CODEWITHSUNDEEP
java
kiwu2
kiwu2

Reputation: 7

(Java)Calculating the number of years to get to a specific balance

Here's the prompt:

There is a bank account that earns 5 percent interest per year. The initial account balance is $10000. Given a target balance (for example, $20000), return the number of years that it takes for the account balance to reach the target balance. The number of years should be an integer.

Here's my code:

public int getNumberOfYears(double targetBalance){
  int years = 1;
  for (double balance = 10000.0; balance <= targetBalance; years++) {
    double multiplier = Math.pow(1.05, years);
    balance *= multiplier;
  }
  return years;
}

When this runs, [multiplier] increases by the following pattern:

1.05^1 --> (1.05^1)^2 --> ((1.05^1)^2)^3 --> etc.

How do I fix my code so that [multiplier] increases by the following pattern:

1.05^1 --> 1.05^2 --> 1.05^3 --> etc?

Upvotes: 0

Views: 735

Answers (4)

Saumye Navarathna
Saumye Navarathna

Reputation: 32

You can use a while loop instead of for.

public int getNumberOfYears(double targetBalance){
    int years = 1;
    double balance = 10000.0;
    while (1) {
        if ( balance * 1.05 * years >= targetBalance) {
            break;
        }
        else {
            years++;
            continue;
        }
    }
    return years;
}

But you should consider on what happens to the interest earned on a year, is it being added to the current balance or added separately. If it is being added to the starting balance of the account consider following code.

 public int getNumberOfYears(double targetBalance){
        int years = 0;
        double balance = 10000.0;
        while (balance < targetBalance ) {
            balance *= 1.05;
            years ++;
        }
        return years;
    }

Upvotes: 0

Chepe
Chepe

Reputation: 77

The following code will give you the answer in years. You can test it in excel by adding some formulas, the result is correct:

public class practice {

public static void main(String[] args) {
    
    
    double interestRate = 0.05;
    double initialAmount = 10000.00;
    double targetValue = 20000.00;
    int numYears = 0;
    
    while(targetValue > initialAmount) {
        
        initialAmount += (initialAmount * interestRate);
        numYears++;
        
    }
    
    System.out.println("Year to hit target: " + numYears);
    System.out.println("Final Amount: " + initialAmount);
    
    
}

}

Upvotes: 0

Mureinik
Mureinik

Reputation: 312219

You don't need to increase the multiplier - every year carries the same interest, and you just need to keep multiplying by the same interest rate every year:

public int getNumberOfYears(double targetBalance) {
  int years = 1;
  double multiplier = 1.05;
  for (double balance = 10000.0; balance < targetBalance; years++) {
    balance *= multiplier;
  }
  return years;
}

EDIT:
 As a side note, this can be solved without a loop, by extracting a log of both sides of the equation:

10000 * 1.05year = target
1.05year = target / 10000
log(1.05year) = log(target / 10000)
year * log(1.05) = log(target / 10000)
year = log(target / 10000) / log(1.05)

Or, in java:

public int getNumberOfYears(double targetBalance) {
    double years = Math.log(targetBalance / 10000) / Math.log(1.05);
    return (int) Math.ceil(year);
}

Upvotes: 0

azro
azro

Reputation: 54168

When you do balance * Math.pow(1.05, years) that means you directly apply the increase for years, you want to go year by year, so just use the 1.05.

Also for that kind of problem, a while loop is prefered

public static int getNumberOfYears(double targetBalance) {
    int years = 1;
    double balance = 10000;
    while (balance <= targetBalance) {
        balance *= 1.05;
        years++;
    }
    return years;
}

Upvotes: 0

Related Questions