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07242007, 12:45 AM

#1

Member
Registered: Oct 2005
Posts: 162
Rep:

Calculate e^x in Java
I'm trying to calculate e^x, without any builtin functions such as Math.exp. This is what I have so far:
Code:
public static double math_ex(int iter, int x) {
double ex = 0;
int i = 1;
while(i <= iter) {
if (i == 1) {
ex += x;
} else {
ex += Math.pow(x,(double)i/factorial(i));
}
i++;
}
ex += 1;
return ex;
}
My input is:
Code:
Enter iterations:
6
Enter x:
3
e^3 = 11.989024
But when I use the calculator to figure it out, it is 20.0855........
Can you see anything thats wrong with my function?
Thanks.



07242007, 07:06 AM

#2

Senior Member
Registered: Jan 2005
Location: Roodepoort, South Africa
Distribution: Slackware 10.1/10.2/12, Ubuntu 12.04, Crunchbang Statler
Posts: 3,786

Not a java programmer, but if Math.exp is a built=in function, I assume that Math.pow is as well.
Don't know what is wrong with your code, but is simple multiplying in a loop not easier.
something like
Code:
result=1;
while(iter)
result *= x;
return result;
This is not bug free.
[edit]
OK, it's still vague but the factorial is probably to calculate e; so calculate e first and fill it in in place of x. Will be a bit faster as well as you only have to calculate e once.[/edit]
Last edited by Wim Sturkenboom; 07242007 at 07:20 AM.



07242007, 07:29 AM

#3

LQ Newbie
Registered: Nov 2006
Location: Russia
Distribution: OpenSuSE 10.2
Posts: 1
Rep:

Quote:
Originally Posted by ckoniecny
Code:
ex += Math.pow(x,(double)i/factorial(i));

This line should be
Code:
ex += Math.pow(x,(double)i)/factorial(i);
according to the series e^x=1+x+x^2/2!+...+x^n/n!
(not x^(n/n!) )



07242007, 07:31 AM

#4

LQ Veteran
Registered: Nov 2005
Location: Annapolis, MD
Distribution: Arch/XFCE
Posts: 17,802

What is the algorithm that you intended to code? (I just ran the numbers by hand and it clearly is not converging on the right answer)



07242007, 11:22 PM

#5

Member
Registered: Jun 2006
Location: Texas
Distribution: Ubuntu
Posts: 207
Rep:

Quote:
Originally Posted by pixellany
What is the algorithm that you intended to code? (I just ran the numbers by hand and it clearly is not converging on the right answer)

It looks like it's supposed to be the MacLaurin series sum(x**n / n! for n in {0, 1, 2, ...}).



07242007, 11:27 PM

#6

Member
Registered: Jun 2006
Location: Texas
Distribution: Ubuntu
Posts: 207
Rep:

BTW, what type does your factorial function use? Because of overflow, "int" is only accurate up to n=12, and even "long" only gets you to n=20.



07252007, 11:28 AM

#7

Senior Member
Registered: Sep 2003
Posts: 3,171
Rep:

In addition to changing it as oami specified, you have to either start i at 0 or add one to the result.



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