What\'s the Most Efficient Way to Sum a Number\'s Digits?

Sum the Digits of a Number: An Efficient Approach

To find the sum of the digits in a number, the most efficient approach is to utilize purely integer operations. The following Python code showcases this technique:

def sum_digits(n):
    s = 0
    while n:
        s += n % 10
        n //= 10
    return s

Here, we repeatedly apply modulo 10 to extract the rightmost digit and add it to the sum. By integer-dividing by 10, we shift to the next digit.

An even faster version uses a single assignment statement:

def sum_digits3(n):
    r = 0
    while n:
        r, n = r + n % 10, n // 10
    return r

Speed comparisons indicate that using integer operations is significantly faster than converting the number to a string and using string manipulation.

%timeit sum_digits(n)  # Pure integer operations
1000000 loops, best of 3: 479 ns per loop

%timeit sum_digits2(n)  # Integer operations with divmod
1000000 loops, best of 3: 716 ns per loop

%timeit sum(map(int, str(n)))  # String manipulation
1000000 loops, best of 3: 1.42 us per loop

%timeit sum([int(digit) for digit in str(n)])  # List comprehension
100000 loops, best of 3: 1.52 us per loop

%timeit sum(int(digit) for digit in str(n))  # Generator expression
100000 loops, best of 3: 2.04 us per loop

Therefore, utilizing integer operations is the most efficient approach for summing the digits of a number.

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