Check the number of samples.

    :param in_prefix: the prefix of the input file.

    :type in_prefix: str

    :returns: the number of sample in ``prefix.fam``.

def compute_nb_samples(in_prefix):
    """Check the number of samples.

    :param in_prefix: the prefix of the input file.

    :type in_prefix: str

    :returns: the number of sample in ``prefix.fam``.

    """
    file_name = in_prefix + ".tfam"
    nb = None
    with open(file_name, 'rb') as input_file:
        nb = len(input_file.readlines())
    return nb