def convert_number(number_with_comma):
    """
    function for modifying number strings so they can be cast into float:
        "4,5" -> "4.5"
    
    input: number_with_comma (str), string representing float number with comma

    output: number_with_period (str), string representing float number with period
    """
    if ',' in number_with_comma:
        int_decimals = number_with_comma.split(',')
        number_with_period = int_decimals[0]+'.'+int_decimals[1]
    else:
        number_with_period = number_with_comma

    return number_with_period