TheAlgorithms/Python · error · ValueError

Inputs and select signal must be 0 or 1

Error message

Inputs and select signal must be 0 or 1

What it means

Raised by mux() when any of input0, input1, or select is not exactly the int 0 or 1. The multiplexer models a hardware 2-to-1 MUX, so all three signals must be strict binary; the check uses membership `i in (0, 1)` which also rejects 1.0, True-adjacent floats, and any other value.

Source

Thrown at boolean_algebra/multiplexer.py:36

    1
    >>> mux(1, 0, 1)
    0
    >>> mux(2, 1, 0)
    Traceback (most recent call last):
        ...
    ValueError: Inputs and select signal must be 0 or 1
    >>> mux(0, -1, 0)
    Traceback (most recent call last):
        ...
    ValueError: Inputs and select signal must be 0 or 1
    >>> mux(0, 1, 1.1)
    Traceback (most recent call last):
        ...
    ValueError: Inputs and select signal must be 0 or 1
    """
    if all(i in (0, 1) for i in (input0, input1, select)):
        return input1 if select else input0
    raise ValueError("Inputs and select signal must be 0 or 1")


if __name__ == "__main__":
    import doctest

    doctest.testmod()

View on GitHub (pinned to f5988cc097)

Solutions

1. Normalize signals to binary before calling: s = int(bool(s)).
2. Validate all three arguments against (0, 1) in your caller and reject/clip earlier.
3. If using -1/1 encoding, map to 0/1 first: (x + 1) // 2.

Example fix

# before
mux(0, 1, 0.7)  # ValueError: Inputs and select signal must be 0 or 1

# after
mux(0, 1, int(select > 0.5))  # binary select

if not all(i in (0, 1) for i in (input0, input1, select)):
    raise ValueError('mux signals must be binary 0/1')

def is_binary(v: object) -> bool:
    return isinstance(v, int) and v in (0, 1)

When it happens

Trigger: Calling mux(0, 2, 1), mux(0, -1, 0), or mux(0, 1, 1.1) as in the doctests. Also mux(True, 1, 0) passes (bool == int 1) but mux(0, 1, 0.0) fails because 0.0 in (0, 1) is True — careful: 0.0 == 0 so floats 0.0/1.0 actually pass; values like 2, -1, 1.1 fail.

Common situations: Feeding unnormalized boolean data (e.g. -1/1 encoding, probabilities, or numpy ints that are fine but floats like 1.5 that are not), or passing results of arithmetic that can exceed 1.

Related errors

• Input list must contain at least two elements
• number must be positive
• The value of input must be non-negative
• plain must contain only lowercase letters (a-z)
• plaintext is empty

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/6be8c70ae8f44342. Report an issue: GitHub.