import string #importing some useful string methods

"""
This function returns True if my_string can be safely converted
to an integer using the int function.
"""
def check_integer(my_string):
    # this line removes whitespace from the beginning and the end.
    my_string = string.strip(my_string)
    position = 0

    # we will set illegal_flag to true if we find proof that the string is
    # not a valid integer
    illegal_flag = False

    # skip any minus sign at the beginning
    if position < len(my_string):
        character = my_string[position]
        if character == "-": #skip minus sign
            position = position + 1

    # we now should get a string of consecutive digits. Let's count how many.
    digit_count = 0
    while position < len(my_string):
        character = my_string[position]
        if character in "0123456789":
            position = position + 1
            digit_count = digit_count + 1
        else:
            illegal_flag = True
            break

    if digit_count == 0:
        illegal_flag = True

    if (illegal_flag == True):
        return False
    else:
        return True


"""
This function can be used as a substitute for the built-in
Python input function, when we want to get an integer from
the user. The difference is that the built-in function crashes
if the user does not enter a number. The input_integer function
just keeps asking the user to enter a number.
"""
def input_integer(message):
    while(True):
        text = raw_input(message)
        if (check_integer(text) == True):
            result = int(text)
            return result

        print "you need to type in an integer!"
    


def list_to_string(list1):
    if not(type(list1) is list):
        return None

    result = ""
    for i in list1:
        if not(type(i) is str):
            return None
        if not(len(i) == 1):
            return None

        result = result + i;

    return result