ilogb() function in C++ STL

The ilogb(x) function in C++ STL returns the integral part of the logarithm of |x|, by using the FLT_RADIX as base for the logarithm. In general, the value of FLT_RADIX is 2, so ilogb() is equivalent to ilog2()(for positive values only).

Syntax:

ilogb(x)

Parameter: The function accepts a single mandatory parmaeter x whose ilogb() is to be calculated. The data-type can be of double, float, long double or int.

Return Value: The function returns an integral part of the logarithm of |x|, using FLT_RADIX as base for the logarithm. The function returns three exception values:

If the argument is NaN, it returns FP_LOGBNAN.
If the argument is infinite, it returns INT_MAX
If the argument is 0, it returns FP_LOGB0.

Below programs illustrate the above function:

Program 1:

// C++ program to illustrate the ilogb() 
// function when input is an integer 
#include <cfloat> 
#include <cmath> 
#include <iostream> 
  
using namespace std; 
  
int main() 
{ 
    int result, x = 25; 
  
    // Function to calculate ilogb(25) 
    result = ilogb(x); 
    cout << "ilogb (" << x << ") = " << result << endl; 
  
    // Function to calculate ilogb(50) 
    x = 50; 
    result = ilogb(x); 
    cout << "ilogb (" << x << ") = " << result << endl; 
  
    return 0; 
} 

Output:

ilogb (25) = 4
ilogb (50) = 5

Program 2:
Program for non-integral type

// C++ program to illustrate the ilogb() 
// function when input is a double value 
#include <cfloat> 
#include <cmath> 
#include <iostream> 
  
using namespace std; 
  
int main() 
{ 
    int result, x = 11.11; 
  
    result = ilogb(x); 
    cout << "ilogb (" << x << ") = " << result << endl; 
  
    x = 41.11; 
    result = ilogb(x); 
    cout << "ilogb (" << x << ") = " << result << endl; 
  
    return 0; 
} 

Output:

ilogb (11) = 3
ilogb (41) = 5

Program 3:

// C++ program to illustrate the ilogb() 
// function when input is 0 
#include <cfloat> 
#include <cmath> 
#include <iostream> 
#include <iostream> 
  
using namespace std; 
  
int main() 
{ 
    int result, x = 0; 
  
    result = ilogb(x); 
    cout << "ilogb (" << x << ") = " << result << endl; 
  
    return 0; 
} 

Output:

ilogb (0) = -2147483648

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