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# log2() function in C++ with Examples

• Last Updated : 04 May, 2020

The function log2() of cmath header file in C++ is used to find the logarithmic value with base 2 of the passed argument.

Syntax:

`log2(x)`

Parameters: This function takes a value x, in the range [0, ∞] whose log value is to be found.

Return Type: It returns the logarithmic value, as double, float or long double type, based on the following conditions:

• If x > 1: It returns the positive logarithmic value of x.
• If x is equals to 1: It returns 0.
• If 0 < x < 1: It returns the negative logarithmic value of x.
• If x is equals to 0: It returns the negative infinity(-∞).
• If x < 0: It returns NaN(Not a Number).

Below examples demonstrate the use of log2() method:

Example 1:

 `// C++ program to illustrate log2() function`` ` `#include ``using` `namespace` `std;`` ` `// Driver Code``int` `main()``{``    ``long` `b = 16;``    ``float` `c = 2.5;``    ``double` `d = 10.35;``    ``long` `double` `e = 25.5;`` ` `    ``// Logarithmic value of long datatype``    ``cout << log2(b) << ``"\n"``;`` ` `    ``// Logarithmic value of float datatype``    ``cout << log2(c) << ``"\n"``;`` ` `    ``// Logarithmic value of double datatype``    ``cout << log2(d) << ``"\n"``;`` ` `    ``// Logarithmic value of long double datatype``    ``cout << log2(e) << ``"\n"``;`` ` `    ``return` `0;``}`
Output:
```4
1.32193
3.37156
4.67243
```

Example 2:

 `// C++ program to illustrate log2() function`` ` `#include ``using` `namespace` `std;`` ` `// Driver Code``int` `main()``{``    ``// To show extreme cases``    ``int` `a = 0;``    ``int` `b = -16;`` ` `    ``// Logarithmic value of 0``    ``cout << log2(a) << ``"\n"``;`` ` `    ``// Logarithmic value of negative value``    ``cout << log2(b) << ``"\n"``;`` ` `    ``return` `0;``}`
Output:
```-inf
nan
```
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