AtsushiSakai/PythonRobotics · error · ValueError

x coordinates must be sorted in ascending order

Error message

x coordinates must be sorted in ascending order

What it means

CubicSpline's constructor computes np.diff(x) and rejects any negative difference, i.e. x points not sorted ascending. Spline math requires monotonically increasing knots.

Source

Thrown at PathPlanning/CubicSpline/cubic_spline_planner.py:50

    >>> y = [1.7, -6, 5, 6.5, 0.0]
    >>> sp = CubicSpline1D(x, y)
    >>> xi = np.linspace(0.0, 5.0)
    >>> yi = [sp.calc_position(x) for x in xi]
    >>> plt.plot(x, y, "xb", label="Data points")
    >>> plt.plot(xi, yi , "r", label="Cubic spline interpolation")
    >>> plt.grid(True)
    >>> plt.legend()
    >>> plt.show()

    .. image:: cubic_spline_1d.png

    """

    def __init__(self, x, y):

        h = np.diff(x)
        if np.any(h < 0):
            raise ValueError("x coordinates must be sorted in ascending order")

        self.a, self.b, self.c, self.d = [], [], [], []
        self.x = x
        self.y = y
        self.nx = len(x)  # dimension of x

        # calc coefficient a
        self.a = [iy for iy in y]

        # calc coefficient c
        A = self.__calc_A(h)
        B = self.__calc_B(h, self.a)
        self.c = np.linalg.solve(A, B)

        # calc spline coefficient b and d
        for i in range(self.nx - 1):
            d = (self.c[i + 1] - self.c[i]) / (3.0 * h[i])
            b = 1.0 / h[i] * (self.a[i + 1] - self.a[i]) \

View on GitHub (pinned to 1fe4fb980f)

Solutions

1. Sort points by x before construction: order = np.argsort(x); CubicSpline(x[order], y[order]).
2. Fix the data source to emit monotonically increasing x.
3. Validate np.all(np.diff(x) > 0) before calling.

Example fix

# before
sp = CubicSpline(x, y)  # x unsorted

# after
order = np.argsort(x)
sp = CubicSpline(x[order], y[order])

import numpy as np
assert np.all(np.diff(x) > 0), 'x must be strictly ascending'

def sorted_ascending(x) -> bool:
    import numpy as np
    return bool(np.all(np.diff(x) > 0))

When it happens

Trigger: Constructing CubicSpline(x, y) with x out of order, e.g. [0, 2, 1, 3], or with duplicate/unsorted waypoint data from a file.

Common situations: Waypoints collected from GPS or user input in arbitrary order, or data shuffled during preprocessing before spline fitting.

Related errors

• Input array should only contain 0 and 1

AI-assisted analysis of AtsushiSakai/PythonRobotics@1fe4fb980f (2026-08-28). Data as JSON: /api/errors/3d2e1c3a1b090b13. Report an issue: GitHub.