TheAlgorithms/Python · error · ValueError

The word parameter should be a string of length greater than

Error message

The word parameter should be a string of length greater than 0.

What it means

Raised by doppler_effect() in physics/doppler_frequency.py when the computed observed frequency is <= 0. With f = f0*(v + v0)/(v - vs), a non-positive result means the numerator or denominator flipped sign: the source outruns the wave toward the observer (vs > v) or the observer recedes faster than the wave propagates (v0 < -v with the source behind). Physically the simple Doppler formula has broken down, so the library refuses the value.

Source

Thrown at backtracking/word_search.py:144

    board_error_message = (
        "The board should be a non empty matrix of single chars strings."
    )

    len_board = len(board)
    if not isinstance(board, list) or len(board) == 0:
        raise ValueError(board_error_message)

    for row in board:
        if not isinstance(row, list) or len(row) == 0:
            raise ValueError(board_error_message)

        for item in row:
            if not isinstance(item, str) or len(item) != 1:
                raise ValueError(board_error_message)

    # Validate word
    if not isinstance(word, str) or len(word) == 0:
        raise ValueError(
            "The word parameter should be a string of length greater than 0."
        )

    len_board_column = len(board[0])
    for i in range(len_board):
        for j in range(len_board_column):
            if exits_word(
                board, word, i, j, 0, {get_point_key(len_board, len_board_column, i, j)}
            ):
                return True

    return False


if __name__ == "__main__":
    import doctest

    doctest.testmod()

View on GitHub (pinned to f5988cc097)

Solutions

1. Check the sign convention: obs_vel + toward source, src_vel + toward observer; verify your signs before calling.
2. Reject or special-case inputs where src_vel > wave_vel or obs_vel < -wave_vel (supersonic regimes need shock physics, not this formula).
3. Catch ValueError and flag the input pair as out of model range.

Example fix

# before
f = doppler_effect(100, 330, 10, 340)  # supersonic source -> ValueError

# after
if src_vel >= wave_vel or obs_vel <= -wave_vel:
    raise ValueError("supersonic regime: Doppler formula invalid")
f = doppler_effect(100, 330, 10, src_vel)

if src_vel > wave_vel or obs_vel < -wave_vel:
    raise ValueError("supersonic regime outside Doppler model")
f = doppler_effect(org_freq, wave_vel, obs_vel, src_vel)

try:
    f = doppler_effect(f0, v, v0, vs)
except ValueError as e:
    if "Non-positive frequency" in str(e):
        sign_hint = 'src_vel too large' if vs > v else 'obs_vel too negative (sign convention?)'
        raise ValueError(sign_hint) from e
    raise

When it happens

Trigger: doppler_effect(100, 330, 10, 340): source at 340 m/s > 330 m/s wave speed. doppler_effect(100, 330, -340, 10): observer receding at 340 m/s > wave speed. Any supersonic closing speed or observer receding faster than v.

Common situations: Supersonic aircraft / Mach > 1 simulations where the classical Doppler formula does not apply; sign-convention mistakes — obs_vel is positive when moving toward the source, src_vel positive when moving toward the observer, and swapped signs can flip the result negative.

Related errors

• Input value must be a positive integer
• Impossible fluid density
• Impossible bulk modulus
• Expected a_coeffs to have {self.order + 1} elements for {sel
• Expected b_coeffs to have {self.order + 1} elements for {sel

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/4a530142437de45d. Report an issue: GitHub.