C++ Standard Library C++ STL Library

C++ <cmath> - logb() Function

The C++ <cmath> logb() function returns the logarithm of |x| (mod of argument), using FLT_RADIX as base for the logarithm.

On most platforms, FLT_RADIX is 2, which makes this function equivalent to log2() function for positive values.


double logb (double x);
float logb (float x);
long double logb (long double x);
double logb (T x);                    


x Specify the value to calculate logarithm.

Return Value

Returns the base-FLT_RADIX logarithm of |x|.


In the example below, logb() function is used to calculate the base-FLT_RADIX logarithm of absolute value of given numbers.

#include <iostream>
#include <cmath>

using namespace std;
int main (){
  cout<<"logb(10) = "<<logb(10)<<"\n";
  cout<<"logb(-10) = "<<logb(-10)<<"\n";
  return 0;

The output of the above code will be:

logb(10) = 3
logb(-10) = 3

❮ C++ <cmath> Library