Section 4 - The Mortgage Calculator

5A.4.1 The Program Listing

Now we can look at the full program.  You can copy/paste this into your C++ IDE and try it out.    As I said,  the definition of each of the three methods appears right after the main().

Here is the listing.  See how much you can understand before reading on.

#include <iostream>
#include <string>
#include <cmath>
using namespace std;

// method prototypes
void stateInstructions();
void getInputAndComputeMonthlyPayment();
void sayGoodbye();

int main()
{
stateInstructions();
getInputAndComputeMonthlyPayment();
sayGoodbye();
}

// gives an overview to user
void stateInstructions()
{
string instructions;

instructions =
"\nThe following program will calculate the \n"
"monthly payment  required for a loan of D dollars \n"
"over a period of Y years at an annual \n"
"interest rate of R%.\n";
cout << instructions;
}

// does all the work - gets input, computes and reports answer
void getInputAndComputeMonthlyPayment()
{
string prompt;
double dblPrincipal, dblRate, dblYears, dblMoRt, dblMonths;
double dblTemp, dblPmt;

// get principal
prompt = "\nEnter amount of the loan. (only use numbers, \n"
"please, no commas or characters like '$')\n"
"Your loan amount: ";
cout << prompt;
cin >> dblPrincipal;

// get interest
prompt = "\nNow enter the interest rate (If the quoted rate is 6.5%, \n"
"for example, enter 6.5 without the %.)\n"
"Your annual interest rate: ";
cout << prompt;
cin >> dblRate;

// get length of loan
prompt = "\nEnter term of the loan in years: ";
cout << prompt;
cin >> dblYears;

// convert years to months
dblMonths = dblYears * 12;

// convert rate to decimal and months
dblMoRt = dblRate / (100 * 12);

// use formula to get result
dblTemp = pow(1 + dblMoRt, dblMonths);
dblPmt = dblPrincipal * dblMoRt * dblTemp
/ ( dblTemp - 1 );

cout <<  "\nYour Monthly Payment: " << dblPmt << endl;
}

// sign off
void sayGoodbye()
{
string signoff;
signoff =
"\nThanks for using the Foothill Mortgage Calculator. \n"
"We hope you'll come back and see us again, soon.\n\n";
cout << signoff;
}

5A.4.2 Discussion of the Program

We already discussed what stateInstructions() does.  You can see that sayGoodbye() works exactly the same way, but with a different phrase to send to the screen.  That leaves the method getInputAndComputeMonthlyPayment() to discuss.

• First, notice that there is nothing major inside this method that is new (not counting pow() which we'll get to in a moment).  These are all statements that we could have placed directly in main() and we would have (or should have) been able to understand them even last week. 
• Second, notice that this method declares variables (both string objects and primitive doubles).  In fact, all of our methods declare variables inside their definitions.
• Third, because we don't know (yet!) how to transfer a variable or its value from one method to another, we had to do a lot inside getInputAndComputeMonthlyPayment().  It would have been better to have one method get the input, a second compute the payment and a third report the result.  We'll get there soon.
• Fourth, there is a method call inside the definition of getInputAndComputeMonthlyPayment() -- a call to a method pow().  This is a function that raises one number, base, to an exponent, power.  pow(A, B) will raise the number A to the power B.  This is useful if B is a large value or a fractional number.  However, if B is 2 or 3, it is easier to use ordinary multiplication rather than invoking this function.  In order to use pow() and the many other math functions like sqrt(), abs(), cos(), sin(), etc. , you have to #include <cmath> as we did at the top of the program.
• Fifth, because the formula was a substantially complicated one, I broke up the computation into a few lines.  If you study these lines and compare them to the formula I gave earlier, you should be able to see that this gives the right answer.

That's all I can think to say about this program.  You should meditate on these comments as you run the program and examine its source.  You might try changing the wording around but you'll have to keep the math as is if you want it to be accurate. While you're at it, you can see if you can afford that home you want.