The function **double log(double x);** returns the natural logarithm of x. The natural logarithm is the base-e logarithm.

## Function prototype of log

*double log(double x);***x**: A floating point value whose logarithm to be calculated. It must be positive.

## Return value of log

It returns the natural logarithm of x. If x is negative, a domain error occurs.

## C program using log function

The following program shows the use of log function.

#include <stdio.h> #include <math.h> int main () { double x, result; /* Logarithm is only defined for positive numbers */ printf("Enter a positive number\n"); scanf("%lf", &x); result = log(x); printf("Logarithm of %lf with base e is : %lf", x, result); return 0; }

Output

Enter a positive number 5.6 Logarithm of 5.600000 with base e is : 1.722767