In this C++ tutorial, you will learn how to compute natural logarithm of (x+1), using log1p() function of cmath, with syntax and examples.

C++ log1p()

C++ log1p() returns natural logarithm of (x+1), where x is the given number as argument.


The syntax of C++ log1p() is



xA double, float, long double, or any integral type value.


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

The return value of log1p(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 log1p() function is

double log1p (double x);
float log1p (float x);
long double log1p (long double x);
double log1p (T x); // for integral type argument values

log1p() is a function of cmath library. Include cmath library at the start of program, if using log1p() function.



In this example, we read a value from user into variable x, and find the base-e logarithm value of (x+1) using log1p() function.

C++ Program

#include <iostream>
using namespace std;

int main() {
    double x;
    cout << "Enter a number : ";
    cin >> x;
    double result = log1p(x);
    cout << "log1p(" << x << ") : " << result << endl;


Enter a number : 10
log1p(10) : 2.3979
Program ended with exit code: 0
Enter a number : 0
log1p(0) : 0
Program ended with exit code: 0
Enter a number : -5
log1p(-5) : nan
Program ended with exit code: 0


In this C++ Tutorial, we learned the syntax of C++ log1p(), and how to use this function to find the natural logarithm of (x+1), with the help of examples.