logb() function in C++ STL

The logb() is a builtin function in C++ STL which returns the logarithm of |x|, using FLT_RADIX as base for the logarithm. In general, the value of FLT_RADIX is 2, so logb() is equivalent to log2()(for positive values only).

Syntax:

logb(val)

Parameter: The function accepts a single mandatory parameter val which specifies the val whose logb() is to be calculated. The data-type can be of double, float, long double or int.

Return Type: The function returns the logarithm of |x|.

Below programs illustrate the above function:

Program 1:

// C++ program to illustrate 
// to implement logb() function 
// when data-type is integer 
#include <cmath> 
#include <iostream> 
using namespace std; 
int main() 
{ 
    double result; 
    int x = -10; 
  
    result = logb(x); 
    cout << "logb(" << x << ") = "
         << "log(|" << x << "|) = " << result << endl; 
  
    x = 10; 
  
    result = logb(x); 
    cout << "logb(" << x << ") = "
         << "log(|" << x << "|) = " << result << endl; 
  
    return 0; 
} 

Output:

logb(-10) = log(|-10|) = 3
logb(10) = log(|10|) = 3

Program2:

// C++ program to illustrate 
// to implement logb() function 
// when data-type is double 
#include <cmath> 
#include <iostream> 
  
using namespace std; 
  
int main() 
{ 
    double x = 70.56, result; 
  
    result = logb(x); 
    cout << "logb(" << x << ") = "
         << "log(|" << x << "|) = " << result << endl; 
  
    x = 17.6; 
  
    result = logb(x); 
    cout << "logb(" << x << ") = "
         << "log(|" << x << "|) = " << result << endl; 
  
    return 0; 
} 

Output:

logb(70.56) = log(|70.56|) = 6
logb(17.6) = log(|17.6|) = 4

Program3:

// C++ program to illustrate 
// to implement logb() function 
// when input is 0 
#include <cmath> 
#include <iostream> 
  
using namespace std; 
  
int main() 
{ 
    double result; 
    int x = 0; 
  
    result = logb(x); 
    cout << "logb(" << x << ") = "
         << "log(|" << x << "|) = " << result << endl; 
  
    return 0; 
} 

Output:

logb(0) = log(|0|) = -inf

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