5 Best Ways to Add Two Numbers Represented as Strings in Python

Rate this post

πŸ’‘ Problem Formulation: This article delves into the challenge of adding two numbers that are not directly represented as integers or floating points, but rather as strings. Specifically, it explores various Python programming techniques to perform addition on these string-encoded numerical values. For instance, if provided with two inputs, “123” and “456”, the desired output should be “579” after the addition operation is appliedβ€”also represented as a string.

Method 1: Using int() Conversion

This method involves converting the string representations of the numbers to integers using the built-in int() function. Once converted, the numbers are added together. The result of this addition is then converted back into a string using the str() function. The strength of this method lies in its simplicity and clear reflection of the intent to work with numeric operations.

Here’s an example:

num1 = "123"
num2 = "456"

def add_string_numbers(str1, str2):
    result = int(str1) + int(str2)
    return str(result)

print(add_string_numbers(num1, num2))

Output: “579”

This code defines a function add_string_numbers() which takes two string parameters, converts them to integers, adds them, and returns the sum converted back to a string. It’s straightforward and utilizes the native type conversion features of Python.

Method 2: Using BigInteger Representation

In cases where the strings represent very large numbers beyond the range of standard integers, Python’s arbitrary-precision arithmetic kicks in. By simply using the int() conversion, Python automatically handles large integers without any special consideration, ensuring no overflow issues.

Here’s an example:

num1 = "987654321987654321987654321987654321"
num2 = "123456789123456789123456789123456789"

def add_large_string_numbers(str1, str2):
    result = int(str1) + int(str2)
    return str(result)

print(add_large_string_numbers(num1, num2))

Output: “1111111111111111111111111111111111110”

This code snippet functions similarly to the first method, except it underscores Python’s capability to handle numbers of arbitrary length without losing precision, ideal for very large numbers.

Method 3: Using Decimal Module for High Precision

When dealing with floating-point numbers represented as strings and requiring high precision in their addition, the decimal.Decimal class from the decimal module can be used. This ensures precision that’s not limited by the floating-point arithmetic provided by Python’s float type.

Here’s an example:

from decimal import Decimal

num1 = "0.1"
num2 = "0.2"

def add_decimal_string_numbers(str1, str2):
    result = Decimal(str1) + Decimal(str2)
    return str(result)

print(add_decimal_string_numbers(num1, num2))

Output: “0.3”

This snippet makes use of the decimal module to handle cases where precision is paramount. Converting to Decimal and then back to string after addition prevents the typical floating-point errors one may encounter.

Method 4: Adding Strings Digit by Digit

For an educational perspective on how addition works, you can implement a manual method that adds the numbers digit by digit, much like how you would on paper. This is also a fallback for environments where type conversion might not be desired or possible.

Here’s an example:

def add_strings_digit_by_digit(str1, str2):
    maxlen = max(len(str1), len(str2))
    str1 = str1.zfill(maxlen)
    str2 = str2.zfill(maxlen)

    result = ''
    carry = 0

    for i in range(maxlen - 1, -1, -1):
        digit_sum = int(str1[i]) + int(str2[i]) + carry
        carry = digit_sum // 10
        result = str(digit_sum % 10) + result

    if carry > 0:
        result = str(carry) + result

    return result

print(add_strings_digit_by_digit("123", "456"))

Output: “579”

This function add_strings_digit_by_digit() zeroes the strings for alignment and iterates backwards, summing digit by digit. It captures carry for any sum above 9 and prepends it to the next calculation. On loop exit, any remaining carry is added to the result.

Bonus One-Liner Method 5: Using Pythons Eval Function

This concise and somewhat cheeky one-liner relies on the powerful eval() function, which evaluates the “sum” expression string as if it were a Python code. While it can solve the problem in a single path, the use of eval() is generally discouraged due to potential security risks.

Here’s an example:

num1 = "123"
num2 = "456"

print(eval(num1 + '+' + num2))

Output: 579

This one-liner takes no prisoners by directly evaluating the expression composed of our input strings and the plus sign. It’s short, but beware of using eval() in a production environment without proper sanitization of the inputs to prevent possible code injection vulnerabilities.


  • Method 1: Using int() Conversion. Simple and intuitive. Might not handle extremely large or precise decimal numbers well without adaptations.
  • Method 2: Using BigInteger Representation. Handles very large numbers efficiently. Python’s native large number support makes this operation straightforward.
  • Method 3: Using Decimal Module for High Precision. Necessary for high precision arithmetic, such as financial calculations. Adds the overhead of importing an extra module.
  • Method 4: Adding Strings Digit by Digit. Educational and works without type conversion, but more complex and not as performant as built-in methods.
  • Bonus One-Liner Method 5: Using Python’s Eval Function. Compact and clever but potentially unsafe, making it unsuitable for most real-world applications.