Converting Strings to Floating-Point Numbers in Java

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I wrote this some time ago, when I was using Java 1.0 and 1.1. Those early versions had no library routines for loading Double/Float values from a String, so I rolled my own (see below). Since Java 1.2 however, all you have to do is try { double a = Double.parseDouble(“1.5311243e-23”).doubleValue(); } catch( NumberFormatException e ) { panic(e); } I’m not sure whether the parseDouble method copes with using a ‘D’ as the seperator between the mantissa and exponent, but I suspect it does. Check the Java API for more info.

Parsing Floating-Point Numbers in Java 1.0 and 1.1

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While Java does have methods such as Integer.parseInt(), for converting strings into integers, there doesn’t appear to be any built-in methods to convert a string to a floating point number (or perhaps I just haven’t found them). While writing a routine to do it is not that difficult, it is rather fiddly, and so I have included my code on these pages (see below). The output of the program looks like this: Routine to show parseDouble in action… Should correctly interpret inputstr = -.4322e+02 value = -43.22 2*value = -86.44 The routine will translate strings of the form (e.g. 123.02, 121, -.321), and of the form (e.g. 0.123e4, 121.23d-5, .2D-001, 12E04).

Please feel free to copy and use the parseDouble routine, and If anyone knows a better way of doing it, e-mail me about it so I can improve my code. One known problem is that if the routine is made to translate a string it doesn’t recognise, it just writes ‘Bad data!’ to the standard output and returns (double) 0.0. Whenever I get around to seriously using this routine, I’ll alter it so it throws a NumberFormatException if there is a problem.

The Code

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Included here verbatim, and can also be downloaded from the link below. // –> parseDoubleEG.java // Example stand-alone application which uses the parseDouble routine: // By A N Jackson.

import java.io.*;

class parseDoubleEG {
public static void main(String args[]) {
  String filename,inputstr;
  float indat;

  inputstr = " -.4322e+02 ";

  System.out.println("Routine to show parseDouble in action...");
  System.out.println("Should correctly interpret inputstr = " + inputstr);

  indat = (float) parseDouble(inputstr);
  System.out.println("value = " + indat);

  indat *= 2.0;
  System.out.println("2*value = " + indat);

}

/* --> parseDouble v1.0
 Routine to parse a floating point number from a string.
 Written by A N Jackson (c) 27th January 1998.
*/
public static double parseDouble(String ipstr) {
  double num = 0.0, mant = 0.0, exp = 0.0;
  int mantsn = 1,expsn = 1,dp = -1,i,len,inum = 0;
  boolean inmant = true,inexp = false, skip = false;
  char ichr;
  String istr;

  len = ipstr.length();

  for (i=0; i<len; i++) {
    skip = false;
    ichr = ipstr.charAt(i);

    if (inexp) {
      if (ichr == ' ') {
        skip = true;
      }
      if (ichr == '+') {
        skip = true;
      }
      if (ichr == '-') {
        expsn = -1;
        skip = true;
      }
    }
    if (inmant) {
      if (ichr == ' ') {
        skip = true;
      }
      if (ichr == '+') {
        skip = true;
      }
      if (ichr == '-') {
        mantsn = -1;
        skip = true;
      }
      if (ichr == '.') {
        dp = 0;
        skip = true;
      }
      if (ichr == 'E' || ichr == 'e' || ichr == 'D' || ichr == 'd') {
        inmant = false;
        inexp = true;
        skip = true;
      }
    }
    if (skip != true) {
      istr = "x";
      istr = istr.replace('x',ichr);
      try {
        inum = Integer.parseInt(istr);
      } catch (NumberFormatException NFE) {
        // Should throw an appropriate NumberFormatException
        System.out.println("Bad data!");
        return (double) 0.0;
      }

      if (inmant && dp>-1) dp++;
      if (inmant) mant = mant*10.0 + inum;
      if (inexp) exp = exp*10.0 + inum;
    }

  }

  if (dp==-1) dp = 0;
  num = mantsn*mant*Math.pow(10.0, expsn*exp - dp);
  return num;
}
}