C# replace in long variable

Question

I have a large number like:

1012223457

I need to set the 3rd-most-significant digit in this number to 3. How can I achieve that? Is there any way to do it without converting to string and without adding "20000000" to it?

Answer 1

This would do it without a string. Most of the convolution is determining the length of the number without using string. So rather I use a mathematical trick:

long number = 1012223457;
number += (long)(2*Math.Pow(10.0,(double)((int)(Math.Log10(Math.Max(Math.Abs(number), 0.5)) + 1)-3)));

The 2* is the value you wanted to add.

The solution is fugly, but it should work.

I didn't even think that the trick for the length of a number would be in StackOverflow but you can find information on it here . Coincidentally the code provided was in C#.

So one has to go with a pretty complex method without using strings, but would require math functions to compensate. I'd probably go with something that uses string because readability would be much improved.

Answer 2

This is not the best way to do what you asked for OP, you have to use ToString for the best and most reliable solution, but nevertheless, here's a sample that does what you asked for:

var a = 10000000L;
var pows = Enumerable.Range(0, 16).Select(x => (long)Math.Pow(10, x));
var d = pows.First(x => a / x == 0);
var thirdDigit = (a / (d / 1000)) % 100;
a += (long)((3 - thirdDigit) * (d / 1000));

Answer 3

See this answer:

How can I get a count of the total number of digits in a number?

Thus it is posssible to do like this:

int digitpos = 5;
long number = 1080511231123;
int numbertoset = 3;

int digitcount = (int)Math.Floor(Math.Log10(number) + 1);
long exp = (long)Math.Pow(10, digitcount - digitpos);
if (exp == 0) throw new ArgumentException("The number does not contain required digit.");
int targetdigit = (int)(number / exp) % 10;
int multiplier = numbertoset - targetdigit;
long result = number + multiplier * exp;

To replace the target digit I had to calculate its value first.

Answer 4

The small usage of ToString results in a much cleaner code. Keep in mind that I am still using mathematical functions to do the change, no string manipulation involved.

   var number = 123456789;
   var length = number.ToString().Length;
   var multiplier = Math.Pow(10, length - 3):
   var oldDigit = Math.Floor(number / multiplier) % 10;
   var change = 3 - oldDigit;
   number += change * multiplier;