Develop Reference

Aproximation of log(10^k)

I am trying to find the result of log(10^k) , where k is big number like 10000. For example :
BigDecimal first = BigDecimal.TEN.pow(10000);
double result = Math.log(first.doubleValue());
However "result" becomes Infinity , however on wolphram approximates it to 23025.85.Any suggestion how to find the result? As a result the number with the first two digits after the decimal point are enough for me.

Use the fact that
log(10^k) = k*log(10)
So:
System.out.println(10000 * Math.log(10));
Prints:
23025.850929940458

The problem you are likely having, is that Wolphram is able to either hold the powered value or it is doing the log operation first.
When running this like your example, you will have an extremely large number that goes past the maximum value for a BigDecimal, which should result in an error or an "infinity", because it overflows the capability of the data type, I would suggest doing the operation the other way arround, perhaphs process the log first on a base 1 value for example and only then multiply it by whatever powered number you are tying to use.

See, there is a simple property of logarithms that you can use:
log(x^y) = y*log(x)
So what you can do is:
double y = y*log(x);
System.out.println(Math.round(y));
Hope this helps!

How to keep n decimal places in Java

Hi All it might be a trivial question but as for now I could not find any solution.So asking for your help!
What I am trying to get is a specific encoding table that looks like this:
0.000
0.100
0.200
I need to keep track of zeroes as I will use them to reconstruct a part of an specific array. Original loop that was creating those numbers is:
for(int k=0;k<m_0;k++){
for(int l=0;l<m_1;l++){
for(int a=0;a<a1;a++){
Y[x-1]=0.1*k+0.01*l+0.001*a;
x++;
}
}
}
Problem! I could not fix zeros after decimal place and rather then getting table described above I am getting following:
0.0
0.1
0.2
As a solution I have tried to use BigDecimal and DecimalFormat but no success. Any suggestions?
UPD Few words what I am trying to do. I am encoding specific array to array and back index correspondence. For example 0.100 will be decomposed into 1 and 0 and 0 and used as array index labeling like:
Array1[Method(1,0,0,Y(i)][Method(1,0,0,Y(i))]=Array2[1][0][0]
So that I need an output suitable for assigning array index and string will not do the deal.

The DecimalFormat class is the correct place to look. You just need the correct format.
DecimalFormat df = new DecimalFormat("0.000");
System.out.println(df.format(0.1));
Output:
0.100

As an alternative to the DecimalFormat class, I would like to propose the following (which I use quite regularly):
Step 1: Create a function that allows me to specify the number of units to keep. Here is a copy of this function.
public static String format(Number n) {
NumberFormat format = DecimalFormat.getInstance();
format.setRoundingMode(RoundingMode.FLOOR);
format.setMinimumFractionDigits(0);
format.setMaximumFractionDigits(2);
return format.format(n);
}
Step 2: Call the function whenever you have any output to format. Below is a simple example using this function to set the appropriate decimal place length:
System.out.println("Based on this, the wind chill index was calculated to be " + format(chill));
Note that you could simply change the line:
format.setMaximumFractionDigits(2);
to
format.setMaximumFractionDigits(n);
depending on your desired decimal length.

When you are printing the numbers, you can use this:
System.out.format("%.3f", yourDecimalNumber);

How to format numbers on Android

i am learning to program mobile aplications on Android. My first app is a unit converter. Everithing is working for now, but i have a question about formating numbers. I hava this code to get text from buttons and to convert the appropriet output:
if (bPrevodZ.getText() == "milimeter"){
if (bPrevodDo.getText()=="kilometer"){
String PomocnaPremenna = jednotkaZ.getText().toString();
double cisloNaPrevod = Double.parseDouble(PomocnaPremenna);
cisloNaPrevod = cisloNaPrevod*0.0000001;
vysledok.setText(Double.toString(cisloNaPrevod));
}
The final result is "cisloNaPrevod", but i have problems to show a good format of that number. For example:
12345 mm = 0,0012345 km this is good right ? :)
but if i convert:
563287 mm = 0.05632869999999995 this is bad :) i need it to show 0.0563287
Thx for any help

Use String.format:
String.format("%.6f", cisloNaPrevod);

If you want your number to always have 6 significant figures, use
vysledok.setText(String.format("%.6g", cisloNaPrevod));
giving the result 0.0563287.
If you want to round to 6 numbers after the decimal place, use
vysledok.setText(String.format("%.6f", cisloNaPrevod));
giving the result 0.056329.
Here's some good resources that cover number formatting:
Floating-point cheat sheet for Java
java.util.Formatter

If it's something you're going to do often, perhaps you should use DecimalFormat.
DecimalFormat df = new DecimalFormat("#.#######");
Then call:
df.format(someDoubleValue);

Java Decimal Format - as much precision as given

I'm working with DecimalFormat, I want to be able to read and write decimals with as much precision as given (I'm converting to BigDecimal).
Essentially, I want a DecimalFormat which enforces the following pattern "\d+(\.\d+)?" i.e. "at least one digit then, optionally, a decimal separator followed by at least one digit".
I'm struggling to be able to implement this using DecimalFormat, I've tried several patterns but they seem to enforced fixed number of digits.
I'm open to alternative ways of achieving this too.
Edit:
For a little more background, I'm parsing user-supplied data in which decimals could be formatted in any way, and possibly not in the locale format. I'm hoping to let them supply a decimal format string which I can use the parse the data.

Since you noted in a comment that you need Locale support:
Locale locale = //get this from somewhere else
DecimalFormat df = new DecimalFormat();
df.setDecimalFormatSymbols(new DecimalFormatSymbols(locale));
df.setMaximumFractionDigits(Integer.MAX_VALUE);
df.setMinimumFractionDigits(1);
df.setParseBigDecimal(true);
And then parse.

This seems to work fine:
public static void main(String[] args) throws Exception{
DecimalFormat f = new DecimalFormat("0.#");
f.setParseBigDecimal(true);
f.setDecimalFormatSymbols(new DecimalFormatSymbols(Locale.US));// if required
System.out.println(f.parse("1.0")); // 1.0
System.out.println(f.parse("1")); // 1
System.out.println(f.parse("1.1")); // 1.1
System.out.println(f.parse("1.123")); // 1.123
System.out.println(f.parse("1.")); // 1
System.out.println(f.parse(".01")); // 0.01
}
Except for the last two that violate your "at least one digit" requirement. You may have to check that separately using a regex if it's really important.

help regarding rounding off numbers

float per = (num / (float)totbrwdbksint) * 100;
i m getting the value of per as say 29.475342 . i want it to round off upto two decimal places only.like 29.48 .how to achieve this?

You should do this as part of the formatting - the floating point number itself doesn't have any concept of "two decimal places".
For example, you can use a DecimalFormat with a pattern of "#0.00":
import java.text.*;
public class Test
{
public static void main(String[] args)
{
float y = 12.34567f;
NumberFormat formatter = new DecimalFormat("#0.00");
System.out.println(formatter.format(y));
}
}

As Jon implies, format for display. The most succinct way to do this is probably using the String class.
float f = 70.9999999f;
String toTwoDecPlaces = String.format("%.2f", f);
This will result in the string "71.00"

If you need to control how rounding is done you should check BigDecimal ist has several rounding modes. http://java.sun.com/j2se/1.5.0/docs/api/java/math/BigDecimal.html

You need to be careful here, this answer is not related to java, it relates to all aspects of decimals in many programming languages hence it is generic. The danger lies with rounding numbers, is this, and it has happened in my experience and know that it can be tricky to deal with:
Supposing you are dealing with prices on items, the pricing you get from a retail supplier may be different to the price the computer tells you, sure it is marginally small, but it could add up to big money.
Adding a sales tax on a price can either be positive or negative, it can impact the operating margin of the profit/loss balance sheets...
If you are in this kind of arena of development, then my advice is not to adjust by rounding up/down...it may not show up on small sales of the items, but it could show up elsewhere...an accountant would spot it...Best thing to do is to simply, truncate it,
e.g. 29.475342 -> 29.47 and leave it at that, why?, the .005 can add up to big profit/loss.
In conjunction to what is discussed here...electronic tills and registers use their own variety of handling this scenario, instead of dealing with XX.XXXXXXXXXX (like computers, which has 27/28 decimal places), it deals with XX.XX.
Its something to keep in mind...
Hope this helps,
Best regards,
Tom.

you can use the formatted print method System.out.printf to do the formatted printing if that's what you need