How to find the log of a number?

Question

I have a number (num1) which is 18 digits long. I am storing it in an integer array. I have another number (num2) which is also 18 digits long. This is also being stored in an integer array.

I have to find the log of the first number to the base of the second number (log num1 to base num2).

How to do this in C++? I can't use the log function as the numbers are being stored in arrays.

Do you have a big numbers library you're using? If not - get one. — littleadv, Apr 02 '12 at 08:07
If you want to do it manually, see the answer to this question http://stackoverflow.com/questions/739532/logarithm-of-a-bigdecimal — Jeff Foster, Apr 02 '12 at 08:08
18 digits of what? Stored in these integers how? What endianness? Packed binary? ASCII hex? ASCII decimal? What? In what form do you want the output? — Martin James, Apr 02 '12 at 08:14
Show us what you have so far, especially how the numbers are stored. And the result, must it be exact or can it be an approximation (with, say, 53 bits of precision)? — Mr Lister, Apr 02 '12 at 08:27
This sounds like a very confused and incomplete question on the [discrete log problem](http://en.wikipedia.org/wiki/Discrete_log). — President James K. Polk, Apr 03 '12 at 00:14

score 2 · Answer 1 · answered Apr 02 '12 at 08:12

They key term to google for is bigint. There are various C++ libraries which support bigints (that is, number which can be as long as your memory permits).

The only bigint library I've used myself is GMP. However, if you just need a single function on bigints (log, in your case), then maybe taking some smaller library (is more practical).

Just checked the GMP page and incidentally, it calls them bignums all over. So that may be another useful term to use when searching for a solution. ;-)

How to find the log of a number?

1 Answers1