Question

How would I go about calculating a compound interest rate, so far I have:

 double principal = [[principalLabel text] doubleValue];
 NSLog(@"Principal: %lf",principal);
 double years = [[yearsLabel text] doubleValue];
 NSLog(@"Years: %lf",years);
 double months = [[monthsLabel text] doubleValue] / 12; 
 NSLog(@"Months: %lf",months);
 double days = ([[daysLabel text] doubleValue] / 365) / 10;
 NSLog(@"Days :%lf",days);
 double rate = ([[rateLabel text] doubleValue] / 100);
 NSLog(@"Rate: %lf",rate);
 double time = years + days + months;
 NSLog(@"Time: %lf",time);

 double total = pow(principal * (1 - rate), time);
 NSLog(@"Total: %lf",total);
 double interest = pow(principal * (1 - rate), time) - principal;
 NSLog(@"Interest: %lf",interest);
 NSString *interestString = [[NSString alloc] initWithFormat:@"%lf",interest];
 NSString *totalString = [[NSString alloc] initWithFormat:@"%lf",total];
 [interestLabel setText:interestString];
 [totalLabel setText:totalString];

So as you can see I have 5 UITextFields for the: principal, rate, years, months, days. At the moment I keep getting some answer that is no where near the actual answer I am after even though my math seems correct I have reviewed my code thoroughly and found no solution.

My desired result is: E.g.     
M = P * (1+R)^Y              
M = 1000 * (1+0.10)^2       
M = 1210

Answer 1

If you place the output from the NSLog messages in your question also, it will be even more helpful to answer your question. Right now an obvious mistake is mentioned below:

In this line of code

double total = pow(principal * (1 - rate), time);

you have 1 – rate, while you need to have

double total = pow(principal * (1 + rate), time);