py_number_base_converter — Convert Between Number Bases

Difficulty: medium

Write a function that converts a number from one base to another. Support bases from 2 to 36 inclusive, using digits 0-9 and letters A-Z for values 10-35. Return "ERROR" for invalid inputs (base, digits).

def number_base_converter(number: str, from_base: int, to_base: int) -> str:

Examples

>>> number_base_converter("1010", 2, 10)
"10"

>>> number_base_converter("FF", 16, 10)
"255"

>>> number_base_converter("255", 10, 16)
"FF"

>>> number_base_converter("123", 10, 2)
"1111011"

>>> number_base_converter("Z", 36, 10)
"35"

>>> number_base_converter("35", 10, 36)
"Z"

>>> number_base_converter("123", 1, 10)
"ERROR"

>>> number_base_converter("G", 16, 10)
"ERROR"

Solution

Download py_number_base_converter.py

def number_base_converter(number: str, from_base: int, to_base: int) -> str:
    if not (2 <= from_base <= 36) or not (2 <= to_base <= 36):
        return "ERROR"
    if not number:
        return "ERROR"

    digits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    value = 0
    for ch in number.upper():
        if ch not in digits:
            return "ERROR"
        d = digits.index(ch)
        if d >= from_base:
            return "ERROR"
        value = value * from_base + d

    if value == 0:
        return "0"

    result = ""
    while value > 0:
        result = digits[value % to_base] + result
        value //= to_base
    return result

How It Works

Goal: Convert a number string from one base to another, with validation.

Approach: Parse the input into an integer using the source base, then build the output string by repeatedly dividing by the target base.

Step by step:

1. Validate both bases are within [2, 36]; otherwise return "ERROR".
2. For each character of the input, find its digit value. Reject characters that are out of the digit alphabet or out of range for the source base.
3. Accumulate the integer value: value = value * from_base + digit.
4. Convert back by repeatedly taking value % to_base (rightmost digit) and integer-dividing.

Key concept: Positional numeral systems — a number in base b is Σ dᵢ · bⁱ, so parsing multiplies by b and converting back divides by b.