log2

I'm unable to get log2 to work in my program.
does anyone have an idea to get this to work.
i have read the following

Code:

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#include<cmath>

double logResult=0;

logResult=log2(128);

cout<<logResult;

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i get an error of
log2 undeclared identifier

There is no log2 in the c standard. It's either log or log10. If you want to get a base two log value, you'd have to

Code:

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log(128.0)/log(2.0);

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It is normally a good idea to check if a function exists before you attempt to use it. log2() is not a standard function in C or C++.

The only standard functions are log() which computes the natural logarithm (to base 2.71828...) and log10() which computes the logarithm to base 10.

The log of x in some arbitrary base can be computed as log(x)/log(base).