Results 1 to 7 of 7
Thread: Years to Double
 10102011, 05:28 AM #1Member
 Join Date
 Sep 2011
 Posts
 29
 Rep Power
 0
Years to Double
Hey everyone,
I have a code here that has to check how long it takes for a value to double at any given interest rate. This is what i have so far, however it won't print anything except for rate(percent and years to double
Any ideas on what i'm doing wrong
/////////////////////////////////////////////////////////////////////////
class Hw05
{
//
public static void main ( String [] args ) throws Exception
{
System.out.println("rate(percent) years to double");
for ( int percent : new int[] {1,2,3,4,6,9,12,24} ) line(percent);
}
//
public static void line ( int percent )
{
System.out.printf("%7d%17d\n",percent,yearsToDoubl e(percent));
}
//
public static int yearsToDouble ( int percent )
{
int years = 0;
int initialAmount = 100;
while (initialAmount <= 200)
{
initialAmount *= percent; //compound interest
years++;
}
return years;
}
}
//
// end class Hw05
/////////////////////////////////////////////////////////////////////////
Thanks in advance!
 10102011, 05:49 AM #2Member
 Join Date
 May 2011
 Posts
 35
 Rep Power
 0
Re: Years to Double
yearsToDoubl e?
Is the code above that you posted have any syntax error?
 10102011, 05:56 AM #3Member
 Join Date
 Sep 2011
 Posts
 29
 Rep Power
 0
Re: Years to Double
Hmm don't know why it did that on here. On my code there is no space. no syntax errors.
do i need to multiply percent by .01 and then do
newAmount = ((intererest rate in percent form) * initial amount) + intial amount
 10102011, 06:14 AM #4Member
 Join Date
 Sep 2011
 Posts
 29
 Rep Power
 0
Re: Years to Double
Here is what i got now and it returns all 2's for years to double
{
//
public static void main ( String [] args ) throws Exception
{
System.out.println("rate(percent) years to double");
for ( int percent : new int[] {1,2,3,4,6,9,12,24} ) line(percent);
}
//
public static void line ( int percent )
{
System.out.printf("%7d%17d\n",percent,yearsToDoubl e(percent));
}
//
public static int yearsToDouble ( int percent )
{
int yearsToDouble = 0;
int initialAmount = 100;
double total= initialAmount * .01;
while (total <= 200)
{
total = (total *initialAmount) + initialAmount; //compound interest
yearsToDouble++;
}
return yearsToDouble;
}
}
 10102011, 06:53 AM #5Member
 Join Date
 May 2011
 Posts
 35
 Rep Power
 0
Re: Years to Double
Try this
public static int yearsToDouble(int percent) {
int yearsToDouble = 0;
double total = 100;
while (total <= 200) {
total = total * (100+percent)/100;
yearsToDouble++;
}
return yearsToDouble;
}
 10102011, 07:20 AM #6Member
 Join Date
 Sep 2011
 Posts
 29
 Rep Power
 0
Re: Years to Double
Wow you are a life saver! Thank you so much for your help.
could you explain what you did with the total = total * (100+percent)/100; ?
again thank you so much.
 10102011, 08:03 AM #7
 Join Date
 Sep 2008
 Location
 Voorschoten, the Netherlands
 Posts
 14,271
 Blog Entries
 7
 Rep Power
 24
Re: Years to Double
Oh dear, oh dear, this sure is a forum full of mathematically challenged folks ... where's my blackboard ...
If you have an interest percentage i then the multiplication rate r is 1+i/100 or (100+i)/100 (check this), so if you have an amount of money p, after one year you have an amount of money r*p, after two years you have r*r*p and after n years you have r**n*p (where ** is the power operator).
The question is, when have you doubled your present amount of money p? So:
2*p == r**n*p where n is the unknown value; continuing:
log(2*p) == log(r**n*p) >
log(2*p) == n*log(r)+log(p) >
log(2)+log(p) == n*log(r)+log(p) >
log(2) == n*log(r) >
n == log(2)/log(r)
As you can see there is no mention of the present amount p in the equation, i.e. every amount of money doubles in log(2)/log(r) years. There is no need for loops whatsoever; the above equation is enough.
kind regards,
JosThe only person who got everything done by Friday was Robinson Crusoe.
Similar Threads

How many years have you been using Java?
By derrickD in forum Jobs DiscussionReplies: 41Last Post: 05062011, 07:15 AM 
for loop leap years
By tabako in forum New To JavaReplies: 4Last Post: 10142010, 02:22 AM 
Java/JEE developer with more than 6 Years experience
By nusolid in forum Jobs WantedReplies: 3Last Post: 07222010, 02:21 PM 
Java is 15 years old  what is the Future?
By mikeVB in forum Jobs DiscussionReplies: 1Last Post: 03042010, 05:21 AM
Bookmarks