Converting Strings to Floating-Point Numbers in Java

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

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

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-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;
  }
 }