Unexpected value when multiplying a double

[John Ryan]

Hi,

We are converting a monetary value (say dollars and cents) which is stored in a Double into an integer which stores the number in cents by multiplying the value by 100. I would have expected this to work without a problem but I recently discovered that we do not always get the value we expected e.g. if a double contains the value 2.07 and is multiplied by 100 the result is not 207.000 but 206.99999999999997
But if 2.18 is multiplied by 100 the result is 218.00000000000003.

This can be seen by executing the following code

  
Double a = new Double("2.07");
System.out.println( "a is -> " +  a);
double b = a.doubleValue() * 100;
System.out.println( "b is ->" + b );
 
Double c = new Double("2.18");
System.out.println( "c is -> " +  c);
double d = c.doubleValue() * 100;
System.out.println( "d is ->" + d );

Im sure that this is some feature of the IEEE floating point standard that I dont fully understand but what now concerns me is how I can fix this problem so that the cent value is always correct. Would it be enough to round my double value up before converting it to an int as follows

  
Double a = new Double("2.07");
System.out.println( "a is -> " +  a);
double b = a.doubleValue() * 100;
System.out.println( "b is ->" + b );
int c = new java.math.BigDecimal( b ).setScale(2, java.math.BigDecimal.ROUND_CEILING ).doubleValue();
System.out.println( "c is -> " + c );

All comments appreciated.
John

[Jesper de Jong]

Computers simply cannot represent any arbitrary floating point number with infinite precision. Use java.text.DecimalFormat to print numbers on screen with a specified number of digits.

Read this if you want to know all the technical details:
What Every Computer Scientist Should Know About Floating-Point Arithmetic
[ August 24, 2005: Message edited by: Jesper de Jong ]

[Scheepers de Bruin]

The Double implementation actually has a "bug", because not all numbers can be represented in a binary form.

http://forum.java.sun.com/thread.jspa?threadID=386078&messageID=1660010
http://forum.java.sun.com/thread.jspa?threadID=618476&start=0&tstart=0

At the company I work for (a bank) we use the java.math.BigDecimal to handle currency. It is slighly better at it.

[Scheepers de Bruin]

hmmm... what he said. Never leave a post editor open over lunch =)