TheAlgorithms/Python · error · ValueError

Step size must be positive.

Error message

Step size must be positive.

What it means

Error "Step size must be positive." thrown in TheAlgorithms/Python.

Source

Thrown at maths/numerical_analysis/runge_kutta_gills.py:66

    >>> y = runge_kutta_gills(f, 0, 0, 0.2, -1)
    Traceback (most recent call last):
        ...
    ValueError: The final value of x must be greater than initial value of x.

    >>> def f(x, y):
    ...     return x
    >>> y = runge_kutta_gills(f, -1, 0, -0.2, 0)
    Traceback (most recent call last):
        ...
    ValueError: Step size must be positive.
    """
    if x_initial >= x_final:
        raise ValueError(
            "The final value of x must be greater than initial value of x."
        )

    if step_size <= 0:
        raise ValueError("Step size must be positive.")

    n = int((x_final - x_initial) / step_size)
    y = np.zeros(n + 1)
    y[0] = y_initial
    for i in range(n):
        k1 = step_size * func(x_initial, y[i])
        k2 = step_size * func(x_initial + step_size / 2, y[i] + k1 / 2)
        k3 = step_size * func(
            x_initial + step_size / 2,
            y[i] + (-0.5 + 1 / sqrt(2)) * k1 + (1 - 1 / sqrt(2)) * k2,
        )
        k4 = step_size * func(
            x_initial + step_size, y[i] - (1 / sqrt(2)) * k2 + (1 + 1 / sqrt(2)) * k3
        )

        y[i + 1] = y[i] + (k1 + (2 - sqrt(2)) * k2 + (2 + sqrt(2)) * k3 + k4) / 6
        x_initial += step_size
    return y

View on GitHub (pinned to f5988cc097)

Solutions

1. Use a positive step size h > 0.

When it happens

Trigger: Thrown at maths/numerical_analysis/runge_kutta_gills.py:66 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/cadd58bafef81e7c. Report an issue: GitHub.