how to check if double variable is almost equal to zero in C++?

Question

I want to find approximate polynomial roots of a polynomial equation.

#include <fstream>
#include <cmath>
#include <iostream>

using namespace std;

bool polynomial_algorithm(double a, double b, double c, double d, double e,int& sum, int i1, int i2)
{
    double y = 0;
    for (double i = i1; i <= i2; i += 0.0000001)
    {
        double x = (a * pow(i, 4) + b * pow(i, 3) + c * pow(i, 2) + d * i + e);
        
    }
    return true;
}

int main()
{
    int i1,i2,sum=0;
    double a, b, c, d, e;

    //interval configuration:

    i1 = -1;
    i2 = 2;

    cout << "interval (" << i1 << ";" << i2 << ")" << endl;

    //configuration of values:

    a = -1;
    b = 3;
    c = static_cast<double>(-4) / 9;
    d = static_cast<double>(-4) / 3;
    e = static_cast<double>(32) / 81;

    cout << a << " " << b << " " << c << " " << d << " " << e << endl;

    if (i2 < i1)
    {
        cout << "invalid interval";
    }

    cout << endl << "compiling" << endl;
    cout << endl << "compilation success: "<<polynomial_algorithm(a,b,c,d,e,sum,i1,i2) << endl;
}

This is the code I currently have, how do I got about checking if double x ≈ 0 in bool polynomial_algorithm?

I tried reading articles about the std::abs function and other stuff but it just really fried my brain, I appreciate the help if I get any.

Answer 1

I figured it out by using a bool variable to determine when a polynomial hits 0 watching when it becomes negative or positive by reading from a file.

bool rootofpolynome(int sum)
{
  fstream data("s.txt");
  double numb,numb1;
  bool ispositive;
  data >>numb>>numb1;
  if (numb < 0)
  {
    ispositive=0;
  }
  while (!data.eof())
    {
      data >>numb>>numb1;
      if (ispositive == false && numb > 0)
      {
        cout << "root " << numb1 << endl;
        result << "root " << numb1 << endl;
        sum++;
        ispositive=1; 
      }
      if (ispositive==true && numb < 0)
      {
        cout << "root " << numb1 << endl;
        result << "root " << numb1 << endl;
        ispositive=0;
        sum++;
      }
    }
  cout << "found " << sum << " polynomial roots" << endl;
  return true;
}