How can you get the first digit in an int (C#)?

Question

In C#, what\'s the best way to get the 1st digit in an int? The method I came up with is to turn the int into a string, find the 1st char of the string, then turn it back to

Answer 1

This is what I usually do ,please refer my function below :

This function can extract first number occurance from any string you can modify and use this function according to your usage

   public static int GetFirstNumber(this string strInsput)
    {
        int number = 0;
        string strNumber = "";
        bool bIsContNo = true;
        bool bNoOccued = false;

        try
        {
            var arry = strInsput.ToCharArray(0, strInsput.Length - 1);

            foreach (char item in arry)
            {
                if (char.IsNumber(item))
                {
                    strNumber = strNumber + item.ToString();

                    bIsContNo = true;

                    bNoOccued = true;
                }
                else
                {
                    bIsContNo = false;
                }

                if (bNoOccued && !bIsContNo)
                {
                    break;
                }


            }

            number = Convert.ToInt32(strNumber);

        }
        catch (Exception ex)
        {

            return 0;
        }

        return number;

    }

Answer 2

int i = 4567789;
int digit1 = int.Parse(i.ToString()[0].ToString());

Answer 3

int myNumber = 8383;
char firstDigit = myNumber.ToString()[0];
// char = '8'

Answer 4

Very simple (and probably quite fast because it only involves comparisons and one division):

if(i<10)
   firstdigit = i;
else if (i<100)
   firstdigit = i/10;
else if (i<1000)
   firstdigit = i/100;
else if (i<10000)
   firstdigit = i/1000;
else if (i<100000)
   firstdigit = i/10000;
else (etc... all the way up to 1000000000)

Answer 5

Had the same idea as Lennaert

int start = number == 0 ? 0 : number / (int) Math.Pow(10,Math.Floor(Math.Log10(Math.Abs(number))));

This also works with negative numbers.

Answer 6

Benchmarks

Firstly, you must decide on what you mean by "best" solution, of course that takes into account the efficiency of the algorithm, its readability/maintainability, and the likelihood of bugs creeping up in the future. Careful unit tests can generally avoid those problems, however.

I ran each of these examples 10 million times, and the results value is the number of ElapsedTicks that have passed.

Without further ado, from slowest to quickest, the algorithms are:

Converting to a string, take first character

int firstDigit = (int)(Value.ToString()[0]) - 48;

Results:

12,552,893 ticks

Using a logarithm

int firstDigit = (int)(Value / Math.Pow(10, (int)Math.Floor(Math.Log10(Value))));

Results:

9,165,089 ticks

Looping

while (number >= 10)
    number /= 10;

Results:

6,001,570 ticks

Conditionals

int firstdigit;
if (Value < 10)
     firstdigit = Value;
else if (Value < 100)
     firstdigit = Value / 10;
else if (Value < 1000)
     firstdigit = Value / 100;
else if (Value < 10000)
     firstdigit = Value / 1000;
else if (Value < 100000)
     firstdigit = Value / 10000;
else if (Value < 1000000)
     firstdigit = Value / 100000;
else if (Value < 10000000)
     firstdigit = Value / 1000000;
else if (Value < 100000000)
     firstdigit = Value / 10000000;
else if (Value < 1000000000)
     firstdigit = Value / 100000000;
else
     firstdigit = Value / 1000000000;

Results:

1,421,659 ticks

Unrolled & optimized loop

if (i >= 100000000) i /= 100000000;
if (i >= 10000) i /= 10000;
if (i >= 100) i /= 100;
if (i >= 10) i /= 10;

Results:

1,399,788 ticks

Note:

each test calls Random.Next() to get the next int