C++ ilogb() function usage and example

C++ Library Function <cmath>

C ++The ilogb() function in uses FLT_RADIX as the base of the logarithm and returns the integer part of the logarithm of |x|.

This is<cmath>defined in the header file.

Mathematically

x = significand　*　FLT_RADIXexponent

significand is in [1.0,2.0) within the range of floating-point values, x is the parameter passed to ilogb(), and exponent is the integer value returned by ilogb(). The value of FLT_RADIX is generally2.

The value returned by ilogb() is greater thanfrexp()The exponent generated by the function is smaller than1 , because the number of significant digits is in [ 1.0,2.0] range, rather than frexp() in [0.5,1.0] range.

ilogb() prototype [from C ++ 11Standard beginning]

int ilogb(double x);
int ilogb(float x);
int ilogb(long double x);
int ilogb(T x);　//　For integral type

The ilogb() parameter

The ilogb() function accepts a parameter, whose ilogb is calculated.

The return value of ilogb()

The ilogb() function uses FLT_RADIX as the base of the logarithm and returns the integer part of the logarithm of |x|.

• If the parameter is 0, it returns FP_LOGB0.

• If the parameter is NaN, it returns FP_LOGBNAN.

• If the parameter is infinite, it returns INT_MAX.

Example1：How the ilogb() function works in C ++How does it work in

#include <iostream>
#include <cmath>
#include <cfloat>
using namespace std;
int main()
{
　　　　int result;
　　　　double significand;
　　　　double x =　16.81;
　　　　result = ilogb(x);
　　　　significand = x　/　pow(FLT_RADIX, result);
　　　　cout << "ilogb (" << x << ") = " << result << endl;
　　　　cout << x << " = " << significand << ""　*　cout << " << FLT_RADIX << "^" << result << endl << endl;
　　　　return 0;
}

When the program is run, the output is:

ilogb (16.81) =　4
16.81　=　1.05062　*　2^4

Example2：The ilogb() function with integer type

#include <iostream>
#include <cmath>
#include <cfloat>
using namespace std;
int main()
{
　　　　int result, x =　19;
　　　　result = ilogb(x);
　　　　double significand = x/pow(FLT_RADIX, result);
　　　　cout << "ilogb (" << x << ") = " << result << endl;
　　　　cout << x << " = " << significand << ""　*　cout << " << FLT_RADIX << "^" << result << endl << endl;
　　　　
　　　　return 0;
}

When the program is run, the output is:

ilogb (19) =　4
19　=　1.1875　*　2^4

C++ Library Function <cmath>