Problem with fractionally increasing numbers

[vonbismarck]

Hello all. I am trying to write a program that would allow me to find the highest interest rate I could afford based off of length of the loan, amount borrowed and the most I could pay a month. My problem is that I don't know how to continually increase the interest fractionally until I reach the desired number. I know that if I do ++ it continually increases by 1 but I need to increase continually by only .001. Also, when this number is reached, how do I display it? Lastly, an exact number might not be able to be reached (i.e. .0625 is too small but .0626 is too large). If this is the case, I would need it to display the largest number without going over (i.e. .0625 from the example above).

Now that I write this, it seems like it is probably more complicated than I had originally thought. Here is the code that I have so far:

#include <cstdlib>
#include <iostream>

using namespace std;



#include <math.h>
int main(int argc, char *argv[])
{
    float d;
    float a;
    float B;
    
cout<<"How many years is the loan for? ";
cin>>d;
cout<<"How much do you want to borrow? ";
cin>>a;
cout<<"What is the most you will pay a month? ";
cin>>B;
  
  float i=d*12;
  float r=.001;
  float b=r/12;
  float c=b+1;
  float j=pow(c,i);
  float f=1/j;
  float g=1-f;
  float h=g/b;
  float z=a/h;
  while (z<B)
  {
        ;
        }
  cout<<"The highest rate you can afford is "<<endl;        
    system("PAUSE");
        return EXIT_SUCCESS;
}

I left blank the spots where I think the code should go. I apologize for what is probably sloppy code, I wrote this after a day of reading tutorials and only made it as far as functions and also from what I remembered from reading tutorials from a year or more ago.

If there are any questions, feel free to ask.

Thank you.

[Faldrax]

One option for getting an increment of 0.001 is to have code such as

double interestRate;
for (int i=60; i<70; i++)      //Rates from 0.060 to 0.070
{
    interestRate = double(i)/1000;  //case i to double otherwise calculation will
                          //use integer arithmetic, return 0 then convert to double:-(
    //calculation using rate here
}

To stop when you have found the limit you want there are a couple of options.
You could have multiple conditions in the for loop

for (int i=60; i<70 && monthlyPayment<maxICanAfford ; i++)   
...

or you can use a break statement to jump out of a for loop.
To get the value you want you can use a second set of varaibles

for ...
{
   //do calculations
   if (monthlyPayment<maxICanAfford)
   {
       //copy values to second set
   }
   else
   {
       //exit loop 
   }
}
//Print second set