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Last Two Digit Sum in Python


Given a non-negative integer n with at least two digits, compute and return the sum of the last two digits

Example:

Input: n = 2379

Output: 16

Explanation:
Last two digits of n are 7 and 9, so our answer is 7 + 9 = 16

Understanding the Problem

The core challenge of this problem is to extract the last two digits of a given number and compute their sum. This problem is significant in various applications such as checksum calculations, digital root computations, and more. A common pitfall is to overcomplicate the extraction of the last two digits, but with the right approach, it can be simplified.

Approach

To solve this problem, we can use the modulo operator to extract the last two digits of the number. Here's a step-by-step approach:

  1. Use the modulo operator to get the last two digits of the number.
  2. Extract the last digit using modulo 10.
  3. Extract the second last digit by performing integer division by 10.
  4. Sum the two extracted digits.

Let's break down the algorithm:

Algorithm

  1. Compute last_two_digits = n % 100 to get the last two digits.
  2. Compute last_digit = last_two_digits % 10 to get the last digit.
  3. Compute second_last_digit = last_two_digits // 10 to get the second last digit.
  4. Return the sum of last_digit and second_last_digit.

Code Implementation

def sum_last_two_digits(n):
    # Step 1: Get the last two digits
    last_two_digits = n % 100
    
    # Step 2: Get the last digit
    last_digit = last_two_digits % 10
    
    # Step 3: Get the second last digit
    second_last_digit = last_two_digits // 10
    
    # Step 4: Return the sum of the last two digits
    return last_digit + second_last_digit

# Example usage
n = 2379
print(sum_last_two_digits(n))  # Output: 16

Complexity Analysis

The time complexity of this approach is O(1) because the operations involved (modulo and integer division) are constant time operations. The space complexity is also O(1) as we are using a fixed amount of extra space.

Edge Cases

Consider the following edge cases:

  • n = 10: The last two digits are 1 and 0, so the sum is 1.
  • n = 99: The last two digits are 9 and 9, so the sum is 18.
  • n = 100: The last two digits are 0 and 0, so the sum is 0.

Testing

To test the solution comprehensively, consider a variety of test cases:

def test_sum_last_two_digits():
    assert sum_last_two_digits(2379) == 16
    assert sum_last_two_digits(10) == 1
    assert sum_last_two_digits(99) == 18
    assert sum_last_two_digits(100) == 0
    assert sum_last_two_digits(12345) == 8
    print("All tests passed.")

test_sum_last_two_digits()

Thinking and Problem-Solving Tips

When approaching such problems, consider the following tips:

  • Break down the problem into smaller, manageable parts.
  • Use mathematical operations like modulo and integer division to simplify the extraction of digits.
  • Think about edge cases and test your solution against them.
  • Practice similar problems to improve your problem-solving skills.

Conclusion

In this blog post, we discussed how to solve the problem of summing the last two digits of a given number. We explored the problem definition, approach, algorithm, code implementation, complexity analysis, edge cases, and testing. Understanding and solving such problems is crucial for developing strong problem-solving skills in programming.

Additional Resources

For further reading and practice, consider the following resources: