In this C++ tutorial, you will learn how to find integral part of logarithm of |x|, using ilogb() function of cmath, with syntax and examples.

## C++ ilogb()

C++ ilogb() returns integral part of logarithm of |x|, where x is the argument to ilogb() function.

## Syntax

The syntax of C++ ilogb() is

`ilogb(x)`

where

Returns

The return value depends on the type of value passed for parameter x.

The return value of ilogb(x) is

• double if x is double or integral type.
• float if x is float.
• long double if x is long double.

The synopsis of ilogb() function is

```double ilogb(double x);
float ilogb(float x);
long double ilogb(long double x);
double ilogb(T x); // for integral type argument values```

ilogb() is a function of cmath library. Include cmath library at the start of program, if using ilogb() function.

## Example

In this example, we read a value from user into variable x, and find the integral part of the calculation: logarithm value of |x| with FLT_RADIX as base; using ilogb() function.

C++ Program

```#include <iostream>
#include<cmath>
using namespace std;

int main() {
double x;
cout << "Enter a number : ";
cin >> x;

double result = ilogb(x);
cout << "ilogb(" << x << ") : " << result << endl;
}```

Output

```Enter a number : 19
ilogb(19) : 4
Program ended with exit code: 0```
```Enter a number : -18
ilogb(-18) : 4
Program ended with exit code: 0```
```Enter a number : 0
ilogb(0) : -2.14748e+09
Program ended with exit code: 0```

## Conclusion

In this C++ Tutorial, we learned the syntax of C++ ilogb(), and how to use this function, with the help of examples.