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!

yearsToDoubl e?

Is the code above that you posted have any syntax error?

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

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;
}
}

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;
}

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.

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,

Jos