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 an int.

int start = Convert.ToInt32(curr.ToString().Substring(0, 1));

While this does the job, it feels like there is probably a good, simple, math-based solution to such a problem. String manipulation feels clunky.

Edit: irrespective of speed differences, mystring[0] instead of Substring() is still just string manipulation

Answer 1

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