Java: How to format String number with tenth exponent

Question

I have a number as a string like this: "9.756088256835938E-4" but I only can use a specified number of characters (in this special case 9 char). So I want to have something like this: "9.7561E-4". I already tried to convert the String to a Double and then used the format method to get a less characters but I don't got a correct solution.

The problem is that I need ten exponential output since some numbers are longer than the number of characters I have. If it is possible, the number should be displayed with no ten exponent, if not just use the ten exponent.

Also correct rounding would be good.

And it should work for negative numbers. (minus needs one character!!!)

Is there a format function where I can define the maximum length of the output string? Any ideas?

Numerous similar posts: http://stackoverflow.com/q/4429163/122607, http://stackoverflow.com/q/4457722/122607, http://stackoverflow.com/q/649447/122607, http://stackoverflow.com/q/2525492/122607, http://stackoverflow.com/q/3008800/122607 — Pops, Mar 17 '11 at 14:04
@LordTorg, I think the OP is asking about using an tenth exponent, which none of these question address, they are all focused on number of decimal places. — jzd, Mar 17 '11 at 14:08
@jzd, that's possible... it depends on what the OP is trying to say with "ten exponential output"/"ten exponent." — Pops, Mar 17 '11 at 14:12
@LordTorg, I agree, the OP should include several examples with edge cases to be more clear. — jzd, Mar 17 '11 at 14:14
I'm not sure how it is called in english (tenth exponent) but jzd is right that my focus was on this exponent format. I already found most of the posts but they didn't help with my problem. thanks anyway — fpdragon, Mar 18 '11 at 07:49

Mark Peters · Accepted Answer · 2011-03-17T14:48:52.847

I'm having trouble findind a single format pattern that will cover all of the cases that you described. But here's a combination of logic that I think works:

   public static void main(String[] args) throws Exception {
      // 97.560883
      System.out.println(formatNum(Double.parseDouble("9.756088256835938E+1")));
      // 9.756E+11
      System.out.println(formatNum(Double.parseDouble("9.756088256835938E+11"))); 
      // 0.0009756
      System.out.println(formatNum(Double.parseDouble("9.756088256835938E-4")));  
      // -9.8E+111
      System.out.println(formatNum(Double.parseDouble("-9.756088256835938E+111")));
   }

   private static final int MAX_LENGTH = 9;

   private static String formatNum(double number) {
      String out = null;
      for ( int i = 0; i < MAX_LENGTH; i++ ) {
         String format = "%." + i + "G";
         out = String.format(format, number);
         if ( out.length() == MAX_LENGTH ) {
            return out;
         }
      }
      return out; //the best we can do
   }

The "G" in the pattern instructs the formatter to forego the use of the exponent when it will allow for the same or better precision. We grow up to the maximum length and stop when our output string is 10 characters. I think you could take the same approach with a DecimalFormat, but I'm more familiar with Formatter.

Nice function but do you know why the String.format(format, number); throws an java.lang.ArrayIndexOutOfBoundsException when number = 0? — fpdragon, Mar 18 '11 at 08:07

score 2 · Answer 2 · edited May 23 '17 at 12:26

Seeing the Mark's example meet your requirements, I updated my answer to show the DecimalFormat implementation. I used Mark's test cases. It is definitely an uglier option because there is no easy way to turn on/off exponents. The only advantage over the String.format option is that it handles very small numbers well.

public static void main(String[] args) throws Exception {
    // 97.560883
    System.out.println(formatNum(Double.parseDouble("9.756088256835938E+1")));
    // 9.756E+11
    System.out.println(formatNum(Double.parseDouble("9.756088256835938E+11")));
    // 0.0009756
    System.out.println(formatNum(Double.parseDouble("9.756088256835938E-4")));
    // -9.8E+111
    System.out.println(formatNum(Double.parseDouble("-9.756088256835938E+111")));
}

private static final int MAX_LENGTH = 9;

private static String formatNum(double number) {
    int digitsAvailable = MAX_LENGTH - 2;
    if (Math.abs(number) < Math.pow(10, digitsAvailable)
            && Math.abs(number) > Math.pow(10, -digitsAvailable)) {
        String format = "0.";
        double temp = number;
        for (int i = 0; i < digitsAvailable; i++) {
            if ((temp /= 10) < 1) {
                format += "#";
            }
        }
        return new DecimalFormat(format).format(number);
    }
    String format = "0.";
    for (int i = 0; i < digitsAvailable; i++) {
            format += "#";
    }
    String r = new DecimalFormat(format + "E0").format(number);
    int lastLength = r.length() + 1;
    while (r.length() > MAX_LENGTH && lastLength > r.length()) {
        lastLength = r.length();
        r = r.replaceAll("\\.?[0-9]E", "E");
    }
    return r;
}

This reminded me of a similar question where the OP only had 5 or so spaces for a number and wanted to show a decimal only when there was enough space. But instead of exponents, wanted to use a suffix of (k,m, etc)

Yes your link seems to be similarly implemented. I didn't found this post in my context and I initially was looking about a out of box java function (seems there is none) because the standard double conversions also seems to switch automatically between tenth exponent and the normal format. I thought of a possibility to configure the standard behavior. thx — fpdragon, Mar 18 '11 at 07:55

Java: How to format String number with tenth exponent

2 Answers2