# 5 Best Ways to Program to Find Sum of Digits Until It Is a One-Digit Number in Python

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π‘ Problem Formulation: Given an integer, the objective is to repeatedly sum its digits until the result is a single-digit number. For instance, if the input is `9875`, the process would be: `9+8+7+5=29`, and then `2+9=11`, and finally `1+1=2`, which is a single-digit number.

## Method 1: Using a While Loop

This method involves parsing the number into its individual digits and summing them within a while loop. The loop continues to execute until a one-digit number is obtained. The function specification is simple: take an integer as input and return a single-digit integer after successive summations of its digits.

Here’s an example:

```def sum_digits_until_one(num):
while num > 9:
num = sum(int(digit) for digit in str(num))
return num

print(sum_digits_until_one(9875))
```

The output of this code snippet is:

`2`

The code defines a function `sum_digits_until_one()` that continues to sum the digits of the input number `num` until `num` is a single-digit number. This is a straightforward and easy-to-understand approach.

## Method 2: Using Recursion

Recursion is a technique where a function calls itself to reduce the problem into smaller instances. In this context, we use recursion to sum the digits of the number, and call the same function again if the result is not a single-digit number.

Here’s an example:

```def sum_digits_until_one_recursive(num):
if num < 10:
return num
else:
return sum_digits_until_one_recursive(sum(int(digit) for digit in str(num)))

print(sum_digits_until_one_recursive(9875))
```

The output of this code snippet is:

`2`

This snippet illustrates a recursive function `sum_digits_until_one_recursive()` which calls itself with the digit sum of the input number until the sum is a one-digit number. This method is elegant and involves fewer lines of code, but can be harder to understand for beginners.

## Method 3: Using a Digital Root Concept

The digital root of a non-negative integer is the single-digit value obtained by an iterative process of summing digits, on each iteration using the result from the previous iteration to compute the digit sum. This process continues until a single-digit number is produced. This single-digit result is the digital root of the original number.

Here’s an example:

```def digital_root(num):
return (num - 1) % 9 + 1 if num > 0 else 0

print(digital_root(9875))
```

The output of this code snippet is:

`2`

In this code, `digital_root()` computes the digital root of the input number using a simple mathematical formula which avoids iteration or recursion, making it the most efficient method.

## Method 4: Iterative Process with String Conversion

This approach uses iteration with string conversion to perform the digit summing process. It converts the number to a string to facilitate splitting into individual digits, then converts the digits back to integers to sum them.

Here’s an example:

```def sum_digits_iterative(num):
while len(str(num)) > 1:
num = sum(map(int, str(num)))
return num

print(sum_digits_iterative(9875))
```

The output of this code snippet is:

`2`

This code uses an iterative approach similar to Method 1, but differentiates by explicitly checking the length of the number as a string, thus ensuring it reduces the number to a single digit.

## Bonus One-Liner Method 5: Using Generator Expressions

For those who prefer concise code, a one-liner can achieve the same result using a generator expression within a recursive function. This method makes use of Python’s lazy evaluation and expressive capabilities to minimize the code footprint.

Here’s an example:

```f = lambda x: x if x < 10 else f(sum(int(d) for d in str(x)))
print(f(9875))
```

The output of this code snippet is:

`2`

This one-liner defines a lambda function `f` that recursively sums the digits of its input until it becomes a one-digit number. It is an elegant and compact solution that leverages the power of lambda functions and generator expressions.

## Summary/Discussion

• Method 1: While Loop. It is easy to understand and implement. However, it’s not as elegant as a mathematical solution, and may not be as efficient as other methods.
• Method 2: Recursion. Offers a cleaner, more straightforward code and is as efficient as the while loop. The downside is its potential for stack overflow on very large inputs.
• Method 3: Digital Root Concept. It’s the most efficient method as it avoids loops and recursion. It could be less intuitive unless one is familiar with the digital root concept. Good for large numbers.
• Method 4: Iterative Process with String Conversion. Similarly straightforward but checks string length repeatedly, which could be slightly less efficient than the while loop approach.
• Method 5: One-Liner. Offers elegance and brevity. However, it might sacrifice readability and could lead to stack overflow with very large numbers due to recursion.