main.py

"""
Task: Write a  method that takes a  string as an input and converts it  to an integer.
Do not use any type of built in methods like int.Parse or atoi.

Solution:
Strategy:
1. Use regular expression to check if the input is a valid numeric string
2. Use decimal number definition to construct the integer (e.g., 23 = 2*10+3, 134 = (1*10+3)*10+4), -3 = -(3))
"""
import re


def numstring_to_integer(numstring, allowed_leading_zeros=False):
    """
    Convert a numeric string input to an integer without using built-in parse functions
    The function only convert valid numeric strings. e.g.,
    Valid numeric strings: "4094499383228822200494837", "-193", "0004532", "-009284" (when allowing leading zeros)
    Invalid numeric strings: "d993", "00-343", "+4544", "--34"

    :param numstring: str, numeric string input
    :param allowed_leading_zeros: bool, if True allowing leading zeros
    :return: int or None, if the input is valid numeric string. Otherwise return None
    """
    # Check if the input string is a valid integer (use regular expression)
    if not allowed_leading_zeros:  # NOT allow leading 0s
        is_integer = re.match("[-]?[1-9]+[0-9]*$", numstring)
    else:
        is_integer = re.match("[-]?[0-9]+$", numstring)

    # Convert the string input to int if the input is a valid numeric string.
    # If not a valid numeric string. Print out the error.
    if not is_integer:
        print(f"ERROR: The input {numstring} is NOT a valid integer!!!")
    else:
        dict_str_to_int = {'0': 0, '1': 1, '2': 2, '3': 3, '4': 4, '5': 5, '6': 6, '7': 7, '8': 8, '9': 9}
        result_int = 0
        for num_str in numstring:
            if num_str != '-':      # handle negative value
                result_int = 10 * result_int + dict_str_to_int.get(num_str)     # use decimal number definition
        if numstring.startswith('-'):
            result_int = -result_int

        return result_int


if __name__ == "__main__":
    string_input = input("""
    Convert a numeric string to an integer.

    TYPE a NUMBER and PRESS \"ENTER\" (Leave empty to test the default cases):  
    """)

    print()
    # Allow leading zeros
    if string_input:
        result = numstring_to_integer(string_input, allowed_leading_zeros=True)
        print(f"The numeric string input is: {string_input}, type: {type(string_input)}")
        print(f"Convert to integer:          {result}, type: {type(result)}")
    else:
        default_test_cases = ["4094499383228822200494837",
                              "-193",
                              "d993",
                              "0004532",
                              "-009284",
                              "00-343",
                              "+4544",
                              "--34"]
        for i, case in enumerate(default_test_cases):
            result = numstring_to_integer(case, allowed_leading_zeros=True)
            print(f"Test case {i}")
            print(f"The numeric string input is: {case}")
            print(f"Convert to integer:          {result}")
            print("-----------------------")