Homework Help: C++ Question: Calculate the Investment at time n

1. Oct 4, 2016

Hughng

1. The problem statement, all variables and given/known data
Pretend you have some money (ha!) that you want to invest in the stock market. Ask the user for:

1. The initial investment

2. Yearly estimated market growth

3. How much (if any) you wish to withdraw from the account per year

4. The desired level of money you wish to get
The growth formula per year is simply:
I_new = I_old x (1+g)
Assume you withdraw money from the account after it grows (you cannot withdraw more than the account has!). Find out how long it takes to reach the desired amount of money (if it is reachable at all).

2. Relevant equations
I completed the program but when I ran g =-0.1 with other given values, I was supposed to obtain 340.11 as my instructor required, but I got a whole number: 360.

3. The attempt at a solution
Code (C):
#include <iostream>
#include <cmath>
using namespace std;
int main()
{
double I_0, g, W, BAL, I;
cout << " Initial Investment? ";
cin >> I_0;
cout << " Yearly growth rate? ";
cin >> g;
cout << " Yearly withdraw amount? ";
cin >> W;
cout << " Desired balance? ";
cin >> BAL;
I = I_0*(1 + g);
int i = 1;
if (W <= I_0 && g >= 0)
{
for (i = 1; I < BAL; i++)
{
I = I - W;
I = I*(1 + g);
}
cout << " Years: " <<  i << endl;
cout << " Balance at the end: "<< I << endl;
cout << " Amount withdrawn over period: " << W*i << endl;
}
else if (W >= I_0)
{
cout << " Years: " <<  i << endl;
cout << " Balance at the end: "<< 0 << endl;
cout << " Amount withdrawn over period: " << I << endl;
}

else if (W <= I_0 && g <= 0)
{
for (i = 1; I >= 0; i++)
{
I = I - W;
I = I*(1 + g);
}
cout << " Years: " <<  ceil(i) << endl;
cout << " Balance at the end: "<< 0 << endl;
cout << " Amount withdrawn over period: " <<W*i << endl;
}
return 0;
}

Last edited by a moderator: Oct 4, 2016
2. Oct 4, 2016

Staff: Mentor

What were the other input values you used?

They look like this:
[code=c]
[/code]

3. Oct 4, 2016

Hughng

Here are the data:

Initial investment? 1000
Yearly growth rate? 0.1
Yearly withdraw amount? 20
Desired balance? 1000000
Years: 18
Balance at end: 0
Amount withdrawn over period: 340.114

4. Oct 5, 2016

Staff: Mentor

You said in an earlier post that you were using -0.1 as the growth rate.
Which is it, a growth rate of 0.1 or -0.1?

5. Oct 5, 2016

Hughng

The growth rate is -0.1. I am sorry I did not notice. Thank you.

6. Oct 5, 2016

Staff: Mentor

If the growth rate is -0.1 and you start with 1000 and withdraw 20 each year, you'll never get to 1,000,000.

7. Oct 6, 2016

Staff: Mentor

@Hughng, any progress on this problem?

8. Oct 6, 2016

Hughng

Hello Mark,
Yes, if you keep withdrawing the money given the initial amount above, you will not get the desire investment. The assignment will ask you to "cout" O for final balance, but you still have to print out the number of years that will take to exhaust your account, and the total amount you can withdraw until you have nothing to withdraw. However, I had an issue which was I could not print out the desired withdrawn amount as the assignment require: 340.11. I got a whole number which is 360.
Thank you.

9. Oct 6, 2016

Staff: Mentor

Why do you think that $340.11 is the correct amount? If you withdraw$20 per year for 18 years, that's \$360.

10. Oct 7, 2016

Hughng

I did not think so, but the assignment asked me to print out the desired result. I submitted the assignment, but I really appreciate your help.