nan()

The nan() function returns a quiet NaN (not a number) value of type double.

nan(x)

The x argument is a character string that can be used by library implementations to distinguish different NaN values.

The following example uses the nan() function to return a NaN value:

#include <iostream>
#include <cmath>

using namespace std;

int main() {
  double number1 = 1.0;
  double number2 = nan("");

  if (isnan(number1)) {
    cout << "number1 is not a number";
  }

  if (isnan(number2)) {
    cout << "number2 is not a number";
  }

  return 0;
}

This produces the following output:

number2 is not a number

The following example is runnable and uses the nan() function to return a NaN value:

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