Finding Log of a large BigInteger not working correctly?

Question

I'm trying to find the logarithm of a very large BigInteger in C#. I don't care what the base of the logarithm is. When I try this:

BigInteger b = 1000; // the base
// myBigInt is a huge BigInt i want to find the Log of.

exponent = BigInteger.Log(myBigInt, 1000); //Find the Log
// Re-create the orignal BigInt now that I know base and exponent
BigInteger.Pow(b, Convert.ToInt32(exponent)); 

I get an overflow exception, because Int32 cannot hold the result of the Log. Increasing the value of base doesn't work.

Answer 1

You are not using the same basis in the log and pow

BigInteger bigInt = 10000000000000000000;  // myBigInt is a huge BigInt i want to find the Log of.
double log;
log = BigInteger.Log(bigInt, 1000); //Find the Log
System.Diagnostics.Debug.WriteLine(log.ToString());
System.Diagnostics.Debug.WriteLine(((Int32)log).ToString());
BigInteger bigIntRecreate;
// unless log is an integer value it will round down and will not recreate to proper value
bigIntRecreate = BigInteger.Pow(1000, (Int32)log);
System.Diagnostics.Debug.WriteLine(bigInt.ToString("N0"));
System.Diagnostics.Debug.WriteLine(bigIntRecreate.ToString("N0"));
System.Diagnostics.Debug.WriteLine((bigInt - bigIntRecreate).ToString("N0"));