incorrect output

[UncleAl]

Hello I am working a C++ program and the calculation is giving me the wrong total of 1317.2 when it should be 1167.1. Can you look at my code and algorithm to see what I am missing or doing wrong? I cant seem to figure this out. Thanks very much.

#include <iostream> 
#include <math.h> 
using namespace std; 
int main() 
{ 
int loan = 200000; 
int y = 30; 
int monthlyterm = y*12; 
double i = .00575; //interest rate 
double monthlypayment; 
 
monthlypayment = (loan*i)/(1-pow(1+i,-monthlyterm)); 
cout<<"monthly payment is $"<<monthlypayment; 
return 0;
 

[Banfa]

I canconfirm that when compiled and run this code it prints $1317.2

However checking by hand that is correct for the data and algorithm in the program.

This means that either

1. You have not implemented your algorithm correctly

2. You have not got you constant values (e.g. the interest rate) correct

3. The method you have used to calculate the expected result of the program is wrong.


Since all you have posted is the program code and not any background information it is impossible to tell which one of these is the case in question.

[UncleAl]

I canconfirm that when compiled and run this code it prints $1317.2

However checking by hand that is correct for the data and algorithm in the program.

This means that either

1. You have not implemented your algorithm correctly

2. You have not got you constant values (e.g. the interest rate) correct

3. The method you have used to calculate the expected result of the program is wrong.



Since all you have posted is the program code and not any background information it is impossible to tell which one of these is the case in question.

Thanks I think 1 or 3 is my problem. If I use a mortgage calulator with 200,000 at 5.75 for 30 years it is 1,167 payment but my program gives 1317.2. How can I tell if 1 or 3 is my problem?

[Banfa]

Right I see 2 problems which explain why you are not getting the same result.

1. Your program looks like it may be attempting compound interest but my calculations suggest that your mortgage calculator is doing simple interest.

In compound interest

Year Interest Rate = pow(1+ monthly interest rate, 12) - 1

In simple interest

Year Interest Rate = 12 x monthly interest rate

I also think that you have made the same mistake as me.

Mortgage calculators normally work in years but your calculation is in months.

I did this input to mortgage calculator

principle: $200000
term: 30
rate: 5.75%

but you are calculating in months with a monthly interest rate of 0.575%. Using simple interest this equates to 0.575x12 = 6.9%

When you put

principle: $200000
term: 30
rate: 6.9%

into a mortgage calculator it spits out the monthly paymemt of $1317.20

Which is what your program says.

So the problem was in 3, calculating the expected result :D

[UncleAl]

Right I see 2 problems which explain why you are not getting the same result.

1. Your program looks like it may be attempting compound interest but my calculations suggest that your mortgage calculator is doing simple interest.

In compound interest

Year Interest Rate = pow(1+ monthly interest rate, 12) - 1

In simple interest

Year Interest Rate = 12 x monthly interest rate

I also think that you have made the same mistake as me.

Mortgage calculators normally work in years but your calculation is in months.

I did this input to mortgage calculator

principle: $200000
term: 30
rate: 5.75%

but you are calculating in months with a monthly interest rate of 0.575%. Using simple interest this equates to 0.575x12 = 6.9%

When you put

principle: $200000
term: 30
rate: 6.9%

into a mortgage calculator it spits out the monthly paymemt of $1317.20

Which is what your program says.

So the problem was in 3, calculating the expected result :D

Thanks very very much. I have been trying to still get this to work with your guidance and can't seem to. At the risk of souding foolish, if my variables are as follows, can you help me get the algorithm right?

int loan = 200000;
int y = 30;
int monthlyterm = y*12;
double i = .00575; //interest rate
double monthlypayment;

monthlypayment = (loan*i)/(1-pow(1+i,-monthlyterm)); CAN'T FIGURE OUT !!!

[Banfa]

Erm, did you not understand my previous post? Assuming that the interest rate you are using is the monthly rate (and it seems in the correct order of magnitude to be the monthly rate) then you program was working correctly.

Your calculation of the expect result was wrong.