C++ if command reading true despite false

I've been trying to write a program that factors number for me but my if statement keeps running despite the condition being false. In function factor, I use modf to separate the decimal from the integer and store it as c. The if statement checks if c = 0 which means the number divided evenly.

Below is my source code with results below it.

#include <iostream>
#include <math.h>
using namespace std;

int factor(double b);
int i = 1;
int factors[] = {1};

int main()
{
    int a;
    double b;
    cout << "Please enter a integer to Factor: ";
    cin >> a;
    b = a;
    factor(b);
    return 0;
}

int factor(double b)
{
    double c;
    double d;

    for ( ; c != 0 ; i++)
    {
        cout << "for loop executed" << endl;
        c = modf (b/3, &d);
        cout << c << endl;
        if (c = 0.00);
        {
            cout << 3 << endl;
            factors[i] = 3;
            continue;
        }
        c = modf (b/5, &d);
        if (c = 0);
        {
            cout << 5 << endl;
            factors[i] = 5;
            continue;
        }
        c = modf (b/7, &d);
        if (c = 0);
        {
            cout << 7 << endl;
            factors[i] = 7;
            continue;
        }
        c = modf (b/11, &d);
        if (c = 0);
        {
            cout << 11 << endl;
            factors[i] = 11;
            continue;
        }
        c = modf (b/13, &d);
        if (c = 0);
        {
            cout << 13 << endl;
            factors[i] = 13;
            continue;
        }
        c = modf (b/17, &d);
        if (c = 0);
        {
            cout << 17 << endl;
            factors[i] = 17;
            continue;
        }
    }
    return c;
}

In cmd it prints

Please enter a integer to Factor: 50

for loop executed

0.666667

3

50/3=16.6 repeating
modf of 16.6666666 outputs 16 and 0.666667
0.666667 does not equal 0 so im confused.