# math question (natural log - ln)

Tom Clement

Greenhorn

Posts: 26

posted 11 years ago

Hi, working on a project which must calculate natural logarithms,

I see calculators do it all over, but my luck with source code evaluation hasn't turned up what I need.

Example:

ln 27 = 3.29583686600432907418573571076758

Must be a relatively easy way to do it, closest I came was the rounded 3:

with the following code

import java.math.*;

public class Test2 {

static double xDouble;

public static void main(String[] args) {

double d = 1.0;

BigInteger target = BigInteger.valueOf(27);

target = doLog(target,d);

System.out.println(target);

}

static public BigInteger doLog (BigInteger target, double divisor)

{

xDouble = logBig(target.abs())/divisor;

return BigInteger.valueOf((long)(xDouble));

}

static public double logBig(BigInteger z) {

// Needed because Math.log(z.doubleValue()) returns infinite for z bigger than MAX DOUBLE

BigInteger b;

int temp = z.bitLength() - 1000;

if (temp > 0) {

// b=z.divide(BigInteger.valueOf(2).pow(temp)); //replaced with below

b=z.shiftRight(temp);

return (Math.log(b.doubleValue()) + temp*Math.log(2));

}

else {return (Math.log(z.doubleValue()));}

}

}

Many thanks......................Tom

[ February 26, 2005: Message edited by: Tom Clement ]

I see calculators do it all over, but my luck with source code evaluation hasn't turned up what I need.

Example:

ln 27 = 3.29583686600432907418573571076758

Must be a relatively easy way to do it, closest I came was the rounded 3:

with the following code

import java.math.*;

public class Test2 {

static double xDouble;

public static void main(String[] args) {

double d = 1.0;

BigInteger target = BigInteger.valueOf(27);

target = doLog(target,d);

System.out.println(target);

}

static public BigInteger doLog (BigInteger target, double divisor)

{

xDouble = logBig(target.abs())/divisor;

return BigInteger.valueOf((long)(xDouble));

}

static public double logBig(BigInteger z) {

// Needed because Math.log(z.doubleValue()) returns infinite for z bigger than MAX DOUBLE

BigInteger b;

int temp = z.bitLength() - 1000;

if (temp > 0) {

// b=z.divide(BigInteger.valueOf(2).pow(temp)); //replaced with below

b=z.shiftRight(temp);

return (Math.log(b.doubleValue()) + temp*Math.log(2));

}

else {return (Math.log(z.doubleValue()));}

}

}

Many thanks......................Tom

[ February 26, 2005: Message edited by: Tom Clement ]

M Beck

Ranch Hand

Posts: 323

posted 11 years ago

you're seeing your results rounded to the nearest integer value because you're storing them in a Big

**Integer**variable. have you tried using BigDecimal instead?
Tom Clement

Greenhorn

Posts: 26

posted 11 years ago

tried BigDecimal, it's not a straightforward swap, it'll take more time than I

have tonight, I'll definitely try tomorrow. With all those calculators out there, I'm hoping someone's done this already.

have tonight, I'll definitely try tomorrow. With all those calculators out there, I'm hoping someone's done this already.

David Harkness

Ranch Hand

Posts: 1646

posted 11 years ago

Why can't you use java.lang.Math.log(double)? Since you're taking the double value of your BigInteger (and eventually BigDecimal), why bother with those classes at all?

Layne Lund

Ranch Hand

Posts: 3061

posted 11 years ago

As their names imply, BigInteger and BigDecimal are meant to be used for arbitrarily large numbers. Unless, you are using numbers that are over 9*10^19 (the maximum value for a long), then you should probably just use the built-in types like double and long. The Math class from the standard API provides methods to perform just about any mathematical operation you need. These include the basic trigonometric functions and logarithms, to name a few.

HTH

Layne

HTH

Layne

Ilja Preuss

author

Sheriff

Sheriff

Posts: 14112

posted 11 years ago

Somehow this discussion seems to be continued at http://www.coderanch.com/t/326805/java/java/natural-logarithms-ln

Closing this thread...

Closing this thread...

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