In this C++ tutorial, you will learn how to find logarithm of absolute value of given number using logb() function of cmath, with syntax and examples.

## C++ logb()

C++ logb() returns logarithm of absolute value of given argument. FLT_RADIX is used as base for the logarithm calculation.

FLT_RADIX is 2 or greater, but generally 2. Please check this value before using logb() function.

## Syntax

The syntax of C++ logb() is

`logb(x)`

where

Returns

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

The return value of logb(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 logb() function is

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

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

## Example

In this example, we read a value from user into variable x, and find the logarithm value of |x| using logb() function.

C++ Program

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

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

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

Output

```Enter a number : 16
logb(16) : 4
Program ended with exit code: 0```
```Enter a number : -16
logb(-16) : 4
Program ended with exit code: 0```
```Enter a number : 0
logb(0) : -inf
Program ended with exit code: 0```

## Conclusion

In this C++ Tutorial, we learned the syntax of C++ logb(), and how to use this function to find the logarithm of absolute value of given number, with base value of FLT_RADIX, with the help of examples.