C++ log()

The log() function in C++ returns the natural logarithm (base-e logarithm) of the argument.

This function is defined in <cmath> header file.


[Mathematics] logex = log(x) [In C++ Programming]

log() prototype [As of C++ 11 standard]

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

log() Parameters

The log() function takes a single mandatory argument in the range [0, ∞].

If the value is less than zero, log() returns NaN (Not a Number).


log() Return Value

The log() function returns the natural logarithm of a number.


Parameter (x) Return VALUE
x > 1 Positive
x = 1 0
0 > x > 1 Negative
x = 0 -∞ (- infinity)
x < 0 NaN (Not a Number)

Example 1: How log() works?

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

int main ()
{
	double x = 13.056, result;
	result = log (x);
	cout << "log(x) = " << result << endl;
	
	x = -3.591;
	result = log (x);
	cout << "log(x) = " << result << endl;
	
	return 0;
}

When you run the program, the output will be:

log(x) = 2.56925
log(x) = nan

Example 2: log() With Integral Type

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

int main ()
{
	int x = 2;
	double result;

	result = log (x);
	cout << "log(x) = " << result << endl;
	
	return 0;
}

When you run the program, the output will be:

log(x) = 0.693147