CODEWITHSUNDEEP
c++
NosVader
NosVader

Reputation: 5

Compound Interest algorithm producing weird answer? (5-7 hours of troubleshooting already)

Very new to C++. Doing my best to get a good handle on this.

The extra couts after the read statements were for troubleshooting purposes to make sure it was actually reading user input.

I feel like this is a dumb question but iv been scouring the forums to find something similar to my issue.

I cannot for the life of me figure out how to get this program to start calculating the correct interest. When comparing to an online interest calculator it comes up with a very different number than what the program is calculating.

I have spent tons of hours troubleshooting this and texting different algorithms but I have yet to come out on-top. I dont normally like asking for help because I learn by struggling through something but this one is throwing me for a loop...

So I think I might either be having an issue with the libraries I am using or a calculation error. I know the code is a bit messy and I am learning how to clean it up but as for now I just want to get the code calculating compound interest based off of user input. I have added comments to explain what each section of code does to help with my messiness.

I dont believe its a syntax error but with the way I am either using a few of the commas in the algorithm or my parentheses are somehow wrong.

the formula I am using is,

A = P (1 + r/n)^(nt)

Amount = principle (1 + interest rate/times compounded)^(rate, times compounded)

#include <iostream>
#include <cmath>


using namespace std;

int main()
{

    double rate;
    double time;
    double principle;
    double amount;
    double amount2;

//Asking for the amount of money that the user would like to invest

    cout << "What is the amount of money you would like to invest? ";

    cin >> principle;
    cout << principle << endl;

//Asking for the interest rate that will be compounded annually

    cout << "What is the interest rate you would like to calculate? "; 
    cin >> rate;

//interest rate divided by 100 so it can be multiplied by the principle and the amount of time the money will be invested later in code.

    rate /= 100;
    cout << rate << endl;

//Asking for the amount of years that the user will invest. 
cout << "How many times would you like to compound your money? ";

    cin >> time;
    cout << time << endl;

//calculation for amount of money that will be made after all user input is input

    amount2 = pow(rate, time);
    cout << amount2 << endl;

    amount = (principle * (1 + rate/time), amount2); 


//Output data after all user data is input and calculated. 

    cout << "Your will have "; cout << amount; cout << " dollars in 
    interest! "<< endl;  

    return 0;

}

Upvotes: 0

Views: 796

Answers (1)

Mikel F
Mikel F

Reputation: 3671

This line:

amount = (principle * (1 + rate/time), amount2);

Does absolutely nothing with principle * (1 + rate/time). That expression is evaluated, and then the results are discarded, leaving amount to be assigned the value in amount2. I assume you were trying to call a function with those two expressions as parameters.

http://en.cppreference.com/w/cpp/language/operator_other#Built-in_comma_operator

In a comma expression E1, E2, the expression E1 is evaluated, its result is discarded, and its side effects are completed before evaluation of the expression E2 begins.

Now that we have your formula:

A = P (1 + r/n)^(nt)

Amount = principle (1 + interest rate/times compounded)^(rate * times compounded)

I am not certain you are computing the formula you have listed above, and note the correction to the second line, where I replace the comma with multiplication.

amount2 = pow(rate, time);

Computes rate^time, where you should have:

amount2 = 1 * time; // Where one is the times compounded per period (year)

So that your other line can read:

amount = principle * pow((1 + rate, amount2);

Because you are compounding once per year, you are dividing rate by 1, not by the number of years. By the same token, amount2 is now strictly equal to time;

Upvotes: 2

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