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Parsing a number into digits

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Featured Image - "Miss South Carolina Powerset Parse" by official_powerset, used under BY SA / Dropped Quality to 80% from original

I've run into situations where I had to parse a number into digits.

An easy way to do this is to convert a number into a string and returns a character array, and then convert each character into a number

When dealing with large sets of long numbers, it simply is not optimal.

There is a simple way to convert a number into digits, which requires a simple math.

Given an integer  "val =213", one might consider converting it to a string, breaking it to a character array and convert each character into an integer (it's so simple in LINQ, it's just tempting to implement it this way).

private static List<int> GetDigitsUsingConversion(int val)
{
    return val.ToString().ToCharArray().Select(c => (int) c).ToList();
}

conversion cost is too damn high

The cost of type conversion from an integer to a string, to an array, and then back to integer is too high.

Now let's take a look at another way using a simple math.

Given an integer 213, if you

  1. divide it by 1 and mod 10, you get 3
  2. divide it by 10 and mod 10, you get 1
  3. divide it by 100 and mod 10, you get 2

If you look at the returned result, it's each digit returned in reverse order. There is a useful data structure, for holding data in reverse order, Stack.

An algorithm is fairly simple.

While the given number is greater than 0, divide it by 10^digit, put the digit into a stack, and lastly return the stack as a list.

private static List<int> GetDigits(int val)
{
	Stack<int> stack = new Stack<int>();

	int number = val;
	while (number > 0)
	{
		var digit = number % 10;
		stack.Push(digit);

		number /= 10;
	}

	return stack.ToList();
}

During each iteration, "number" is divided by 10 so it is equivalent to dividing by 10^digit.

I did a simple benchmarking (contrived but works for a simple demo) and the one requiring a type conversion to a string ran about 2x as long.

private const int UPTO = 1000000;

public static void Main(string\[\] args)
{
int val = 123456789;

    Stopwatch watch = new Stopwatch();

    watch.Start();
    for (int i = 0; i < UPTO; i++)
    {
    	List<int> digits = GetDigits(val);
    }
    watch.Stop();
    Console.WriteLine("GetDigit took {0}ms", watch.ElapsedMilliseconds);

    watch.Start();
    for (int j = 0; j < UPTO; j++)
    {
    	List<int> digits2 = GetDigitsUsingConversion(val);
    }

    watch.Stop();
    Console.WriteLine("GetDigitsUsingConversion took {0}ms", watch.ElapsedMilliseconds);

}

Result:

GetDigit took 754ms
GetDigitsUsingConversion took 1468ms

The source code is available on GitHub.

Conclusion

By using simple math, you can extract digits from a number.

It requires no type conversion thus saving run time.