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Java Forum / General / September 2007float to int conversion
System.out.println(new Float(21082007.0).intValue()); // -> 21082008 System.out.println(new Double(21082007.0).intValue()); // -> 21082007 Why? System.out.println(new BigDecimal(21082007.0F).intValue()); // -> 21082008 System.out.println(new BigDecimal(21082007.0D).intValue()); // -> 21082007 Again, why? And what's the proper way of doing it, using always a double? Thanks in advance On Sep 25, 8:45 am, diego.mani...@gmail.com wrote: > System.out.println(new Float(21082007.0).intValue()); // -> 21082008 > System.out.println(new Double(21082007.0).intValue()); // -> 21082007 [quoted text clipped - 11 lines] > > Thanks in advance Floating point numbers are inherently inaccurate. I have to comment that those numbers look like dates. If they are dates, you should use either a Date object, or at worse an integral (long or int) value that represents the milliseconds or nanoseconds from some epoch. Luckily for you, Java supports those out of the bag... If I was wrong about it being a date, you still should figure out if you REALLY need it to be a float or double. Even double has the same problem you're seeing, it just doesn't happen at the same time. > On Sep 25, 8:45 am, diego.mani...@gmail.com wrote: > [quoted text clipped - 24 lines] > you REALLY need it to be a float or double. Even double has the same > problem you're seeing, it just doesn't happen at the same time. Hi, thanks for your reply. Yes, those numbers represent dates, and yes, I would have used another data type to store them, but that's not my choice. The fields are declared with a decimal data type in the DDBB I'm accessing to, so I was using resultSet.getFloat() to retrieve the values, and then casting to int and using a DateFormat to parse the values. I think it's probable that resultSet.getInt() will have the same problems. > Hi, thanks for your reply. Yes, those numbers represent dates, and > yes, I would have used another data type to store them, but that's not > my choice. The fields are declared with a decimal data type in the What is the exact type in the data store? What is the date format, DDMMYYYY? > DDBB DBMS? If not, what is a "DDBB"? > I'm accessing to, so I was using resultSet.getFloat() to retrieve > the values, and then casting to int and using a DateFormat to parse > the values. I think it's probable that resultSet.getInt() will have > the same problems. Floating-point types are the worst possible choice here, much, much worse than using an integral type. You couldn't be more mistaken. Note that Java has BigInteger and BigDecimal types. <http://java.sun.com/javase/6/docs/api/java/sql/ResultSet.html#getBigDecimal(java .lang.String)> <http://java.sun.com/javase/6/docs/api/java/sql/PreparedStatement.html#setBigDeci mal(int,%20java.math.BigDecimal)>  SignatureLew diego.manilla wrote: >> System.out.println(new Float(21082007.0).intValue()); // -> 21082008 >> System.out.println(new Double(21082007.0).intValue()); // -> 21082007 [quoted text clipped - 7 lines] >> >> Again, why? This is no different from the first case. >> And what's the proper way of doing it, using always a double? How many digits of accuracy does float support? double? <http://mindprod.com/jgloss/ieee754.html> > Floating point numbers are inherently inaccurate. > I have to comment that those numbers look like dates. If they are [quoted text clipped - 6 lines] > you REALLY need it to be a float or double. Even double has the same > problem you're seeing, it just doesn't happen at the same time. No one can claim to be a programmer without having read <http://docs.sun.com/source/806-3568/ncg_goldberg.html> SignatureLew On Tue, 25 Sep 2007 15:55:23 -0000, Daniel Pitts <googlegroupie@coloraura.com> wrote, quoted or indirectly quoted someone who said : >I have to comment that those numbers look like dates. In that case see http://mindprod.com/jgloss/calendar.html Java has a whole set of special tools for dealing with dates and times. You never use double or float. The human calendar it just too whacky to be described by the regularity of the rational numbers or even Java floating point. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com > System.out.println(new Float(21082007.0).intValue()); // -> 21082008 > System.out.println(new Double(21082007.0).intValue()); // -> 21082007 > > Why? [...] > And what's the proper way of doing it, using always a double? Java floats use a 23-bit mantissa, so can't represent every 25-bit number exactly. Java double uses 52 bits. Why don't you use an int (32 bit) for this? It seems to me your "dates" can't get significantly bigger than about 31 million. /gordon -- >System.out.println(new Float(21082007.0).intValue()); // -> 21082008 see http://mindprod.com/applet/converter.html you can do that more simply without creating a dummy Float object: // to int i from float f // best i = (int)f; // or i = Math.round(f); // or i = (int)Math.ceil(f); // or i = (int)Math.floor(f); // to see the IEEE bits inside a float i = Float.floatToIntBits(f); SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com >System.out.println(new Float(21082007.0).intValue()); // -> 21082008 21082007.0 is an implied double. It gets converted to a float to be passed as a parameter. See http://mindprod.com/jgloss/primitive.html float has only 6 to 7 significant digits, so effectively you are passing 21082000.0 You are "lucky" you got as close as you did (ints do better). SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com >And what's the proper way of doing it, using always a double? It depends how much precision you need. Because modern floating point hardware is all built on double, normally the only time you use float is for float[] to conserve space when you don't need the precision. see http://mindprod.com/jgloss/primitive.html SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com >System.out.println(new Float(21082007.0).intValue()); // -> 21082008 >System.out.println(new Double(21082007.0).intValue()); // -> 21082007 read the essays at http://mindprod.com/jgloss/primitive.html http://mindprod.com/jgloss/floatingpoint.html There are many other lions lying in wait. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. 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