove non finite values
    # indexes = np.isfinite(x)
    # x = x[indexes]
    # y = y[indexes]

    # check if sorted
    if np.any(np.diff(x) < 0):
        indexes = np.argsort(x)
        x = x[indexes]
        y = y[indexes]

    size = len(x)

    xdiff = np.diff(x)
    ydiff = np.diff(y)

    # allocate buffer matrices
    Li = np.empty(size)
    Li_1 = np.empty(size-1)
    z = np.empty(size)

    # fill diagonals Li and Li-1 and solve [L][y] = [B]
    Li[0] = sqrt(2*xdiff[0])
    Li_1[0] = 0.0
    B0 = 0.0 # natural boundary
    z[0] = B0 / Li[0]

    for i in range(1, size-1, 1):
        Li_1[i] = xdiff[i-1] / Li[i-1]
        Li[i] = sqrt(2*(xdiff[i-1]+xdiff[i]) - Li_1[i-1] * Li_1[i-1])
        Bi = 6*(ydiff[i]/xdiff[i] - ydiff[i-1]/xdiff[i-1])
        z[i] = (Bi - Li_1[i-1]*z[i-1])/Li[i]

    i = size - 1
    Li_1[i-1] = xdiff[-1] / Li[i-1]
    Li[i] = sqrt(2*xdiff[-1] - Li_1[i-1] * Li_1[i-1])
    Bi = 0.0 # natural boundary
    z[i] = (Bi - Li_1[i-1]*z[i-1])/Li[i]

    # solve [L.T][x] = [y]
    i = size-1
    z[i] = z[i] / Li[i]
    for i in range(size-2, -1, -1):
        z[i] = (z[i] - Li_1[i-1]*z[i+1])/Li[i]

    # find index
    index = x.searchsorted(x0)
    np.clip(index, 1, size-1, index)

    xi1, xi0 = x[index], x[index-1]
    yi1, yi0 = y[index], y[index-1]
    zi1, zi0 = z[index], z[index-1]
    hi1 = xi1 - xi0

    # calculate cubic
    f0 = zi0/(6*hi1)*(xi1-x0)**3 + \
         zi1/(6*hi1)*(x0-xi0)**3 + \
         (yi1/hi1 - zi1*hi1/6)*(x0-xi0) + \
         (yi0/hi1 - zi0*hi1/6)*(xi1-x0)
    return f0

if __name__ == '__main__':
    import matplotlib.pyplot as plt
    x = np.linspace(0, 10, 11)
    y = np.sin(x)
    plt.scatter(x, y)

    x_new = np.linspace(0, 10, 201)
    plt.plot(x_new, cubic_interp1d(x_new, x, y))

    plt.show()

原文由 raphael valentin 发布，翻译遵循 CC BY-SA 3.0 许可协议