Is there any way to write log(base 2) function?
the c language has 2 built in function -->>
1.log which is base e.
2.log10 base 10;
But i need log function of base 2.How to calculate this.Someone help me.
+19
A:
Simple math:
log2 (x) = logy (x) / logy (2)
where y can be anything, which for standard log functions is either 10 or e.
Adam Crume
2010-06-17 19:27:33
Thanx a lot.I got my answer.
russell
2010-06-17 19:34:40
@russell, you should accept this answer then.
Mark Ransom
2010-06-17 19:39:43
A:
Consult your basic mathematics course, log n / log 2. It doesn't matter whether you choose log or log10in this case, dividing by the log of the new base does the trick.
Pieter
2010-06-17 19:27:45
+2
A:
As stated on http://en.wikipedia.org/wiki/Logarithm:
logb(x) = logk(x) / logk(b)
Which means that:
log2(x) = log10(x) / log10(2)
Patrick
2010-06-17 19:28:33
@Johannes: I doubt the compiler will pre-compute log10(2). The compiler doesn't know that log10 will return the same value every time. For all the compiler knows, log10(2) could return different values on successive calls.
abelenky
2010-06-17 21:24:20
@abelenky: Ok, I take that back. Since the compiler never sees the source to the `log()` implementation it won't do it. My bad.
Joey
2010-06-17 21:42:30
@abelenky: Since `log10()` is a function defined in the C standard, the compiler is free to treat it "specially", including precomputing the result, which I believe was @Johannes' suggestion?
caf
2010-06-18 04:27:59
A:
What you're looking to do is logarithmic change of base, which is well documented and easy to do.
fbrereto
2010-06-17 19:28:36
A:
If you want to compute log2 for integers, try this: http://en.wikipedia.org/wiki/Binary_logarithm#Algorithm
lhf
2010-06-17 19:29:16
+3
A:
If you're looking for an integral result, you can just determine the highest bit set in the value and return its position.
tomlogic
2010-06-17 20:35:12
+1
A:
C99 (the latest version of C), has log2 (as well as log2f and log2l for float and long double).
Matthew Flaschen
2010-06-17 21:12:32